Bison syntax error

Table of Contents

Bison

This manual (23 January 2015) is for GNU Bison (version 3.0.4), the GNU parser generator.

Copyright © 1988-1993, 1995, 1998-2015 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover
texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License.”

(a) The FSF’s Back-Cover Text is: “You have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom.”

Introduction

Bison is a general-purpose parser generator that converts an annotated context-free grammar into a deterministic LR or generalized LR (GLR) parser employing LALR(1) parser tables. As an experimental feature, Bison can also generate IELR(1)
or canonical LR(1) parser tables. Once you are proficient with Bison, you can use it to develop a wide range of language parsers, from those used in simple desk calculators to complex programming languages.

Bison is upward compatible with Yacc: all properly-written Yacc grammars ought to work with Bison with no change. Anyone familiar with Yacc should be able to use Bison with little trouble. You need to be fluent in C or C++ programming in order
to use Bison or to understand this manual. Java is also supported as an experimental feature.

We begin with tutorial chapters that explain the basic concepts of using Bison and show three explained examples, each building on the last. If you don’t know Bison or Yacc, start by reading these chapters. Reference chapters follow, which
describe specific aspects of Bison in detail.

Bison was written originally by Robert Corbett. Richard Stallman made it Yacc-compatible. Wilfred Hansen of Carnegie Mellon University added multi-character string literals and other features. Since then, Bison has grown more robust and evolved
many other new features thanks to the hard work of a long list of volunteers. For details, see the
THANKS and ChangeLog files included in the Bison distribution.

This edition corresponds to version 3.0.4 of Bison.

Conditions for Using Bison

The distribution terms for Bison-generated parsers permit using the parsers in nonfree programs. Before Bison version 2.2, these extra permissions applied only when Bison was generating LALR(1) parsers in C. And before Bison version 1.24, Bison-generated
parsers could be used only in programs that were free software.

The other GNU programming tools, such as the GNU C compiler, have never had such a requirement. They could always be used for nonfree software. The reason Bison was different was not due to a special policy decision; it resulted from applying
the usual General Public License to all of the Bison source code.

The main output of the Bison utility—the Bison parser implementation file—contains a verbatim copy of a sizable piece of Bison, which is the code for the parser’s implementation. (The actions from your grammar are inserted into
this implementation at one point, but most of the rest of the implementation is not changed.) When we applied the GPL terms to the skeleton code for the parser’s implementation, the effect was to restrict the use of Bison output to free
software.

We didn’t change the terms because of sympathy for people who want to make software proprietary. Software should be free. But we concluded that limiting Bison’s use to free software was doing little to encourage people
to make other software free. So we decided to make the practical conditions for using Bison match the practical conditions for using the other GNU tools.

This exception applies when Bison is generating code for a parser. You can tell whether the exception applies to a Bison output file by inspecting the file for text beginning with “As a special exception…”. The text spells out
the exact terms of the exception.

GNU GENERAL PUBLIC LICENSE

Version 3, 29 June 2007

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This is free software, and you are welcome to redistribute it
under certain conditions; type ‘show c’ for details.

The hypothetical commands ‘show w’ and ‘show c’ should show the appropriate parts of the General Public License. Of course, your program’s commands might be different; for a GUI interface,
you would use an “about box”.

You should also get your employer (if you work as a programmer) or school, if any, to sign a “copyright disclaimer” for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see
http://www.gnu.org/licenses/.

The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this
is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read http://www.gnu.org/philosophy/why-not-lgpl.html.

1 The Concepts of Bison

This chapter introduces many of the basic concepts without which the details of Bison will not make sense. If you do not already know how to use Bison or Yacc, we suggest you start by reading this chapter carefully.

1.1 Languages and Context-Free Grammars

In order for Bison to parse a language, it must be described by a
context-free grammar. This means that you specify one or more
syntactic groupings and give rules for constructing them from their parts. For example, in the C language, one kind of grouping is called an ‘expression’. One rule for making an expression might be, “An expression
can be made of a minus sign and another expression”. Another would be, “An expression can be an integer”. As you can see, rules are often recursive, but there must be at least one rule which leads out of the recursion.

The most common formal system for presenting such rules for humans to read is Backus-Naur Form or “BNF”, which was developed in order to specify the language Algol 60. Any grammar expressed in BNF is a context-free grammar.
The input to Bison is essentially machine-readable BNF.

There are various important subclasses of context-free grammars. Although it can handle almost all context-free grammars, Bison is optimized for what are called LR(1) grammars. In brief, in these grammars, it must be possible to tell how to parse
any portion of an input string with just a single token of lookahead. For historical reasons, Bison by default is limited by the additional restrictions of LALR(1), which is hard to explain simply. See Mysterious Conflicts,
for more information on this. As an experimental feature, you can escape these additional restrictions by requesting IELR(1) or canonical LR(1) parser tables. See LR Table Construction, to learn how.

Parsers for LR(1) grammars are deterministic, meaning roughly that the next grammar rule to apply at any point in the input is uniquely determined by the preceding input and a fixed, finite portion (called a lookahead) of the
remaining input. A context-free grammar can be ambiguous, meaning that there are multiple ways to apply the grammar rules to get the same inputs. Even unambiguous grammars can be nondeterministic, meaning that no fixed lookahead
always suffices to determine the next grammar rule to apply. With the proper declarations, Bison is also able to parse these more general context-free grammars, using a technique known as GLR parsing (for Generalized LR). Bison’s GLR
parsers are able to handle any context-free grammar for which the number of possible parses of any given string is finite.

In the formal grammatical rules for a language, each kind of syntactic unit or grouping is named by a symbol. Those which are built by grouping smaller constructs according to grammatical rules are called
nonterminal symbols; those which can’t be subdivided are called
terminal symbols or token types. We call a piece of input corresponding to a single terminal symbol a token, and a piece corresponding to a single nonterminal symbol a grouping.

We can use the C language as an example of what symbols, terminal and nonterminal, mean. The tokens of C are identifiers, constants (numeric and string), and the various keywords, arithmetic operators and punctuation marks. So the terminal symbols
of a grammar for C include ‘identifier’, ‘number’, ‘string’, plus one symbol for each keyword, operator or punctuation mark: ‘if’, ‘return’, ‘const’, ‘static’,
‘int’, ‘char’, ‘plus-sign’, ‘open-brace’, ‘close-brace’, ‘comma’ and many more. (These tokens can be subdivided into characters, but that is a matter of lexicography,
not grammar.)

Here is a simple C function subdivided into tokens:

int             /* keyword ‘int’ */
square (int x)  /* identifier, open-paren, keyword ‘int’,
                   identifier, close-paren */
{               /* open-brace */
  return x * x; /* keyword ‘return’, identifier, asterisk,
                   identifier, semicolon */
}               /* close-brace */

The syntactic groupings of C include the expression, the statement, the declaration, and the function definition. These are represented in the grammar of C by nonterminal symbols ‘expression’, ‘statement’, ‘declaration’
and ‘function definition’. The full grammar uses dozens of additional language constructs, each with its own nonterminal symbol, in order to express the meanings of these four. The example above is a function definition; it contains
one declaration, and one statement. In the statement, each ‘x’ is an expression and so is ‘x * x’.

Each nonterminal symbol must have grammatical rules showing how it is made out of simpler constructs. For example, one kind of C statement is the
return statement; this would be described with a grammar rule which reads informally as follows:

A ‘statement’ can be made of a ‘return’ keyword, an ‘expression’ and a ‘semicolon’.

There would be many other rules for ‘statement’, one for each kind of statement in C.

One nonterminal symbol must be distinguished as the special one which defines a complete utterance in the language. It is called the start
symbol. In a compiler, this means a complete input program. In the C language, the nonterminal symbol ‘sequence of definitions and declarations’ plays this role.

For example, ‘1 + 2’ is a valid C expression—a valid part of a C program—but it is not valid as an entire C program. In the context-free grammar of C, this follows from the fact that ‘expression’
is not the start symbol.

The Bison parser reads a sequence of tokens as its input, and groups the tokens using the grammar rules. If the input is valid, the end result is that the entire token sequence reduces to a single grouping whose symbol is the grammar’s start
symbol. If we use a grammar for C, the entire input must be a ‘sequence of definitions and declarations’. If not, the parser reports a syntax error.

1.2 From Formal Rules to Bison Input

A formal grammar is a mathematical construct. To define the language for Bison, you must write a file expressing the grammar in Bison syntax: a Bison grammar file. See Bison Grammar Files.

A nonterminal symbol in the formal grammar is represented in Bison input as an identifier, like an identifier in C. By convention, it should be in lower case, such as expr, stmt or declaration.

The Bison representation for a terminal symbol is also called a token
type. Token types as well can be represented as C-like identifiers. By convention, these identifiers should be upper case to distinguish them from nonterminals: for example, INTEGER, IDENTIFIER, IF or
RETURN. A terminal symbol that stands for a particular keyword in the language should be named after that keyword converted to upper case. The terminal symbol error is reserved for error recovery. See Symbols.

A terminal symbol can also be represented as a character literal, just like a C character constant. You should do this whenever a token is just a single character (parenthesis, plus-sign, etc.): use that same character in a literal as the terminal
symbol for that token.

A third way to represent a terminal symbol is with a C string constant containing several characters. See Symbols, for more information.

The grammar rules also have an expression in Bison syntax. For example, here is the Bison rule for a C return statement. The semicolon in quotes is a literal character token, representing part of the C syntax for the statement; the
naked semicolon, and the colon, are Bison punctuation used in every rule.

See Syntax of Grammar Rules.

1.3 Semantic Values

A formal grammar selects tokens only by their classifications: for example, if a rule mentions the terminal symbol ‘integer constant’, it means that
any integer constant is grammatically valid in that position. The precise value of the constant is irrelevant to how to parse the input: if ‘
x+4’ is grammatical then ‘x+1’ or ‘x+3989’ is equally grammatical.

But the precise value is very important for what the input means once it is parsed. A compiler is useless if it fails to distinguish between 4, 1 and 3989 as constants in the program! Therefore, each token in a Bison grammar has both a token type
and a semantic value. See Defining Language Semantics, for details.

The token type is a terminal symbol defined in the grammar, such as
INTEGER, IDENTIFIER or ','. It tells everything you need to know to decide where the token may validly appear and how to group it with other tokens. The grammar rules know nothing about tokens except
their types.

The semantic value has all the rest of the information about the meaning of the token, such as the value of an integer, or the name of an identifier. (A token such as ',' which is just punctuation doesn’t need to have any semantic
value.)

For example, an input token might be classified as token type
INTEGER and have the semantic value 4. Another input token might have the same token type INTEGER but value 3989. When a grammar rule says that INTEGER is allowed, either of these tokens is acceptable
because each is an INTEGER. When the parser accepts the token, it keeps track of the token’s semantic value.

Each grouping can also have a semantic value as well as its nonterminal symbol. For example, in a calculator, an expression typically has a semantic value that is a number. In a compiler for a programming language, an expression typically has
a semantic value that is a tree structure describing the meaning of the expression.

1.4 Semantic Actions

In order to be useful, a program must do more than parse input; it must also produce some output based on the input. In a Bison grammar, a grammar rule can have an action made up of C statements. Each time the parser recognizes a match
for that rule, the action is executed. See Actions.

Most of the time, the purpose of an action is to compute the semantic value of the whole construct from the semantic values of its parts. For example, suppose we have a rule which says an expression can be the sum of two expressions. When the
parser recognizes such a sum, each of the subexpressions has a semantic value which describes how it was built up. The action for this rule should create a similar sort of value for the newly recognized larger expression.

For example, here is a rule that says an expression can be the sum of two subexpressions:

expr: expr '+' expr   { $$ = $1 + $3; } ;

The action says how to produce the semantic value of the sum expression from the values of the two subexpressions.

1.5 Writing GLR Parsers

In some grammars, Bison’s deterministic LR(1) parsing algorithm cannot decide whether to apply a certain grammar rule at a given point. That is, it may not be able to decide (on the basis of the input read so far) which of two possible reductions
(applications of a grammar rule) applies, or whether to apply a reduction or read more of the input and apply a reduction later in the input. These are known respectively as reduce/reduce conflicts (see Reduce/Reduce),
and shift/reduce conflicts (see Shift/Reduce).

To use a grammar that is not easily modified to be LR(1), a more general parsing algorithm is sometimes necessary. If you include
%glr-parser among the Bison declarations in your file (see Grammar Outline), the result is a Generalized LR (GLR) parser. These parsers handle Bison grammars that contain no unresolved conflicts
(i.e., after applying precedence declarations) identically to deterministic parsers. However, when faced with unresolved shift/reduce and reduce/reduce conflicts, GLR parsers use the simple expedient of doing both, effectively cloning the
parser to follow both possibilities. Each of the resulting parsers can again split, so that at any given time, there can be any number of possible parses being explored. The parsers proceed in lockstep; that is, all of them consume (shift)
a given input symbol before any of them proceed to the next. Each of the cloned parsers eventually meets one of two possible fates: either it runs into a parsing error, in which case it simply vanishes, or it merges with another parser, because
the two of them have reduced the input to an identical set of symbols.

During the time that there are multiple parsers, semantic actions are recorded, but not performed. When a parser disappears, its recorded semantic actions disappear as well, and are never performed. When a reduction makes two parsers identical,
causing them to merge, Bison records both sets of semantic actions. Whenever the last two parsers merge, reverting to the single-parser case, Bison resolves all the outstanding actions either by precedences given to the grammar rules involved,
or by performing both actions, and then calling a designated user-defined function on the resulting values to produce an arbitrary merged result.

1.5.1 Using GLR on Unambiguous Grammars

In the simplest cases, you can use the GLR algorithm to parse grammars that are unambiguous but fail to be LR(1). Such grammars typically require more than one symbol of lookahead.

Consider a problem that arises in the declaration of enumerated and subrange types in the programming language Pascal. Here are some examples:

type subrange = lo .. hi;
type enum = (a, b, c);

The original language standard allows only numeric literals and constant identifiers for the subrange bounds (‘lo’ and ‘hi’), but Extended Pascal (ISO/IEC 10206) and many other Pascal implementations
allow arbitrary expressions there. This gives rise to the following situation, containing a superfluous pair of parentheses:

type subrange = (a) .. b;

Compare this to the following declaration of an enumerated type with only one value:

(These declarations are contrived, but they are syntactically valid, and more-complicated cases can come up in practical programs.)

These two declarations look identical until the ‘..’ token. With normal LR(1) one-token lookahead it is not possible to decide between the two forms when the identifier ‘
a’ is parsed. It is, however, desirable for a parser to decide this, since in the latter case ‘
a’ must become a new identifier to represent the enumeration value, while in the former case ‘a’ must be evaluated with its current meaning, which may be a constant or even a function call.

You could parse ‘(a)’ as an “unspecified identifier in parentheses”, to be resolved later, but this typically requires substantial contortions in both semantic actions and large parts of the grammar, where
the parentheses are nested in the recursive rules for expressions.

You might think of using the lexer to distinguish between the two forms by returning different tokens for currently defined and undefined identifiers. But if these declarations occur in a local scope, and ‘a’ is defined
in an outer scope, then both forms are possible—either locally redefining ‘a’, or using the value of ‘a’ from the outer scope. So this approach cannot work.

A simple solution to this problem is to declare the parser to use the GLR algorithm. When the GLR parser reaches the critical state, it merely splits into two branches and pursues both syntax rules simultaneously. Sooner or later, one of them
runs into a parsing error. If there is a ‘..’ token before the next ‘
;’, the rule for enumerated types fails since it cannot accept ‘..’ anywhere; otherwise, the subrange type rule fails since it requires a ‘..’ token. So one of the branches
fails silently, and the other one continues normally, performing all the intermediate actions that were postponed during the split.

If the input is syntactically incorrect, both branches fail and the parser reports a syntax error as usual.

The effect of all this is that the parser seems to “guess” the correct branch to take, or in other words, it seems to use more lookahead than the underlying LR(1) algorithm actually allows for. In this example, LR(2) would suffice,
but also some cases that are not LR(k) for any k can be handled this way.

In general, a GLR parser can take quadratic or cubic worst-case time, and the current Bison parser even takes exponential time and space for some grammars. In practice, this rarely happens, and for many grammars it is possible to prove that it
cannot happen. The present example contains only one conflict between two rules, and the type-declaration context containing the conflict cannot be nested. So the number of branches that can exist at any time is limited by the constant 2,
and the parsing time is still linear.

Here is a Bison grammar corresponding to the example above. It parses a vastly simplified form of Pascal type declarations.

%token TYPE DOTDOT ID

%left '+' '-'
%left '*' '/'

%%
type_decl: TYPE ID '=' type ';' ;

type:
  '(' id_list ')'
| expr DOTDOT expr
;

id_list:
  ID
| id_list ',' ID
;

expr:
  '(' expr ')'
| expr '+' expr
| expr '-' expr
| expr '*' expr
| expr '/' expr
| ID
;

When used as a normal LR(1) grammar, Bison correctly complains about one reduce/reduce conflict. In the conflicting situation the parser chooses one of the alternatives, arbitrarily the one declared first. Therefore the following correct input
is not recognized:

The parser can be turned into a GLR parser, while also telling Bison to be silent about the one known reduce/reduce conflict, by adding these two declarations to the Bison grammar file (before the first ‘
%%’):

No change in the grammar itself is required. Now the parser recognizes all valid declarations, according to the limited syntax above, transparently. In fact, the user does not even notice when the parser splits.

So here we have a case where we can use the benefits of GLR, almost without disadvantages. Even in simple cases like this, however, there are at least two potential problems to beware. First, always analyze the conflicts reported by Bison to make
sure that GLR splitting is only done where it is intended. A GLR parser splitting inadvertently may cause problems less obvious than an LR parser statically choosing the wrong alternative in a conflict. Second, consider interactions with the
lexer (see Semantic Tokens) with great care. Since a split parser consumes tokens without performing any actions during the split, the lexer cannot obtain information via parser actions. Some cases of lexer interactions
can be eliminated by using GLR to shift the complications from the lexer to the parser. You must check the remaining cases for correctness.

In our example, it would be safe for the lexer to return tokens based on their current meanings in some symbol table, because no new symbols are defined in the middle of a type declaration. Though it is possible for a parser to define the enumeration
constants as they are parsed, before the type declaration is completed, it actually makes no difference since they cannot be used within the same enumerated type declaration.

1.5.2 Using GLR to Resolve Ambiguities

Let’s consider an example, vastly simplified from a C++ grammar.

%{
  #include <stdio.h>
  #define YYSTYPE char const *
  int yylex (void);
  void yyerror (char const *);
%}

%token TYPENAME ID

%right '='
%left '+'

%glr-parser

%%

prog:
  %empty
| prog stmt   { printf ("n"); }
;

stmt:
  expr ';'  %dprec 1
| decl      %dprec 2
;

expr:
  ID               { printf ("%s ", $$); }
| TYPENAME '(' expr ')'
                   { printf ("%s <cast> ", $1); }
| expr '+' expr    { printf ("+ "); }
| expr '=' expr    { printf ("= "); }
;

decl:
  TYPENAME declarator ';'
                   { printf ("%s <declare> ", $1); }
| TYPENAME declarator '=' expr ';'
                   { printf ("%s <init-declare> ", $1); }
;

declarator:
  ID               { printf (""%s" ", $1); }
| '(' declarator ')'
;

This models a problematic part of the C++ grammar—the ambiguity between certain declarations and statements. For example,

parses as either an expr or a stmt decl (assuming that ‘T’ is recognized as a TYPENAME and ‘
x’ as an ID). Bison detects this as a reduce/reduce conflict between the rules
expr : ID and declarator : ID, which it cannot resolve at the time it encounters x in the example above. Since this is a GLR parser, it therefore splits the problem into two parses, one for each choice
of resolving the reduce/reduce conflict. Unlike the example from the previous section (see Simple GLR Parsers), however, neither of these parses “dies,” because the grammar as it stands is ambiguous.
One of the parsers eventually reduces stmt : expr ';' and the other reduces stmt : decl, after which both parsers are in an identical state: they’ve seen ‘prog stmt’ and have the same
unprocessed input remaining. We say that these parses have merged.

At this point, the GLR parser requires a specification in the grammar of how to choose between the competing parses. In the example above, the two %dprec declarations specify that Bison is to give precedence to the parse that interprets
the example as a
decl, which implies that x is a declarator. The parser therefore prints

"x" y z + T <init-declare>

The %dprec declarations only come into play when more than one parse survives. Consider a different input string for this parser:

This is another example of using GLR to parse an unambiguous construct, as shown in the previous section (see Simple GLR Parsers). Here, there is no ambiguity (this cannot be parsed as a declaration). However,
at the time the Bison parser encounters x, it does not have enough information to resolve the reduce/reduce conflict (again, between x as an expr or a declarator). In this case, no precedence
declaration is used. Again, the parser splits into two, one assuming that x is an expr, and the other assuming x is a declarator. The second of these parsers then vanishes when it sees +,
and the parser prints

Suppose that instead of resolving the ambiguity, you wanted to see all the possibilities. For this purpose, you must merge the semantic actions of the two possible parsers, rather than choosing one over the other. To do so, you could change the
declaration of stmt as follows:

stmt:
  expr ';'  %merge <stmtMerge>
| decl      %merge <stmtMerge>
;

and define the stmtMerge function as:

static YYSTYPE
stmtMerge (YYSTYPE x0, YYSTYPE x1)
{
  printf ("<OR> ");
  return "";
}

with an accompanying forward declaration in the C declarations at the beginning of the file:

%{
  #define YYSTYPE char const *
  static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
%}

With these declarations, the resulting parser parses the first example as both an expr and a decl, and prints

"x" y z + T <init-declare> x T <cast> y z + = <OR>

Bison requires that all of the productions that participate in any particular merge have identical ‘
%merge’ clauses. Otherwise, the ambiguity would be unresolvable, and the parser will report an error during any parse that results in the offending merge.

1.5.3 GLR Semantic Actions

The nature of GLR parsing and the structure of the generated parsers give rise to certain restrictions on semantic values and actions.

1.5.3.1 Deferred semantic actions

By definition, a deferred semantic action is not performed at the same time as the associated reduction. This raises caveats for several Bison features you might use in a semantic action in a GLR parser.

In any semantic action, you can examine yychar to determine the type of the lookahead token present at the time of the associated reduction. After checking that yychar is not set to YYEMPTY or YYEOF,
you can then examine yylval and yylloc to determine the lookahead token’s semantic value and location, if any. In a nondeferred semantic action, you can also modify any of these variables to influence syntax
analysis. See Lookahead Tokens.

In a deferred semantic action, it’s too late to influence syntax analysis. In this case, yychar, yylval, and yylloc are set to shallow copies of the values they had at the time of the associated reduction.
For this reason alone, modifying them is dangerous. Moreover, the result of modifying them is undefined and subject to change with future versions of Bison. For example, if a semantic action might be deferred, you should never write it to
invoke
yyclearin (see Action Features) or to attempt to free memory referenced by yylval.

1.5.3.2 YYERROR

Another Bison feature requiring special consideration is YYERROR (see Action Features), which you can invoke in a semantic action to initiate error recovery. During deterministic GLR operation, the effect
of YYERROR is the same as its effect in a deterministic parser. The effect in a deferred action is similar, but the precise point of the error is undefined; instead, the parser reverts to deterministic operation, selecting an
unspecified stack on which to continue with a syntax error. In a semantic predicate (see Semantic Predicates) during nondeterministic parsing, YYERROR silently prunes the parse that invoked the
test.

1.5.3.3 Restrictions on semantic values and locations

GLR parsers require that you use POD (Plain Old Data) types for semantic values and location types when using the generated parsers as C++ code.

1.5.4 Controlling a Parse with Arbitrary Predicates

In addition to the %dprec and %merge directives, GLR parsers allow you to reject parses on the basis of arbitrary computations executed in user code, without having Bison treat this rejection as an error if there are
alternative parses. (This feature is experimental and may evolve. We welcome user feedback.) For example,

widget:
  %?{  new_syntax } "widget" id new_args  { $$ = f($3, $4); }
| %?{ !new_syntax } "widget" id old_args  { $$ = f($3, $4); }
;

is one way to allow the same parser to handle two different syntaxes for widgets. The clause preceded by %? is treated like an ordinary action, except that its text is treated as an expression and is always evaluated immediately (even
when in nondeterministic mode). If the expression yields 0 (false), the clause is treated as a syntax error, which, in a nondeterministic parser, causes the stack in which it is reduced to die. In a deterministic parser, it acts like YYERROR.

As the example shows, predicates otherwise look like semantic actions, and therefore you must be take them into account when determining the numbers to use for denoting the semantic values of right-hand side symbols. Predicate actions, however,
have no defined value, and may not be given labels.

There is a subtle difference between semantic predicates and ordinary actions in nondeterministic mode, since the latter are deferred. For example, we could try to rewrite the previous example as

widget:
  { if (!new_syntax) YYERROR; }
    "widget" id new_args  { $$ = f($3, $4); }
|  { if (new_syntax) YYERROR; }
    "widget" id old_args   { $$ = f($3, $4); }
;

(reversing the sense of the predicate tests to cause an error when they are false). However, this does not have the same effect if new_args and old_args have overlapping syntax. Since the mid-rule actions testing
new_syntax are deferred, a GLR parser first encounters the unresolved ambiguous reduction for cases where new_args and old_args recognize the same string
before performing the tests of new_syntax. It therefore reports an error.

Finally, be careful in writing predicates: deferred actions have not been evaluated, so that using them in a predicate will have undefined effects.

1.5.5 Considerations when Compiling GLR Parsers

The GLR parsers require a compiler for ISO C89 or later. In addition, they use the inline keyword, which is not C89, but is C99 and is a common extension in pre-C99 compilers. It is up to the user of these parsers to handle portability
issues. For instance, if using Autoconf and the Autoconf macro AC_C_INLINE, a mere

%{
  #include <config.h>
%}

will suffice. Otherwise, we suggest

%{
  #if (__STDC_VERSION__ < 199901 && ! defined __GNUC__ 
       && ! defined inline)
  # define inline
  #endif
%}

1.6 Locations

Many applications, like interpreters or compilers, have to produce verbose and useful error messages. To achieve this, one must be able to keep track of the textual location, or location, of each syntactic construct. Bison provides
a mechanism for handling these locations.

Each token has a semantic value. In a similar fashion, each token has an associated location, but the type of locations is the same for all tokens and groupings. Moreover, the output parser is equipped with a default data structure for storing
locations (see Tracking Locations, for more details).

Like semantic values, locations can be reached in actions using a dedicated set of constructs. In the example above, the location of the whole grouping is @$, while the locations of the subexpressions are @1 and
@3.

When a rule is matched, a default action is used to compute the semantic value of its left hand side (see Actions). In the same way, another default action is used for locations. However, the action for locations is general
enough for most cases, meaning there is usually no need to describe for each rule how @$ should be formed. When building a new location for a given grouping, the default behavior of the output parser is to take the beginning of
the first symbol, and the end of the last symbol.

1.7 Bison Output: the Parser Implementation File

When you run Bison, you give it a Bison grammar file as input. The most important output is a C source file that implements a parser for the language described by the grammar. This parser is called a
Bison parser, and this file is called a Bison parser
implementation file. Keep in mind that the Bison utility and the Bison parser are two distinct programs: the Bison utility is a program whose output is the Bison parser implementation file that becomes part of your program.

The job of the Bison parser is to group tokens into groupings according to the grammar rules—for example, to build identifiers and operators into expressions. As it does this, it runs the actions for the grammar rules it uses.

The tokens come from a function called the lexical analyzer that you must supply in some fashion (such as by writing it in C). The Bison parser calls the lexical analyzer each time it wants a new token. It doesn’t know what is “inside”
the tokens (though their semantic values may reflect this). Typically the lexical analyzer makes the tokens by parsing characters of text, but Bison does not depend on this. See The Lexical Analyzer Function yylex.

The Bison parser implementation file is C code which defines a function named yyparse which implements that grammar. This function does not make a complete C program: you must supply some additional functions. One is the lexical analyzer.
Another is an error-reporting function which the parser calls to report an error. In addition, a complete C program must start with a function called
main; you have to provide this, and arrange for it to call
yyparse or the parser will never run. See Parser
C-Language Interface.

Aside from the token type names and the symbols in the actions you write, all symbols defined in the Bison parser implementation file itself begin with ‘yy’ or ‘YY’. This includes interface functions
such as the lexical analyzer function yylex, the error reporting function yyerror and the parser function
yyparse itself. This also includes numerous identifiers used for internal purposes. Therefore, you should avoid using C identifiers starting with ‘yy’ or ‘YY’ in the Bison grammar
file except for the ones defined in this manual. Also, you should avoid using the C identifiers ‘malloc’ and ‘free’ for anything other than their usual meanings.

In some cases the Bison parser implementation file includes system headers, and in those cases your code should respect the identifiers reserved by those headers. On some non-GNU hosts, <alloca.h>,
<malloc.h>, <stddef.h>, and <stdlib.h> are included as needed to declare memory allocators and related types.
<libintl.h> is included if message translation is in use (see Internationalization). Other system headers may be included if you define YYDEBUG to a nonzero value (see
Tracing Your Parser).

1.8 Stages in Using Bison

The actual language-design process using Bison, from grammar specification to a working compiler or interpreter, has these parts:

1. Formally specify the grammar in a form recognized by Bison (see Bison Grammar Files). For each grammatical rule in the language, describe the action that is to be taken when an instance of that rule is recognized.
   The action is described by a sequence of C statements.

2. Write a lexical analyzer to process input and pass tokens to the parser. The lexical analyzer may be written by hand in C (see The
   Lexical Analyzer Function yylex). It could also be produced using Lex, but the use of Lex is not discussed in this manual.

3. Write a controlling function that calls the Bison-produced parser.
4. Write error-reporting routines.

To turn this source code as written into a runnable program, you must follow these steps:

1. Run Bison on the grammar to produce the parser.
2. Compile the code output by Bison, as well as any other source files.
3. Link the object files to produce the finished product.

1.9 The Overall Layout of a Bison Grammar

The input file for the Bison utility is a Bison grammar file. The general form of a Bison grammar file is as follows:

%{
Prologue
%}

Bison declarations

%%
Grammar rules
%%
Epilogue

The ‘%%’, ‘%{’ and ‘%}’ are punctuation that appears in every Bison grammar file to separate the sections.

The prologue may define types and variables used in the actions. You can also use preprocessor commands to define macros used there, and use
#include to include header files that do any of these things. You need to declare the lexical analyzer yylex and the error printer yyerror here, along with any other global identifiers used by the actions
in the grammar rules.

The Bison declarations declare the names of the terminal and nonterminal symbols, and may also describe operator precedence and the data types of semantic values of various symbols.

The grammar rules define how to construct each nonterminal symbol from its parts.

The epilogue can contain any code you want to use. Often the definitions of functions declared in the prologue go here. In a simple program, all the rest of the program can go here.

2 Examples

Now we show and explain several sample programs written using Bison: a reverse polish notation calculator, an algebraic (infix) notation calculator — later extended to track “locations” — and a multi-function calculator.
All produce usable, though limited, interactive desk-top calculators.

These examples are simple, but Bison grammars for real programming languages are written the same way. You can copy these examples into a source file to try them.

2.1 Reverse Polish Notation Calculator

The first example is that of a simple double-precision reverse polish
notation calculator (a calculator using postfix operators). This example provides a good starting point, since operator precedence is not an issue. The second example will illustrate how operator precedence is handled.

The source code for this calculator is named rpcalc.y. The ‘
.y’ extension is a convention used for Bison grammar files.

2.1.1 Declarations for rpcalc

Here are the C and Bison declarations for the reverse polish notation calculator. As in C, comments are placed between ‘/*…*/’.

/* Reverse polish notation calculator.  */

%{
  #include <stdio.h>
  #include <math.h>
  int yylex (void);
  void yyerror (char const *);
%}

%define api.value.type {double}
%token NUM

%% /* Grammar rules and actions follow.  */

The declarations section (see The prologue) contains two preprocessor directives and two forward declarations.

The #include directive is used to declare the exponentiation function pow.

The forward declarations for yylex and yyerror are needed because the C language requires that functions be declared before they are used. These functions will be defined in the epilogue, but the parser calls them so
they must be declared in the prologue.

The second section, Bison declarations, provides information to Bison about the tokens and their types (see The Bison
Declarations Section).

The %define directive defines the variable api.value.type, thus specifying the C data type for semantic values of both tokens and groupings (see Data Types of Semantic Values). The Bison parser
will use whatever type api.value.type is defined as; if you don’t define it, int is the default. Because we specify ‘
{double}’, each token and each expression has an associated value, which is a floating point number. C code can use YYSTYPE to refer to the value api.value.type.

Each terminal symbol that is not a single-character literal must be declared. (Single-character literals normally don’t need to be declared.) In this example, all the arithmetic operators are designated by single-character literals, so the
only terminal symbol that needs to be declared is NUM, the token type for numeric constants.

2.1.2 Grammar Rules for rpcalc

Here are the grammar rules for the reverse polish notation calculator.

input:
  %empty
| input line
;

line:
  'n'
| exp 'n'      { printf ("%.10gn", $1); }
;

exp:
  NUM           { $$ = $1;           }
| exp exp '+'   { $$ = $1 + $2;      }
| exp exp '-'   { $$ = $1 - $2;      }
| exp exp '*'   { $$ = $1 * $2;      }
| exp exp '/'   { $$ = $1 / $2;      }
| exp exp '^'   { $$ = pow ($1, $2); }  /* Exponentiation */
| exp 'n'       { $$ = -$1;          }  /* Unary minus    */
;

%%

The groupings of the rpcalc “language” defined here are the expression (given the name exp), the line of input (line), and the complete input transcript (input). Each of these nonterminal symbols
has several alternate rules, joined by the vertical bar ‘|’ which is read as “or”. The following sections explain what these rules mean.

The semantics of the language is determined by the actions taken when a grouping is recognized. The actions are the C code that appears inside braces. See Actions.

You must specify these actions in C, but Bison provides the means for passing semantic values between the rules. In each action, the pseudo-variable $$ stands for the semantic value for the grouping that the rule is going to construct.
Assigning a value to $$ is the main job of most actions. The semantic values of the components of the rule are referred to as $1, $2, and so on.

2.1.2.1 Explanation of input

Consider the definition of input:

input:
  %empty
| input line
;

This definition reads as follows: “A complete input is either an empty string, or a complete input followed by an input line”. Notice that “complete input” is defined in terms of itself. This definition is said to be left recursive since input appears always as the leftmost symbol in the sequence. See Recursive Rules.

The first alternative is empty because there are no symbols between the colon and the first ‘|’; this means that input can match an empty string of input (no tokens). We write the rules this way because it
is legitimate to type Ctrl-d right after you start the calculator. It’s conventional to put an empty alternative first and to use the (optional) %empty directive, or to write the comment ‘/* empty
*/’ in it (see Empty Rules).

The second alternate rule (input line) handles all nontrivial input. It means, “After reading any number of lines, read one more line if possible.” The left recursion makes this rule into a loop. Since the first alternative
matches empty input, the loop can be executed zero or more times.

The parser function yyparse continues to process input until a grammatical error is seen or the lexical analyzer says there are no more input tokens; we will arrange for the latter to happen at end-of-input.

2.1.2.2 Explanation of line

Now consider the definition of line:

line:
  'n'
| exp 'n'  { printf ("%.10gn", $1); }
;

The first alternative is a token which is a newline character; this means that rpcalc accepts a blank line (and ignores it, since there is no action). The second alternative is an expression followed by a newline. This is the alternative that
makes rpcalc useful. The semantic value of the exp grouping is the value of $1 because the exp in question is the first symbol in the alternative. The action prints this value, which is the result of
the computation the user asked for.

This action is unusual because it does not assign a value to $$. As a consequence, the semantic value associated with the line is uninitialized (its value will be unpredictable). This would be a bug if that value were
ever used, but we don’t use it: once rpcalc has printed the value of the user’s input line, that value is no longer needed.

2.1.2.3 Explanation of expr

The exp grouping has several rules, one for each kind of expression. The first rule handles the simplest expressions: those that are just numbers. The second handles an addition-expression, which looks like two expressions followed
by a plus-sign. The third handles subtraction, and so on.

exp:
  NUM
| exp exp '+'     { $$ = $1 + $2;    }
| exp exp '-'     { $$ = $1 - $2;    }
…
;

We have used ‘|’ to join all the rules for exp, but we could equally well have written them separately:

exp: NUM ;
exp: exp exp '+'     { $$ = $1 + $2; };
exp: exp exp '-'     { $$ = $1 - $2; };
…

Most of the rules have actions that compute the value of the expression in terms of the value of its parts. For example, in the rule for addition,
$1 refers to the first component exp and $2 refers to the second one. The third component, '+', has no meaningful associated semantic value, but if it had one you could refer to it as
$3. When yyparse recognizes a sum expression using this rule, the sum of the two subexpressions’ values is produced as the value of the entire expression. See Actions.

You don’t have to give an action for every rule. When a rule has no action, Bison by default copies the value of $1 into $$. This is what happens in the first rule (the one that uses NUM).

The formatting shown here is the recommended convention, but Bison does not require it. You can add or change white space as much as you wish. For example, this:

exp: NUM | exp exp '+' {$$ = $1 + $2; } | … ;

means the same thing as this:

exp:
  NUM
| exp exp '+'    { $$ = $1 + $2; }
| …
;

The latter, however, is much more readable.

2.1.3 The rpcalc Lexical Analyzer

The lexical analyzer’s job is low-level parsing: converting characters or sequences of characters into tokens. The Bison parser gets its tokens by calling the lexical analyzer. See The Lexical
Analyzer Function yylex.

Only a simple lexical analyzer is needed for the RPN calculator. This lexical analyzer skips blanks and tabs, then reads in numbers as
double and returns them as NUM tokens. Any other character that isn’t part of a number is a separate token. Note that the token-code for such a single-character token is the character itself.

The return value of the lexical analyzer function is a numeric code which represents a token type. The same text used in Bison rules to stand for this token type is also a C expression for the numeric code for the type. This works in two ways.
If the token type is a character literal, then its numeric code is that of the character; you can use the same character literal in the lexical analyzer to express the number. If the token type is an identifier, that identifier is defined
by Bison as a C macro whose definition is the appropriate number. In this example, therefore, NUM becomes a macro for yylex to use.

The semantic value of the token (if it has one) is stored into the global variable yylval, which is where the Bison parser will look for it. (The C data type of yylval is YYSTYPE, whose value was defined
at the beginning of the grammar via ‘%define api.value.type
{double}’; see Declarations for rpcalc.)

A token type code of zero is returned if the end-of-input is encountered. (Bison recognizes any nonpositive value as indicating end-of-input.)

Here is the code for the lexical analyzer:

/* The lexical analyzer returns a double floating point
   number on the stack and the token NUM, or the numeric code
   of the character read if not a number.  It skips all blanks
   and tabs, and returns 0 for end-of-input.  */

#include <ctype.h>

int
yylex (void)
{
  int c;

  /* Skip white space.  */
  while ((c = getchar ()) == ' ' || c == 't')
    continue;

  /* Process numbers.  */
  if (c == '.' || isdigit (c))
    {
      ungetc (c, stdin);
      scanf ("%lf", &yylval);
      return NUM;
    }

  /* Return end-of-input.  */
  if (c == EOF)
    return 0;
  /* Return a single char.  */
  return c;
}

2.1.4 The Controlling Function

In keeping with the spirit of this example, the controlling function is kept to the bare minimum. The only requirement is that it call
yyparse to start the process of parsing.

int
main (void)
{
  return yyparse ();
}

2.1.5 The Error Reporting Routine

When yyparse detects a syntax error, it calls the error reporting function yyerror to print an error message (usually but not always "syntax error"). It is up to the programmer to supply
yyerror (see Parser C-Language Interface), so here is the definition we will use:

#include <stdio.h>

/* Called by yyparse on error.  */
void
yyerror (char const *s)
{
  fprintf (stderr, "%sn", s);
}

After yyerror returns, the Bison parser may recover from the error and continue parsing if the grammar contains a suitable error rule (see Error Recovery). Otherwise, yyparse returns nonzero.
We have not written any error rules in this example, so any invalid input will cause the calculator program to exit. This is not clean behavior for a real calculator, but it is adequate for the first example.

2.1.6 Running Bison to Make the Parser

Before running Bison to produce a parser, we need to decide how to arrange all the source code in one or more source files. For such a simple example, the easiest thing is to put everything in one file, the grammar file. The definitions of yylex,
yyerror and
main go at the end, in the epilogue of the grammar file (see The Overall Layout of a Bison Grammar).

For a large project, you would probably have several source files, and use
make to arrange to recompile them.

With all the source in the grammar file, you use the following command to convert it into a parser implementation file:

In this example, the grammar file is called rpcalc.y (for “Reverse Polish CALCulator”). Bison produces a parser implementation file named file.tab.c, removing the ‘
.y’ from the grammar file name. The parser implementation file contains the source code for yyparse. The additional functions in the grammar file (yylex, yyerror and main)
are copied verbatim to the parser implementation file.

2.1.7 Compiling the Parser Implementation File

Here is how to compile and run the parser implementation file:

# List files in current directory.
$ ls
rpcalc.tab.c  rpcalc.y

# Compile the Bison parser.
# ‘-lm’ tells compiler to search math library for pow.
$ cc -lm -o rpcalc rpcalc.tab.c

# List files again.
$ ls
rpcalc  rpcalc.tab.c  rpcalc.y

The file rpcalc now contains the executable code. Here is an example session using rpcalc.

$ rpcalc
4 9 +
⇒ 13
3 7 + 3 4 5 *+-
⇒ -13
3 7 + 3 4 5 * + - n              Note the unary minus, ‘n’
⇒ 13
5 6 / 4 n +
⇒ -3.166666667
3 4 ^                            Exponentiation
⇒ 81
^D                               End-of-file indicator
$

2.2 Infix Notation Calculator: calc

We now modify rpcalc to handle infix operators instead of postfix. Infix notation involves the concept of operator precedence and the need for parentheses nested to arbitrary depth. Here is the Bison code for
calc.y, an infix desk-top calculator.

/* Infix notation calculator.  */

%{
  #include <math.h>
  #include <stdio.h>
  int yylex (void);
  void yyerror (char const *);
%}

/* Bison declarations.  */
%define api.value.type {double}
%token NUM
%left '-' '+'
%left '*' '/'
%precedence NEG   /* negation--unary minus */
%right '^'        /* exponentiation */

%% /* The grammar follows.  */

input:
  %empty
| input line
;

line:
  'n'
| exp 'n'  { printf ("t%.10gn", $1); }
;

exp:
  NUM                { $$ = $1;           }
| exp '+' exp        { $$ = $1 + $3;      }
| exp '-' exp        { $$ = $1 - $3;      }
| exp '*' exp        { $$ = $1 * $3;      }
| exp '/' exp        { $$ = $1 / $3;      }
| '-' exp  %prec NEG { $$ = -$2;          }
| exp '^' exp        { $$ = pow ($1, $3); }
| '(' exp ')'        { $$ = $2;           }
;

%%

The functions yylex, yyerror and main can be the same as before.

There are two important new features shown in this code.

In the second section (Bison declarations), %left declares token types and says they are left-associative operators. The declarations
%left and %right (right associativity) take the place of
%token which is used to declare a token type name without associativity/precedence. (These tokens are single-character literals, which ordinarily don’t need to be declared. We declare them here to specify the associativity/precedence.)

Operator precedence is determined by the line ordering of the declarations; the higher the line number of the declaration (lower on the page or screen), the higher the precedence. Hence, exponentiation has the highest precedence, unary minus (NEG)
is next, followed by ‘*’ and ‘/’, and so on. Unary minus is not associative, only precedence matters (%precedence. See Operator
Precedence.

The other important new feature is the %prec in the grammar section for the unary minus operator. The %prec simply instructs Bison that the rule ‘| ‘-‘ exp’ has the same precedence as
NEG—in this case the next-to-highest. See Context-Dependent Precedence.

Here is a sample run of calc.y:

$ calc
4 + 4.5 - (34/(8*3+-3))
6.880952381
-56 + 2
-54
3 ^ 2
9

2.3 Simple Error Recovery

Up to this point, this manual has not addressed the issue of error
recovery—how to continue parsing after the parser detects a syntax error. All we have handled is error reporting with yyerror. Recall that by default yyparse returns after calling
yyerror. This means that an erroneous input line causes the calculator program to exit. Now we show how to rectify this deficiency.

The Bison language itself includes the reserved word error, which may be included in the grammar rules. In the example below it has been added to one of the alternatives for line:

line:
  'n'
| exp 'n'   { printf ("t%.10gn", $1); }
| error 'n' { yyerrok;                  }
;

This addition to the grammar allows for simple error recovery in the event of a syntax error. If an expression that cannot be evaluated is read, the error will be recognized by the third rule for line, and parsing will continue. (The
yyerror function is still called upon to print its message as well.) The action executes the statement
yyerrok, a macro defined automatically by Bison; its meaning is that error recovery is complete (see Error Recovery). Note the difference between yyerrok and yyerror; neither
one is a misprint.

This form of error recovery deals with syntax errors. There are other kinds of errors; for example, division by zero, which raises an exception signal that is normally fatal. A real calculator program must handle this signal and use longjmp to return to main and resume parsing input lines; it would also have to discard the rest of the current line of input. We won’t discuss this issue further because it is not specific to Bison programs.

2.4 Location Tracking Calculator: ltcalc

This example extends the infix notation calculator with location tracking. This feature will be used to improve the error messages. For the sake of clarity, this example is a simple integer calculator, since most of the work needed to use locations
will be done in the lexical analyzer.

2.4.1 Declarations for ltcalc

The C and Bison declarations for the location tracking calculator are the same as the declarations for the infix notation calculator.

/* Location tracking calculator.  */

%{
  #include <math.h>
  int yylex (void);
  void yyerror (char const *);
%}

/* Bison declarations.  */
%define api.value.type {int}
%token NUM

%left '-' '+'
%left '*' '/'
%precedence NEG
%right '^'

%% /* The grammar follows.  */

Note there are no declarations specific to locations. Defining a data type for storing locations is not needed: we will use the type provided by default (see Data Types of Locations), which is a four member structure
with the following integer fields:
first_line, first_column, last_line and
last_column. By conventions, and in accordance with the GNU Coding Standards and common practice, the line and column count both start at 1.

2.4.2 Grammar Rules for ltcalc

Whether handling locations or not has no effect on the syntax of your language. Therefore, grammar rules for this example will be very close to those of the previous example: we will only modify them to benefit from the new information.

Here, we will use locations to report divisions by zero, and locate the wrong expressions or subexpressions.

input:
  %empty
| input line
;

line:
  'n'
| exp 'n' { printf ("%dn", $1); }
;

exp:
  NUM           { $$ = $1; }
| exp '+' exp   { $$ = $1 + $3; }
| exp '-' exp   { $$ = $1 - $3; }
| exp '*' exp   { $$ = $1 * $3; }

| exp '/' exp
    {
      if ($3)
        $$ = $1 / $3;
      else
        {
          $$ = 1;
          fprintf (stderr, "%d.%d-%d.%d: division by zero",
                   @3.first_line, @3.first_column,
                   @3.last_line, @3.last_column);
        }
    }

| '-' exp %prec NEG     { $$ = -$2; }
| exp '^' exp           { $$ = pow ($1, $3); }
| '(' exp ')'           { $$ = $2; }

This code shows how to reach locations inside of semantic actions, by using the pseudo-variables @n for rule components, and the pseudo-variable @$ for groupings.

We don’t need to assign a value to @$: the output parser does it automatically. By default, before executing the C code of each action,
@$ is set to range from the beginning of @1 to the end of @n, for a rule with n components. This behavior can be redefined (see Default Action for
Locations), and for very specific rules, @$ can be computed by hand.

2.4.3 The ltcalc Lexical Analyzer.

Until now, we relied on Bison’s defaults to enable location tracking. The next step is to rewrite the lexical analyzer, and make it able to feed the parser with the token locations, as it already does for semantic values.

To this end, we must take into account every single character of the input text, to avoid the computed locations of being fuzzy or wrong:

int
yylex (void)
{
  int c;

  /* Skip white space.  */
  while ((c = getchar ()) == ' ' || c == 't')
    ++yylloc.last_column;

  /* Step.  */
  yylloc.first_line = yylloc.last_line;
  yylloc.first_column = yylloc.last_column;

  /* Process numbers.  */
  if (isdigit (c))
    {
      yylval = c - '0';
      ++yylloc.last_column;
      while (isdigit (c = getchar ()))
        {
          ++yylloc.last_column;
          yylval = yylval * 10 + c - '0';
        }
      ungetc (c, stdin);
      return NUM;
    }

  /* Return end-of-input.  */
  if (c == EOF)
    return 0;

  /* Return a single char, and update location.  */
  if (c == 'n')
    {
      ++yylloc.last_line;
      yylloc.last_column = 0;
    }
  else
    ++yylloc.last_column;
  return c;
}

Basically, the lexical analyzer performs the same processing as before: it skips blanks and tabs, and reads numbers or single-character tokens. In addition, it updates yylloc, the global variable (of type
YYLTYPE) containing the token’s location.

Now, each time this function returns a token, the parser has its number as well as its semantic value, and its location in the text. The last needed change is to initialize yylloc, for example in the controlling function:

int
main (void)
{
  yylloc.first_line = yylloc.last_line = 1;
  yylloc.first_column = yylloc.last_column = 0;
  return yyparse ();
}

Remember that computing locations is not a matter of syntax. Every character must be associated to a location update, whether it is in valid input, in comments, in literal strings, and so on.

2.5 Multi-Function Calculator: mfcalc

Now that the basics of Bison have been discussed, it is time to move on to a more advanced problem. The above calculators provided only five functions, ‘+’, ‘—’, ‘*’, ‘/’
and ‘^’. It would be nice to have a calculator that provides other mathematical functions such as sin, cos, etc.

It is easy to add new operators to the infix calculator as long as they are only single-character literals. The lexical analyzer yylex passes back all nonnumeric characters as tokens, so new grammar rules suffice for adding a new
operator. But we want something more flexible: built-in functions whose syntax has this form:

At the same time, we will add memory to the calculator, by allowing you to create named variables, store values in them, and use them later. Here is a sample session with the multi-function calculator:

$ mfcalc
pi = 3.141592653589
⇒ 3.1415926536

sin(pi)
⇒ 0.0000000000

alpha = beta1 = 2.3
⇒ 2.3000000000
alpha
⇒ 2.3000000000
ln(alpha)
⇒ 0.8329091229
exp(ln(beta1))
⇒ 2.3000000000
$

Note that multiple assignment and nested function calls are permitted.

2.5.1 Declarations for mfcalc

Here are the C and Bison declarations for the multi-function calculator.

%{
  #include <stdio.h>  /* For printf, etc. */
  #include <math.h>   /* For pow, used in the grammar.  */
  #include "calc.h"   /* Contains definition of 'symrec'.  */
  int yylex (void);
  void yyerror (char const *);
%}

%define api.value.type union /* Generate YYSTYPE from these types:  */
%token <double>  NUM         /* Simple double precision number.  */
%token <symrec*> VAR FNCT    /* Symbol table pointer: variable and function.  */
%type  <double>  exp

%precedence '='
%left '-' '+'
%left '*' '/'
%precedence NEG /* negation--unary minus */
%right '^'      /* exponentiation */

The above grammar introduces only two new features of the Bison language. These features allow semantic values to have various data types (see More Than One Value Type).

The special union value assigned to the %define variable
api.value.type specifies that the symbols are defined with their data types. Bison will generate an appropriate definition of YYSTYPE to store these values.

Since values can now have various types, it is necessary to associate a type with each grammar symbol whose semantic value is used. These symbols are
NUM, VAR, FNCT, and exp. Their declarations are augmented with their data type (placed between angle brackets). For instance, values of NUM are stored in double.

The Bison construct %type is used for declaring nonterminal symbols, just as %token is used for declaring token types. Previously we did not use %type before because nonterminal symbols are normally declared
implicitly by the rules that define them. But exp must be declared explicitly so we can specify its value type. See Nonterminal Symbols.

2.5.2 Grammar Rules for mfcalc

Here are the grammar rules for the multi-function calculator. Most of them are copied directly from calc; three rules, those which mention VAR or FNCT, are new.

%% /* The grammar follows.  */

input:
  %empty
| input line
;

line:
  'n'
| exp 'n'   { printf ("%.10gn", $1); }
| error 'n' { yyerrok;                }
;

exp:
  NUM                { $$ = $1;                         }
| VAR                { $$ = $1->value.var;              }
| VAR '=' exp        { $$ = $3; $1->value.var = $3;     }
| FNCT '(' exp ')'   { $$ = (*($1->value.fnctptr))($3); }
| exp '+' exp        { $$ = $1 + $3;                    }
| exp '-' exp        { $$ = $1 - $3;                    }
| exp '*' exp        { $$ = $1 * $3;                    }
| exp '/' exp        { $$ = $1 / $3;                    }
| '-' exp  %prec NEG { $$ = -$2;                        }
| exp '^' exp        { $$ = pow ($1, $3);               }
| '(' exp ')'        { $$ = $2;                         }
;

/* End of grammar.  */
%%

2.5.3 The mfcalc Symbol Table

The multi-function calculator requires a symbol table to keep track of the names and meanings of variables and functions. This doesn’t affect the grammar rules (except for the actions) or the Bison declarations, but it requires some additional
C functions for support.

The symbol table itself consists of a linked list of records. Its definition, which is kept in the header calc.h, is as follows. It provides for either functions or variables to be placed in the table.

/* Function type.  */
typedef double (*func_t) (double);

/* Data type for links in the chain of symbols.  */
struct symrec
{
  char *name;  /* name of symbol */
  int type;    /* type of symbol: either VAR or FNCT */
  union
  {
    double var;      /* value of a VAR */
    func_t fnctptr;  /* value of a FNCT */
  } value;
  struct symrec *next;  /* link field */
};

typedef struct symrec symrec;

/* The symbol table: a chain of 'struct symrec'.  */
extern symrec *sym_table;

symrec *putsym (char const *, int);
symrec *getsym (char const *);

The new version of main will call init_table to initialize the symbol table:

struct init
{
  char const *fname;
  double (*fnct) (double);
};

struct init const arith_fncts[] =
{
  { "atan", atan },
  { "cos",  cos  },
  { "exp",  exp  },
  { "ln",   log  },
  { "sin",  sin  },
  { "sqrt", sqrt },
  { 0, 0 },
};

/* The symbol table: a chain of 'struct symrec'.  */
symrec *sym_table;

/* Put arithmetic functions in table.  */
static
void
init_table (void)
{
  int i;
  for (i = 0; arith_fncts[i].fname != 0; i++)
    {
      symrec *ptr = putsym (arith_fncts[i].fname, FNCT);
      ptr->value.fnctptr = arith_fncts[i].fnct;
    }
}

By simply editing the initialization list and adding the necessary include files, you can add additional functions to the calculator.

Two important functions allow look-up and installation of symbols in the symbol table. The function putsym is passed a name and the type (
VAR or FNCT) of the object to be installed. The object is linked to the front of the list, and a pointer to the object is returned. The function getsym is passed the name of the symbol to look up. If
found, a pointer to that symbol is returned; otherwise zero is returned.

#include <stdlib.h> /* malloc. */
#include <string.h> /* strlen. */

symrec *
putsym (char const *sym_name, int sym_type)
{
  symrec *ptr = (symrec *) malloc (sizeof (symrec));
  ptr->name = (char *) malloc (strlen (sym_name) + 1);
  strcpy (ptr->name,sym_name);
  ptr->type = sym_type;
  ptr->value.var = 0; /* Set value to 0 even if fctn.  */
  ptr->next = (struct symrec *)sym_table;
  sym_table = ptr;
  return ptr;
}

symrec *
getsym (char const *sym_name)
{
  symrec *ptr;
  for (ptr = sym_table; ptr != (symrec *) 0;
       ptr = (symrec *)ptr->next)
    if (strcmp (ptr->name, sym_name) == 0)
      return ptr;
  return 0;
}

2.5.4 The mfcalc Lexer

The function yylex must now recognize variables, numeric values, and the single-character arithmetic operators. Strings of alphanumeric characters with a leading letter are recognized as either variables or functions depending on
what the symbol table says about them.

The string is passed to getsym for look up in the symbol table. If the name appears in the table, a pointer to its location and its type (
VAR or FNCT) is returned to yyparse. If it is not already in the table, then it is installed as a VAR using
putsym. Again, a pointer and its type (which must be VAR) is returned to yyparse.

No change is needed in the handling of numeric values and arithmetic operators in yylex.

#include <ctype.h>

int
yylex (void)
{
  int c;

  /* Ignore white space, get first nonwhite character.  */
  while ((c = getchar ()) == ' ' || c == 't')
    continue;

  if (c == EOF)
    return 0;

  /* Char starts a number => parse the number.         */
  if (c == '.' || isdigit (c))
    {
      ungetc (c, stdin);
      scanf ("%lf", &yylval.NUM);
      return NUM;
    }

Bison generated a definition of YYSTYPE with a member named
NUM to store value of NUM symbols.

  /* Char starts an identifier => read the name.       */
  if (isalpha (c))
    {
      /* Initially make the buffer long enough
         for a 40-character symbol name.  */
      static size_t length = 40;
      static char *symbuf = 0;
      symrec *s;
      int i;

      if (!symbuf)
        symbuf = (char *) malloc (length + 1);

      i = 0;
      do

        {
          /* If buffer is full, make it bigger.        */
          if (i == length)
            {
              length *= 2;
              symbuf = (char *) realloc (symbuf, length + 1);
            }
          /* Add this character to the buffer.         */
          symbuf[i++] = c;
          /* Get another character.                    */
          c = getchar ();
        }

      while (isalnum (c));

      ungetc (c, stdin);
      symbuf[i] = '';

      s = getsym (symbuf);
      if (s == 0)
        s = putsym (symbuf, VAR);
      *((symrec**) &yylval) = s;
      return s->type;
    }

  /* Any other character is a token by itself.        */
  return c;
}

2.5.5 The mfcalc Main

The error reporting function is unchanged, and the new version of
main includes a call to init_table and sets the yydebug on user demand (See Tracing Your Parser, for details):

/* Called by yyparse on error.  */
void
yyerror (char const *s)
{
  fprintf (stderr, "%sn", s);
}

int
main (int argc, char const* argv[])
{
  int i;
  /* Enable parse traces on option -p.  */
  for (i = 1; i < argc; ++i)
    if (!strcmp(argv[i], "-p"))
      yydebug = 1;
  init_table ();
  return yyparse ();
}

This program is both powerful and flexible. You may easily add new functions, and it is a simple job to modify this code to install predefined variables such as pi or e as well.

2.6 Exercises

1. Add some new functions from math.h to the initialization list.
2. Add another array that contains constants and their values. Then modify init_table to add these constants to the symbol table. It will be easiest to give the constants type VAR.
3. Make the program report an error if the user refers to an uninitialized variable in any way except to store a value in it.

3 Bison Grammar Files

Bison takes as input a context-free grammar specification and produces a C-language function that recognizes correct instances of the grammar.

The Bison grammar file conventionally has a name ending in ‘.y’. See Invoking Bison.

3.1 Outline of a Bison Grammar

A Bison grammar file has four main sections, shown here with the appropriate delimiters:

%{
  Prologue
%}

Bison declarations

%%
Grammar rules
%%

Epilogue

Comments enclosed in ‘/* … */’ may appear in any of the sections. As a GNU extension, ‘//’ introduces a comment that continues until end of line.

3.1.1 The prologue

The Prologue section contains macro definitions and declarations of functions and variables that are used in the actions in the grammar rules. These are copied to the beginning of the parser implementation file so that they precede
the definition of yyparse. You can use ‘#include’ to get the declarations from a header file. If you don’t need any C declarations, you may omit the ‘%{’ and ‘
%}’ delimiters that bracket this section.

The Prologue section is terminated by the first occurrence of ‘%}’ that is outside a comment, a string literal, or a character constant.

You may have more than one Prologue section, intermixed with the
Bison declarations. This allows you to have C and Bison declarations that refer to each other. For example, the %union declaration may use types defined in a header file, and you may wish to prototype functions that
take arguments of type YYSTYPE. This can be done with two Prologue blocks, one before and one after the
%union declaration.

%{
  #define _GNU_SOURCE
  #include <stdio.h>
  #include "ptypes.h"
%}

%union {
  long int n;
  tree t;  /* tree is defined in ptypes.h. */
}

%{
  static void print_token_value (FILE *, int, YYSTYPE);
  #define YYPRINT(F, N, L) print_token_value (F, N, L)
%}

…

When in doubt, it is usually safer to put prologue code before all Bison declarations, rather than after. For example, any definitions of feature test macros like _GNU_SOURCE or
_POSIX_C_SOURCE should appear before all Bison declarations, as feature test macros can affect the behavior of Bison-generated
#include directives.

3.1.2 Prologue Alternatives

The functionality of Prologue sections can often be subtle and inflexible. As an alternative, Bison provides a %code directive with an explicit qualifier field, which identifies the purpose of the code and thus the location(s)
where Bison should generate it. For C/C++, the qualifier can be omitted for the default location, or it can be one of requires, provides,
top. See %code Summary.

Look again at the example of the previous section:

%{
  #define _GNU_SOURCE
  #include <stdio.h>
  #include "ptypes.h"
%}

%union {
  long int n;
  tree t;  /* tree is defined in ptypes.h. */
}

%{
  static void print_token_value (FILE *, int, YYSTYPE);
  #define YYPRINT(F, N, L) print_token_value (F, N, L)
%}

…

Notice that there are two Prologue sections here, but there’s a subtle distinction between their functionality. For example, if you decide to override Bison’s default definition for YYLTYPE, in which Prologue section should you write your new definition? You should write it in the first since Bison will insert that code into the parser implementation file before the default
YYLTYPE definition. In which Prologue section should you prototype an internal function, trace_token, that accepts
YYLTYPE and yytokentype as arguments? You should prototype it in the second since Bison will insert that code
after the YYLTYPE and yytokentype definitions.

This distinction in functionality between the two Prologue sections is established by the appearance of the %union between them. This behavior raises a few questions. First, why should the position of a %union affect definitions related to
YYLTYPE and yytokentype? Second, what if there is no %union? In that case, the second kind of Prologue section is not available. This behavior is not intuitive.

To avoid this subtle %union dependency, rewrite the example using a
%code top and an unqualified %code. Let’s go ahead and add the new YYLTYPE definition and the
trace_token prototype at the same time:

%code top {
  #define _GNU_SOURCE
  #include <stdio.h>

  /* WARNING: The following code really belongs
   * in a '%code requires'; see below.  */

  #include "ptypes.h"
  #define YYLTYPE YYLTYPE
  typedef struct YYLTYPE
  {
    int first_line;
    int first_column;
    int last_line;
    int last_column;
    char *filename;
  } YYLTYPE;
}

%union {
  long int n;
  tree t;  /* tree is defined in ptypes.h. */
}

%code {
  static void print_token_value (FILE *, int, YYSTYPE);
  #define YYPRINT(F, N, L) print_token_value (F, N, L)
  static void trace_token (enum yytokentype token, YYLTYPE loc);
}

…

In this way, %code top and the unqualified %code achieve the same functionality as the two kinds of Prologue sections, but it’s always explicit which kind you intend. Moreover, both kinds are always available
even in the absence of %union.

The %code top block above logically contains two parts. The first two lines before the warning need to appear near the top of the parser implementation file. The first line after the warning is required by YYSTYPE and
thus also needs to appear in the parser implementation file. However, if you’ve instructed Bison to generate a parser header file (see %defines), you probably want that line to appear before the YYSTYPE definition in that header file as well. The YYLTYPE definition should also appear in the parser header file to override the default YYLTYPE definition there.

In other words, in the %code top block above, all but the first two lines are dependency code required by the YYSTYPE and YYLTYPE definitions. Thus, they belong in one or more %code requires:

%code top {
  #define _GNU_SOURCE
  #include <stdio.h>
}

%code requires {
  #include "ptypes.h"
}

%union {
  long int n;
  tree t;  /* tree is defined in ptypes.h. */
}

%code requires {
  #define YYLTYPE YYLTYPE
  typedef struct YYLTYPE
  {
    int first_line;
    int first_column;
    int last_line;
    int last_column;
    char *filename;
  } YYLTYPE;
}

%code {
  static void print_token_value (FILE *, int, YYSTYPE);
  #define YYPRINT(F, N, L) print_token_value (F, N, L)
  static void trace_token (enum yytokentype token, YYLTYPE loc);
}

…

Now Bison will insert #include "ptypes.h" and the new
YYLTYPE definition before the Bison-generated YYSTYPE and YYLTYPE definitions in both the parser implementation file and the parser header file. (By the same reasoning, %code
requires would also be the appropriate place to write your own definition for YYSTYPE.)

When you are writing dependency code for YYSTYPE and
YYLTYPE, you should prefer %code requires over
%code top regardless of whether you instruct Bison to generate a parser header file. When you are writing code that you need Bison to insert only into the parser implementation file and that has no special need to appear at the
top of that file, you should prefer the unqualified %code over %code top. These practices will make the purpose of each block of your code explicit to Bison and to other developers reading your grammar file. Following
these practices, we expect the unqualified %code and %code
requires to be the most important of the four Prologue alternatives.

At some point while developing your parser, you might decide to provide trace_token to modules that are external to your parser. Thus, you might wish for Bison to insert the prototype into both the parser header file and the parser
implementation file. Since this function is not a dependency required by YYSTYPE or
YYLTYPE, it doesn’t make sense to move its prototype to a
%code requires. More importantly, since it depends upon
YYLTYPE and yytokentype, %code requires is not sufficient. Instead, move its prototype from the unqualified
%code to a %code provides:

%code top {
  #define _GNU_SOURCE
  #include <stdio.h>
}

%code requires {
  #include "ptypes.h"
}

%union {
  long int n;
  tree t;  /* tree is defined in ptypes.h. */
}

%code requires {
  #define YYLTYPE YYLTYPE
  typedef struct YYLTYPE
  {
    int first_line;
    int first_column;
    int last_line;
    int last_column;
    char *filename;
  } YYLTYPE;
}

%code provides {
  void trace_token (enum yytokentype token, YYLTYPE loc);
}

%code {
  static void print_token_value (FILE *, int, YYSTYPE);
  #define YYPRINT(F, N, L) print_token_value (F, N, L)
}

…

Bison will insert the trace_token prototype into both the parser header file and the parser implementation file after the definitions for yytokentype, YYLTYPE, and
YYSTYPE.

The above examples are careful to write directives in an order that reflects the layout of the generated parser implementation and header files: %code top, %code requires, %code provides, and then %code.
While your grammar files may generally be easier to read if you also follow this order, Bison does not require it. Instead, Bison lets you choose an organization that makes sense to you.

You may declare any of these directives multiple times in the grammar file. In that case, Bison concatenates the contained code in declaration order. This is the only way in which the position of one of these directives within the grammar file
affects its functionality.

The result of the previous two properties is greater flexibility in how you may organize your grammar file. For example, you may organize semantic-type-related directives by semantic type:

%code requires { #include "type1.h" }
%union { type1 field1; }
%destructor { type1_free ($$); } <field1>
%printer { type1_print (yyoutput, $$); } <field1>

%code requires { #include "type2.h" }
%union { type2 field2; }
%destructor { type2_free ($$); } <field2>
%printer { type2_print (yyoutput, $$); } <field2>

You could even place each of the above directive groups in the rules section of the grammar file next to the set of rules that uses the associated semantic type. (In the rules section, you must terminate each of those directives with a semicolon.)
And you don’t have to worry that some directive (like a %union) in the definitions section is going to adversely affect their functionality in some counter-intuitive manner just because it comes first. Such an organization
is not possible using Prologue sections.

This section has been concerned with explaining the advantages of the four
Prologue alternatives over the original Yacc Prologue. However, in most cases when using these directives, you shouldn’t need to think about all the low-level ordering issues discussed here. Instead, you should
simply use these directives to label each block of your code according to its purpose and let Bison handle the ordering.
%code is the most generic label. Move code to %code requires, %code provides, or %code top as needed.

3.1.3 The Bison Declarations Section

The Bison declarations section contains declarations that define terminal and nonterminal symbols, specify precedence, and so on. In some simple grammars you may not need any declarations. See Bison Declarations.

3.1.4 The Grammar Rules Section

The grammar rules section contains one or more Bison grammar rules, and nothing else. See Syntax of Grammar Rules.

There must always be at least one grammar rule, and the first ‘
%%’ (which precedes the grammar rules) may never be omitted even if it is the first thing in the file.

3.1.5 The epilogue

The Epilogue is copied verbatim to the end of the parser implementation file, just as the Prologue is copied to the beginning. This is the most convenient place to put anything that you want to have in the parser implementation
file but which need not come before the definition of yyparse. For example, the definitions of yylex and yyerror often go here. Because C requires functions to be declared before being used, you often
need to declare functions like yylex and yyerror in the Prologue, even if you define them in the Epilogue. See Parser
C-Language Interface.

If the last section is empty, you may omit the ‘%%’ that separates it from the grammar rules.

The Bison parser itself contains many macros and identifiers whose names start with ‘yy’ or ‘YY’, so it is a good idea to avoid using any such names (except those documented in this manual) in
the epilogue of the grammar file.

3.2 Symbols, Terminal and Nonterminal

Symbols in Bison grammars represent the grammatical classifications of the language.

A terminal symbol (also known as a token type) represents a class of syntactically equivalent tokens. You use the symbol in grammar rules to mean that a token in that class is allowed. The symbol is represented in the Bison parser
by a numeric code, and the yylex function returns a token type code to indicate what kind of token has been read. You don’t need to know what the code value is; you can use the symbol to stand for it.

A nonterminal symbol stands for a class of syntactically equivalent groupings. The symbol name is used in writing grammar rules. By convention, it should be all lower case.

Symbol names can contain letters, underscores, periods, and non-initial digits and dashes. Dashes in symbol names are a GNU extension, incompatible with POSIX Yacc. Periods and dashes make symbol names less convenient to use with named references,
which require brackets around such names (see Named References). Terminal symbols that contain periods or dashes make little sense: since they are not valid symbols (in most programming languages) they are not
exported as token names.

There are three ways of writing terminal symbols in the grammar:

• A named token type is written with an identifier, like an identifier in C. By convention, it should be all upper case. Each such name must be defined with a Bison declaration such as
  %token. See Token Type Names.

• A character token type (or literal character token) is written in the grammar using the same syntax used in C for character constants; for example, '+' is a character token type. A character token type doesn’t
  need to be declared unless you need to specify its semantic value data type (see Data Types of
  Semantic Values), associativity, or precedence (see Operator Precedence).

  By convention, a character token type is used only to represent a token that consists of that particular character. Thus, the token type '+' is used to represent the character ‘+’ as a token. Nothing
  enforces this convention, but if you depart from it, your program will confuse other readers.

  All the usual escape sequences used in character literals in C can be used in Bison as well, but you must not use the null character as a character literal because its numeric code, zero, signifies end-of-input (see Calling Convention
  for yylex). Also, unlike standard C, trigraphs have no special meaning in Bison character literals, nor is backslash-newline allowed.

• A literal string token is written like a C string constant; for example, "<=" is a literal string token. A literal string token doesn’t need to be declared unless you need to specify its semantic
  value data type (see Value Type), associativity, or precedence (see Precedence).

  You can associate the literal string token with a symbolic name as an alias, using the %token declaration (see Token
  Declarations). If you don’t do that, the lexical analyzer has to retrieve the token number for the literal string token from the
  yytname table (see Calling Convention).

  Warning: literal string tokens do not work in Yacc.

  By convention, a literal string token is used only to represent a token that consists of that particular string. Thus, you should use the token type "<=" to represent the string ‘<=’
  as a token. Bison does not enforce this convention, but if you depart from it, people who read your program will be confused.

  All the escape sequences used in string literals in C can be used in Bison as well, except that you must not use a null character within a string literal. Also, unlike Standard C, trigraphs have no special meaning in Bison string literals,
  nor is backslash-newline allowed. A literal string token must contain two or more characters; for a token containing just one character, use a character token (see above).

How you choose to write a terminal symbol has no effect on its grammatical meaning. That depends only on where it appears in rules and on when the parser function returns that symbol.

The value returned by yylex is always one of the terminal symbols, except that a zero or negative value signifies end-of-input. Whichever way you write the token type in the grammar rules, you write it the same way in the definition
of
yylex. The numeric code for a character token type is simply the positive numeric code of the character, so yylex can use the identical value to generate the requisite code, though you may need to convert it to unsigned
char to avoid sign-extension on hosts where char is signed. Each named token type becomes a C macro in the parser implementation file, so yylex can use the name to stand for the code. (This is why periods don’t make
sense in terminal symbols.) See Calling Convention for yylex.

If yylex is defined in a separate file, you need to arrange for the token-type macro definitions to be available there. Use the ‘-d’ option when you run Bison, so that it will write these macro definitions
into a separate header file name.tab.h which you can include in the other source files that need it. See Invoking Bison.

If you want to write a grammar that is portable to any Standard C host, you must use only nonnull character tokens taken from the basic execution character set of Standard C. This set consists of the ten digits, the 52 lower- and upper-case English
letters, and the characters in the following C-language string:

"abtnvfr !"#%&'()*+,-./:;<=>?[\]^_{|}~"

The yylex function and Bison must use a consistent character set and encoding for character tokens. For example, if you run Bison in an ASCII environment, but then compile and run the resulting program in an environment that uses
an incompatible character set like EBCDIC, the resulting program may not work because the tables generated by Bison will assume ASCII numeric values for character tokens. It is standard practice for software distributions to contain C source
files that were generated by Bison in an ASCII environment, so installers on platforms that are incompatible with ASCII must rebuild those files before compiling them.

The symbol error is a terminal symbol reserved for error recovery (see Error Recovery); you shouldn’t use it for any other purpose. In particular, yylex should never return this value.
The default value of the error token is 256, unless you explicitly assigned 256 to one of your tokens with a %token declaration.

3.3 Grammar Rules

A Bison grammar is a list of rules.

3.3.1 Syntax of Grammar Rules

A Bison grammar rule has the following general form:

where result is the nonterminal symbol that this rule describes, and components are various terminal and nonterminal symbols that are put together by this rule (see Symbols).

For example,

says that two groupings of type exp, with a ‘+’ token in between, can be combined into a larger grouping of type exp.

White space in rules is significant only to separate symbols. You can add extra white space as you wish.

Scattered among the components can be actions that determine the semantics of the rule. An action looks like this:

This is an example of braced code, that is, C code surrounded by braces, much like a compound statement in C. Braced code can contain any sequence of C tokens, so long as its braces are balanced. Bison does not check the braced code for
correctness directly; it merely copies the code to the parser implementation file, where the C compiler can check it.

Within braced code, the balanced-brace count is not affected by braces within comments, string literals, or character constants, but it is affected by the C digraphs ‘<%’ and ‘%>’ that represent
braces. At the top level braced code must be terminated by ‘}’ and not by a digraph. Bison does not look for trigraphs, so if braced code uses trigraphs you should ensure that they do not affect the nesting of braces
or the boundaries of comments, string literals, or character constants.

Usually there is only one action and it follows the components. See Actions.

Multiple rules for the same result can be written separately or can be joined with the vertical-bar character ‘|’ as follows:

result:
  rule1-components…
| rule2-components…
…
;

They are still considered distinct rules even when joined in this way.

3.3.2 Empty Rules

A rule is said to be empty if its right-hand side (components) is empty. It means that result can match the empty string. For example, here is how to define an optional semicolon:

It is easy not to see an empty rule, especially when | is used. The
%empty directive allows to make explicit that a rule is empty on purpose:

semicolon.opt:
  %empty
| ";"
;

Flagging a non-empty rule with %empty is an error. If run with
-Wempty-rule, bison will report empty rules without
%empty. Using %empty enables this warning, unless
-Wno-empty-rule was specified.

The %empty directive is a Bison extension, it does not work with Yacc. To remain compatible with POSIX Yacc, it is customary to write a comment ‘/* empty */’ in each rule with no components:

semicolon.opt:
  /* empty */
| ";"
;

3.3.3 Recursive Rules

A rule is called recursive when its result nonterminal appears also on its right hand side. Nearly all Bison grammars need to use recursion, because that is the only way to define a sequence of any number of a particular thing.
Consider this recursive definition of a comma-separated sequence of one or more expressions:

expseq1:
  exp
| expseq1 ',' exp
;

Since the recursive use of expseq1 is the leftmost symbol in the right hand side, we call this left recursion. By contrast, here the same construct is defined using right recursion:

expseq1:
  exp
| exp ',' expseq1
;

Any kind of sequence can be defined using either left recursion or right recursion, but you should always use left recursion, because it can parse a sequence of any number of elements with bounded stack space. Right recursion uses up space on
the Bison stack in proportion to the number of elements in the sequence, because all the elements must be shifted onto the stack before the rule can be applied even once. See The Bison Parser Algorithm, for further
explanation of this.

Indirect or mutual recursion occurs when the result of the rule does not appear directly on its right hand side, but does appear in rules for other nonterminals which do appear on its right hand side.

For example:

expr:
  primary
| primary '+' primary
;

primary:
  constant
| '(' expr ')'
;

defines two mutually-recursive nonterminals, since each refers to the other.

3.4 Defining Language Semantics

The grammar rules for a language determine only the syntax. The semantics are determined by the semantic values associated with various tokens and groupings, and by the actions taken when various groupings are recognized.

For example, the calculator calculates properly because the value associated with each expression is the proper number; it adds properly because the action for the grouping ‘x + y’
is to add the numbers associated with x and y.

3.4.1 Data Types of Semantic Values

In a simple program it may be sufficient to use the same data type for the semantic values of all language constructs. This was true in the RPN and infix calculator examples (see Reverse Polish
Notation Calculator).

Bison normally uses the type int for semantic values if your program uses the same data type for all language constructs. To specify some other type, define the %define variable
api.value.type like this:

%define api.value.type {double}

or

%define api.value.type {struct semantic_type}

The value of api.value.type should be a type name that does not contain parentheses or square brackets.

Alternatively, instead of relying of Bison’s %define support, you may rely on the C/C++ preprocessor and define YYSTYPE as a macro, like this:

This macro definition must go in the prologue of the grammar file (see Outline of a Bison Grammar). If compatibility with POSIX Yacc matters to you, use this. Note however that Bison cannot know YYSTYPE’s
value, not even whether it is defined, so there are services it cannot provide. Besides this works only for languages that have a preprocessor.

3.4.2 More Than One Value Type

In most programs, you will need different data types for different kinds of tokens and groupings. For example, a numeric constant may need type
int or long int, while a string constant needs type
char *, and an identifier might need a pointer to an entry in the symbol table.

To use more than one data type for semantic values in one parser, Bison requires you to do two things:

• Specify the entire collection of possible data types. There are several options:
  • let Bison compute the union type from the tags you assign to symbols;
  • use the %union Bison declaration (see The Union
    Declaration);

  • define the %define variable api.value.type to be a union type whose members are the type tags (see Providing a
    Structured Semantic Value Type);

  • use a typedef or a #define to define YYSTYPE to be a union type whose member names are the type tags.
• Choose one of those types for each symbol (terminal or nonterminal) for which semantic values are used. This is done for tokens with the
  %token Bison declaration (see Token Type Names) and for groupings with the %type Bison declaration (see Nonterminal Symbols).

3.4.3 Generating the Semantic Value Type

The special value union of the %define variable
api.value.type instructs Bison that the tags used with the
%token and %type directives are genuine types, not names of members of YYSTYPE.

For example:

%define api.value.type union
%token <int> INT "integer"
%token <int> 'n'
%type <int> expr
%token <char const *> ID "identifier"

generates an appropriate value of YYSTYPE to support each symbol type. The name of the member of YYSTYPE for tokens than have a declared identifier id (such as INT and ID above, but
not
'n') is id. The other symbols have unspecified names on which you should not depend; instead, relying on C casts to access the semantic value with the appropriate type:

/* For an "integer".  */
yylval.INT = 42;
return INT;

/* For an 'n', also declared as int.  */
*((int*)&yylval) = 42;
return 'n';

/* For an "identifier".  */
yylval.ID = "42";
return ID;

If the %define variable api.token.prefix is defined (see api.token.prefix), then it is also used to prefix the union member names. For instance, with ‘%define api.token.prefix
{TOK_}’:

/* For an "integer".  */
yylval.TOK_INT = 42;
return TOK_INT;

This Bison extension cannot work if %yacc (or
-y/—yacc) is enabled, as POSIX mandates that Yacc generate tokens as macros (e.g., ‘#define INT 258’, or ‘#define
TOK_INT 258’).

This feature is new, and user feedback would be most welcome.

A similar feature is provided for C++ that in addition overcomes C++ limitations (that forbid non-trivial objects to be part of a union): ‘
%define api.value.type variant’, see C++ Variants.

3.4.4 The Union Declaration

The %union declaration specifies the entire collection of possible data types for semantic values. The keyword %union is followed by braced code containing the same thing that goes inside a union in C.

For example:

%union {
  double val;
  symrec *tptr;
}

This says that the two alternative types are double and symrec
*. They are given names val and tptr; these names are used in the %token and %type declarations to pick one of the types for a terminal or nonterminal symbol (see Nonterminal Symbols).

As an extension to POSIX, a tag is allowed after the %union. For example:

%union value {
  double val;
  symrec *tptr;
}

specifies the union tag value, so the corresponding C type is
union value. If you do not specify a tag, it defaults to
YYSTYPE (see api.value.union.name).

As another extension to POSIX, you may specify multiple %union declarations; their contents are concatenated. However, only the first
%union declaration can specify a tag.

Note that, unlike making a union declaration in C, you need not write a semicolon after the closing brace.

3.4.5 Providing a Structured Semantic Value Type

Instead of %union, you can define and use your own union type
YYSTYPE if your grammar contains at least one ‘<type>’ tag. For example, you can put the following into a header file
parser.h:

union YYSTYPE {
  double val;
  symrec *tptr;
};

and then your grammar can use the following instead of %union:

%{
#include "parser.h"
%}
%define api.value.type {union YYSTYPE}
%type <val> expr
%token <tptr> ID

Actually, you may also provide a struct rather that a union, which may be handy if you want to track information for every symbol (such as preceding comments).

The type you provide may even be structured and include pointers, in which case the type tags you provide may be composite, with ‘.’ and ‘->’ operators.

3.4.6 Actions

An action accompanies a syntactic rule and contains C code to be executed each time an instance of that rule is recognized. The task of most actions is to compute a semantic value for the grouping built by the rule from the semantic values associated
with tokens or smaller groupings.

An action consists of braced code containing C statements, and can be placed at any position in the rule; it is executed at that position. Most rules have just one action at the end of the rule, following all the components. Actions in the middle
of a rule are tricky and used only for special purposes (see Actions in Mid-Rule).

The C code in an action can refer to the semantic values of the components matched by the rule with the construct $n, which stands for the value of the nth component. The semantic value for the grouping being
constructed is $$. In addition, the semantic values of symbols can be accessed with the named references construct $name or $[name]. Bison translates both of these constructs into
expressions of the appropriate type when it copies the actions into the parser implementation file. $$ (or $name, when it stands for the current grouping) is translated to a modifiable lvalue, so it can
be assigned to.

Here is a typical example:

exp:
…
| exp '+' exp     { $$ = $1 + $3; }

Or, in terms of named references:

exp[result]:
…
| exp[left] '+' exp[right]  { $result = $left + $right; }

This rule constructs an exp from two smaller exp groupings connected by a plus-sign token. In the action, $1 and $3 (
$left and $right) refer to the semantic values of the two component exp groupings, which are the first and third symbols on the right hand side of the rule. The sum is stored into $$ ($result)
so that it becomes the semantic value of the addition-expression just recognized by the rule. If there were a useful semantic value associated with the ‘+’ token, it could be referred to as $2.

See Named References, for more information about using the named references construct.

Note that the vertical-bar character ‘|’ is really a rule separator, and actions are attached to a single rule. This is a difference with tools like Flex, for which ‘|’ stands for either “or”,
or “the same action as that of the next rule”. In the following example, the action is triggered only when ‘b’ is found:

a-or-b: 'a'|'b'   { a_or_b_found = 1; };

If you don’t specify an action for a rule, Bison supplies a default:
$$ = $1.
Thus, the value of the first symbol in the rule becomes the value of the whole rule. Of course, the default action is valid only if the two data types match. There is no meaningful default action for an empty rule; every empty rule must have
an explicit action unless the rule’s value does not matter.

$n with n zero or negative is allowed for reference to tokens and groupings on the stack before those that match the current rule. This is a very risky practice, and to use it reliably you must be certain
of the context in which the rule is applied. Here is a case in which you can use this reliably:

foo:
  expr bar '+' expr  { … }
| expr bar '-' expr  { … }
;

bar:
  %empty    { previous_expr = $0; }
;

As long as bar is used only in the fashion shown here, $0 always refers to the expr which precedes bar in the definition of foo.

It is also possible to access the semantic value of the lookahead token, if any, from a semantic action. This semantic value is stored in yylval. See Special Features for Use in Actions.

3.4.7 Data Types of Values in Actions

If you have chosen a single data type for semantic values, the $$ and $n constructs always have that data type.

If you have used %union to specify a variety of data types, then you must declare a choice among these types for each terminal or nonterminal symbol that can have a semantic value. Then each time you use $$ or
$n, its data type is determined by which symbol it refers to in the rule. In this example,

exp:
  …
| exp '+' exp    { $$ = $1 + $3; }

$1 and $3 refer to instances of exp, so they all have the data type declared for the nonterminal symbol exp. If
$2 were used, it would have the data type declared for the terminal symbol '+', whatever that might be.

Alternatively, you can specify the data type when you refer to the value, by inserting ‘<type>’ after the ‘$’ at the beginning of the reference. For example, if you have defined
types as shown here:

%union {
  int itype;
  double dtype;
}

then you can write $<itype>1 to refer to the first subunit of the rule as an integer, or $<dtype>1 to refer to it as a double.

3.4.8 Actions in Mid-Rule

Occasionally it is useful to put an action in the middle of a rule. These actions are written just like usual end-of-rule actions, but they are executed before the parser even recognizes the following components.

3.4.8.1 Using Mid-Rule Actions

A mid-rule action may refer to the components preceding it using
$n, but it may not refer to subsequent components because it is run before they are parsed.

The mid-rule action itself counts as one of the components of the rule. This makes a difference when there is another action later in the same rule (and usually there is another at the end): you have to count the actions along with the symbols
when working out which number n to use in
$n.

The mid-rule action can also have a semantic value. The action can set its value with an assignment to $$, and actions later in the rule can refer to the value using $n. Since there is no symbol to name the
action, there is no way to declare a data type for the value in advance, so you must use the ‘$<…>n’ construct to specify a data type each time you refer to this value.

There is no way to set the value of the entire rule with a mid-rule action, because assignments to $$ do not have that effect. The only way to set the value for the entire rule is with an ordinary action at the end of the rule.

Here is an example from a hypothetical compiler, handling a let statement that looks like ‘let (variable) statement’ and serves to create a variable named variable temporarily
for the duration of statement. To parse this construct, we must put
variable into the symbol table while statement is parsed, then remove it afterward. Here is how it is done:

stmt:
  "let" '(' var ')'
    {
      $<context>$ = push_context ();
      declare_variable ($3);
    }
  stmt
    {
      $$ = $6;
      pop_context ($<context>5);
    }

As soon as ‘let (variable)’ has been recognized, the first action is run. It saves a copy of the current semantic context (the list of accessible variables) as its semantic value, using alternative
context in the data-type union. Then it calls
declare_variable to add the new variable to that list. Once the first action is finished, the embedded statement stmt can be parsed.

Note that the mid-rule action is component number 5, so the ‘stmt’ is component number 6. Named references can be used to improve the readability and maintainability (see Named References):

stmt:
  "let" '(' var ')'
    {
      $<context>let = push_context ();
      declare_variable ($3);
    }[let]
  stmt
    {
      $$ = $6;
      pop_context ($<context>let);
    }

After the embedded statement is parsed, its semantic value becomes the value of the entire let-statement. Then the semantic value from the earlier action is used to restore the prior list of variables. This removes the temporary let-variable
from the list so that it won’t appear to exist while the rest of the program is parsed.

In the above example, if the parser initiates error recovery (see Error Recovery) while parsing the tokens in the embedded statement stmt, it might discard the previous semantic context $<context>5 without restoring it. Thus, $<context>5 needs a destructor (see Freeing
Discarded Symbols). However, Bison currently provides no means to declare a destructor specific to a particular mid-rule action’s semantic value.

One solution is to bury the mid-rule action inside a nonterminal symbol and to declare a destructor for that symbol:

%type <context> let
%destructor { pop_context ($$); } let

%%

stmt:
  let stmt
    {
      $$ = $2;
      pop_context ($let);
    };

let:
  "let" '(' var ')'
    {
      $let = push_context ();
      declare_variable ($3);
    };

Note that the action is now at the end of its rule. Any mid-rule action can be converted to an end-of-rule action in this way, and this is what Bison actually does to implement mid-rule actions.

3.4.8.2 Mid-Rule Action Translation

As hinted earlier, mid-rule actions are actually transformed into regular rules and actions. The various reports generated by Bison (textual, graphical, etc., see Understanding Your Parser) reveal this translation,
best explained by means of an example. The following rule:

exp: { a(); } "b" { c(); } { d(); } "e" { f(); };

is translated into:

$@1: %empty { a(); };
$@2: %empty { c(); };
$@3: %empty { d(); };
exp: $@1 "b" $@2 $@3 "e" { f(); };

with new nonterminal symbols $@n, where n is a number.

A mid-rule action is expected to generate a value if it uses $$, or the (final) action uses $n where n denote the mid-rule action. In that case its nonterminal is rather named @n:

exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };

is translated into

@1: %empty { a(); };
@2: %empty { $$ = c(); };
$@3: %empty { d(); };
exp: @1 "b" @2 $@3 "e" { f = $1; }

There are probably two errors in the above example: the first mid-rule action does not generate a value (it does not use $$ although the final action uses it), and the value of the second one is not used (the final action does not
use
$3). Bison reports these errors when the
midrule-value warnings are enabled (see Invoking
Bison):

$ bison -fcaret -Wmidrule-value mid.y

mid.y:2.6-13: warning: unset value: $$
 exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };
      ^^^^^^^^

mid.y:2.19-31: warning: unused value: $3
 exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };
                   ^^^^^^^^^^^^^

3.4.8.3 Conflicts due to Mid-Rule Actions

Taking action before a rule is completely recognized often leads to conflicts since the parser must commit to a parse in order to execute the action. For example, the following two rules, without mid-rule actions, can coexist in a working parser
because the parser can shift the open-brace token and look at what follows before deciding whether there is a declaration or not:

compound:
  '{' declarations statements '}'
| '{' statements '}'
;

But when we add a mid-rule action as follows, the rules become nonfunctional:

compound:
  { prepare_for_local_variables (); }
     '{' declarations statements '}'

|    '{' statements '}'
;

Now the parser is forced to decide whether to run the mid-rule action when it has read no farther than the open-brace. In other words, it must commit to using one rule or the other, without sufficient information to do it correctly. (The open-brace
token is what is called the lookahead token at this time, since the parser is still deciding what to do about it. See Lookahead Tokens.)

You might think that you could correct the problem by putting identical actions into the two rules, like this:

compound:
  { prepare_for_local_variables (); }
    '{' declarations statements '}'
| { prepare_for_local_variables (); }
    '{' statements '}'
;

But this does not help, because Bison does not realize that the two actions are identical. (Bison never tries to understand the C code in an action.)

If the grammar is such that a declaration can be distinguished from a statement by the first token (which is true in C), then one solution which does work is to put the action after the open-brace, like this:

compound:
  '{' { prepare_for_local_variables (); }
    declarations statements '}'
| '{' statements '}'
;

Now the first token of the following declaration or statement, which would in any case tell Bison which rule to use, can still do so.

Another solution is to bury the action inside a nonterminal symbol which serves as a subroutine:

subroutine:
  %empty  { prepare_for_local_variables (); }
;

compound:
  subroutine '{' declarations statements '}'
| subroutine '{' statements '}'
;

Now Bison can execute the action in the rule for subroutine without deciding which rule for compound it will eventually use.

3.5 Tracking Locations

Though grammar rules and semantic actions are enough to write a fully functional parser, it can be useful to process some additional information, especially symbol locations.

The way locations are handled is defined by providing a data type, and actions to take when rules are matched.

3.5.1 Data Type of Locations

Defining a data type for locations is much simpler than for semantic values, since all tokens and groupings always use the same type.

You can specify the type of locations by defining a macro called
YYLTYPE, just as you can specify the semantic value type by defining a YYSTYPE macro (see Value Type). When YYLTYPE is not defined, Bison uses a default structure type with four
members:

typedef struct YYLTYPE
{
  int first_line;
  int first_column;
  int last_line;
  int last_column;
} YYLTYPE;

When YYLTYPE is not defined, at the beginning of the parsing, Bison initializes all these fields to 1 for yylloc. To initialize
yylloc with a custom location type (or to chose a different initialization), use the %initial-action directive. See Performing Actions before Parsing.

3.5.2 Actions and Locations

Actions are not only useful for defining language semantics, but also for describing the behavior of the output parser with locations.

The most obvious way for building locations of syntactic groupings is very similar to the way semantic values are computed. In a given rule, several constructs can be used to access the locations of the elements being matched. The location of
the
nth component of the right hand side is
@n, while the location of the left hand side grouping is
@$.

In addition, the named references construct @name and
@[name] may also be used to address the symbol locations. See Named References, for more information about using the named references construct.

Here is a basic example using the default data type for locations:

exp:
  …
| exp '/' exp
    {
      @$.first_column = @1.first_column;
      @$.first_line = @1.first_line;
      @$.last_column = @3.last_column;
      @$.last_line = @3.last_line;
      if ($3)
        $$ = $1 / $3;
      else
        {
          $$ = 1;
          fprintf (stderr, "%d.%d-%d.%d: division by zero",
                   @3.first_line, @3.first_column,
                   @3.last_line, @3.last_column);
        }
    }

As for semantic values, there is a default action for locations that is run each time a rule is matched. It sets the beginning of @$ to the beginning of the first symbol, and the end of @$ to the end of the last symbol.

With this default action, the location tracking can be fully automatic. The example above simply rewrites this way:

exp:
  …
| exp '/' exp
    {
      if ($3)
        $$ = $1 / $3;
      else
        {
          $$ = 1;
          fprintf (stderr, "%d.%d-%d.%d: division by zero",
                   @3.first_line, @3.first_column,
                   @3.last_line, @3.last_column);
        }
    }

It is also possible to access the location of the lookahead token, if any, from a semantic action. This location is stored in yylloc. See Special Features for Use in Actions.

3.5.3 Default Action for Locations

Actually, actions are not the best place to compute locations. Since locations are much more general than semantic values, there is room in the output parser to redefine the default action to take for each rule. The YYLLOC_DEFAULT macro is invoked each time a rule is matched, before the associated action is run. It is also invoked while processing a syntax error, to compute the error’s location. Before reporting an unresolvable syntactic ambiguity, a GLR parser
invokes
YYLLOC_DEFAULT recursively to compute the location of that ambiguity.

Most of the time, this macro is general enough to suppress location dedicated code from semantic actions.

The YYLLOC_DEFAULT macro takes three parameters. The first one is the location of the grouping (the result of the computation). When a rule is matched, the second parameter identifies locations of all right hand side elements of the
rule being matched, and the third parameter is the size of the rule’s right hand side. When a GLR parser reports an ambiguity, which of multiple candidate right hand sides it passes to YYLLOC_DEFAULT is undefined. When processing
a syntax error, the second parameter identifies locations of the symbols that were discarded during error processing, and the third parameter is the number of discarded symbols.

By default, YYLLOC_DEFAULT is defined this way:

# define YYLLOC_DEFAULT(Cur, Rhs, N)                      
do                                                        
  if (N)                                                  
    {                                                     
      (Cur).first_line   = YYRHSLOC(Rhs, 1).first_line;   
      (Cur).first_column = YYRHSLOC(Rhs, 1).first_column; 
      (Cur).last_line    = YYRHSLOC(Rhs, N).last_line;    
      (Cur).last_column  = YYRHSLOC(Rhs, N).last_column;  
    }                                                     
  else                                                    
    {                                                     
      (Cur).first_line   = (Cur).last_line   =            
        YYRHSLOC(Rhs, 0).last_line;                       
      (Cur).first_column = (Cur).last_column =            
        YYRHSLOC(Rhs, 0).last_column;                     
    }                                                     
while (0)

where YYRHSLOC (rhs, k) is the location of the kth symbol in rhs when k is positive, and the location of the symbol just before the reduction when k and n are both zero.

When defining YYLLOC_DEFAULT, you should consider that:

• All arguments are free of side-effects. However, only the first one (the result) should be modified by YYLLOC_DEFAULT.
• For consistency with semantic actions, valid indexes within the right hand side range from 1 to n. When n is zero, only 0 is a valid index, and it refers to the symbol just before the reduction. During error processing
  n is always positive.

• Your macro should parenthesize its arguments, if need be, since the actual arguments may not be surrounded by parentheses. Also, your macro should expand to something that can be used as a single statement when it is followed by a semicolon.

3.6 Named References

As described in the preceding sections, the traditional way to refer to any semantic value or location is a positional reference, which takes the form $n, $$, @n, and @$.
However, such a reference is not very descriptive. Moreover, if you later decide to insert or remove symbols in the right-hand side of a grammar rule, the need to renumber such references can be tedious and error-prone.

To avoid these issues, you can also refer to a semantic value or location using a named reference. First of all, original symbol names may be used as named references. For example:

invocation: op '(' args ')'
  { $invocation = new_invocation ($op, $args, @invocation); }

Positional and named references can be mixed arbitrarily. For example:

invocation: op '(' args ')'
  { $$ = new_invocation ($op, $args, @$); }

However, sometimes regular symbol names are not sufficient due to ambiguities:

exp: exp '/' exp
  { $exp = $exp / $exp; } // $exp is ambiguous.

exp: exp '/' exp
  { $$ = $1 / $exp; } // One usage is ambiguous.

exp: exp '/' exp
  { $$ = $1 / $3; } // No error.

When ambiguity occurs, explicitly declared names may be used for values and locations. Explicit names are declared as a bracketed name after a symbol appearance in rule definitions. For example:

exp[result]: exp[left] '/' exp[right]
  { $result = $left / $right; }

In order to access a semantic value generated by a mid-rule action, an explicit name may also be declared by putting a bracketed name after the closing brace of the mid-rule action code:

exp[res]: exp[x] '+' {$left = $x;}[left] exp[right]
  { $res = $left + $right; }

In references, in order to specify names containing dots and dashes, an explicit bracketed syntax $[name] and @[name] must be used:

if-stmt: "if" '(' expr ')' "then" then.stmt ';'
  { $[if-stmt] = new_if_stmt ($expr, $[then.stmt]); }

It often happens that named references are followed by a dot, dash or other C punctuation marks and operators. By default, Bison will read ‘
$name.suffix’ as a reference to symbol value $name followed by ‘
.suffix’, i.e., an access to the suffix field of the semantic value. In order to force Bison to recognize ‘name.suffix’ in its entirety as the name of a semantic value, the bracketed
syntax ‘
$[name.suffix]’ must be used.

The named references feature is experimental. More user feedback will help to stabilize it.

3.7 Bison Declarations

The Bison declarations section of a Bison grammar defines the symbols used in formulating the grammar and the data types of semantic values. See Symbols.

All token type names (but not single-character literal tokens such as
'+' and '*') must be declared. Nonterminal symbols must be declared if you need to specify which data type to use for the semantic value (see More Than One Value Type).

The first rule in the grammar file also specifies the start symbol, by default. If you want some other symbol to be the start symbol, you must declare it explicitly (see Languages
and Context-Free Grammars).

3.7.1 Require a Version of Bison

You may require the minimum version of Bison to process the grammar. If the requirement is not met, bison exits with an error (exit status 63).

3.7.2 Token Type Names

The basic way to declare a token type name (terminal symbol) is as follows:

Bison will convert this into a #define directive in the parser, so that the function yylex (if it is in this file) can use the name name to stand for this token type’s code.

Alternatively, you can use %left, %right,
%precedence, or
%nonassoc instead of %token, if you wish to specify associativity and precedence. See Operator
Precedence.

You can explicitly specify the numeric code for a token type by appending a nonnegative decimal or hexadecimal integer value in the field immediately following the token name:

%token NUM 300
%token XNUM 0x12d // a GNU extension

It is generally best, however, to let Bison choose the numeric codes for all token types. Bison will automatically select codes that don’t conflict with each other or with normal characters.

In the event that the stack type is a union, you must augment the
%token or other token declaration to include the data type alternative delimited by angle-brackets (see More
Than One Value Type).

For example:

%union {              /* define stack type */
  double val;
  symrec *tptr;
}
%token <val> NUM      /* define token NUM and its type */

You can associate a literal string token with a token type name by writing the literal string at the end of a %token declaration which declares the name. For example:

For example, a grammar for the C language might specify these names with equivalent literal string tokens:

%token  <operator>  OR      "||"
%token  <operator>  LE 134  "<="
%left  OR  "<="

Once you equate the literal string and the token name, you can use them interchangeably in further declarations or the grammar rules. The
yylex function can use the token name or the literal string to obtain the token type code number (see Calling Convention). Syntax error messages passed to yyerror from the parser
will reference the literal string instead of the token name.

The token numbered as 0 corresponds to end of file; the following line allows for nicer error messages referring to “end of file” instead of “$end”:

%token END 0 "end of file"

3.7.3 Operator Precedence

Use the %left, %right, %nonassoc, or
%precedence declaration to declare a token and specify its precedence and associativity, all at once. These are called precedence declarations. See Operator Precedence, for general information
on operator precedence.

The syntax of a precedence declaration is nearly the same as that of
%token: either

or

And indeed any of these declarations serves the purposes of %token. But in addition, they specify the associativity and relative precedence for all the symbols:

• The associativity of an operator op determines how repeated uses of the operator nest: whether ‘x op y op
  z’ is parsed by grouping x with y first or by grouping y with z first. %left specifies left-associativity (grouping x with y first) and
  %right specifies right-associativity (grouping y with
  z first). %nonassoc specifies no associativity, which means that ‘x op y op z’ is considered a syntax error.

  %precedence gives only precedence to the symbols, and defines no associativity at all. Use this to define precedence only, and leave any potential conflict due to associativity enabled.

• The precedence of an operator determines how it nests with other operators. All the tokens declared in a single precedence declaration have equal precedence and nest together according to their associativity. When two tokens declared in different
  precedence declarations associate, the one declared later has the higher precedence and is grouped first.

For backward compatibility, there is a confusing difference between the argument lists of %token and precedence declarations. Only a %token can associate a literal string with a token type name. A precedence declaration
always interprets a literal string as a reference to a separate token. For example:

%left  OR "<="         // Does not declare an alias.
%left  OR 134 "<=" 135 // Declares 134 for OR and 135 for "<=".

3.7.4 Nonterminal Symbols

When you use %union to specify multiple value types, you must declare the value type of each nonterminal symbol for which values are used. This is done with a %type declaration, like this:

%type <type> nonterminal…

Here nonterminal is the name of a nonterminal symbol, and
type is the name given in the %union to the alternative that you want (see The Union Declaration). You can give any number of nonterminal symbols in the same %type declaration,
if they have the same value type. Use spaces to separate the symbol names.

You can also declare the value type of a terminal symbol. To do this, use the same <type> construction in a declaration for the terminal symbol. All kinds of token declarations allow
<type>.

3.7.5 Performing Actions before Parsing

Sometimes your parser needs to perform some initializations before parsing. The %initial-action directive allows for such arbitrary code.

Directive: %initial-action { code }

Declare that the braced code must be invoked before parsing each time
yyparse is called. The code may use $$ (or
$<tag>$) and @$ — initial value and location of the lookahead — and the %parse-param.

For instance, if your locations use a file name, you may use

%parse-param { char const *file_name };
%initial-action
{
  @$.initialize (file_name);
};

3.7.6 Freeing Discarded Symbols

During error recovery (see Error Recovery), symbols already pushed on the stack and tokens coming from the rest of the file are discarded until the parser falls on its feet. If the parser runs out of memory, or if
it returns via YYABORT or YYACCEPT, all the symbols on the stack must be discarded. Even if the parser succeeds, it must discard the start symbol.

When discarded symbols convey heap based information, this memory is lost. While this behavior can be tolerable for batch parsers, such as in traditional compilers, it is unacceptable for programs like shells or protocol implementations that may
parse and execute indefinitely.

The %destructor directive defines code that is called when a symbol is automatically discarded.

Directive: %destructor { code } symbols

Invoke the braced code whenever the parser discards one of the
symbols. Within code, $$ (or $<tag>$) designates the semantic value associated with the discarded symbol, and
@$ designates its location. The additional parser parameters are also available (see The Parser Function
yyparse).

When a symbol is listed among symbols, its %destructor is called a per-symbol %destructor. You may also define a per-type %destructor by listing a semantic type tag among symbols.
In that case, the parser will invoke this code whenever it discards any grammar symbol that has that semantic type tag unless that symbol has its own per-symbol %destructor.

Finally, you can define two different kinds of default %destructors. (These default forms are experimental. More user feedback will help to determine whether they should become permanent features.) You can place each of <*> and <> in the symbols list of exactly one %destructor declaration in your grammar file. The parser will invoke the code associated with one of these whenever it discards any user-defined
grammar symbol that has no per-symbol and no per-type
%destructor. The parser uses the code for <*> in the case of such a grammar symbol for which you have formally declared a semantic type tag (%type counts as such a declaration,
but
$<tag>$ does not). The parser uses the code for <> in the case of such a grammar symbol that has no declared semantic type tag.

For example:

%union { char *string; }
%token <string> STRING1 STRING2
%type  <string> string1 string2
%union { char character; }
%token <character> CHR
%type  <character> chr
%token TAGLESS

%destructor { } <character>
%destructor { free ($$); } <*>
%destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
%destructor { printf ("Discarding tagless symbol.n"); } <>

guarantees that, when the parser discards any user-defined symbol that has a semantic type tag other than <character>, it passes its semantic value to free by default. However, when the parser discards a STRING1 or a string1, it also prints its line number to stdout. It performs only the second %destructor in this case, so it invokes
free only once. Finally, the parser merely prints a message whenever it discards any symbol, such as TAGLESS, that has no semantic type tag.

A Bison-generated parser invokes the default %destructors only for user-defined as opposed to Bison-defined symbols. For example, the parser will not invoke either kind of default
%destructor for the special Bison-defined symbols $accept,
$undefined, or $end (see Bison Symbols), none of which you can reference in your grammar. It also will not invoke either for the error token (see error),
which is always defined by Bison regardless of whether you reference it in your grammar. However, it may invoke one of them for the end token (token 0) if you redefine it from $end to, for example, END:

Finally, Bison will never invoke a %destructor for an unreferenced mid-rule semantic value (see Actions in Mid-Rule). That is, Bison does not consider a mid-rule to have a semantic value if you
do not reference $$ in the mid-rule’s action or $n (where n is the right-hand side symbol position of the mid-rule) in any later action in that rule. However, if you do reference either,
the Bison-generated parser will invoke the <> %destructor whenever it discards the mid-rule symbol.

Discarded symbols are the following:

• stacked symbols popped during the first phase of error recovery,
• incoming terminals during the second phase of error recovery,
• the current lookahead and the entire stack (except the current right-hand side symbols) when the parser returns immediately, and
• the current lookahead and the entire stack (including the current right-hand side symbols) when the C++ parser (lalr1.cc) catches an exception in
  parse,
• the start symbol, when the parser succeeds.

The parser can return immediately because of an explicit call to
YYABORT or YYACCEPT, or failed error recovery, or memory exhaustion.

Right-hand side symbols of a rule that explicitly triggers a syntax error via YYERROR are not discarded automatically. As a rule of thumb, destructors are invoked only when user actions cannot manage the memory.

3.7.7 Printing Semantic Values

When run-time traces are enabled (see Tracing Your Parser), the parser reports its actions, such as reductions. When a symbol involved in an action is reported, only its kind is displayed, as the parser cannot know how semantic
values should be formatted.

The %printer directive defines code that is called when a symbol is reported. Its syntax is the same as %destructor (see Freeing Discarded Symbols).

Directive: %printer { code } symbols

Invoke the braced code whenever the parser displays one of the
symbols. Within code, yyoutput denotes the output stream (a FILE* in C, and an std::ostream& in C++), $$ (or
$<tag>$) designates the semantic value associated with the symbol, and @$ its location. The additional parser parameters are also available (see The Parser Function
yyparse).

The symbols are defined as for %destructor (see Freeing Discarded Symbols.): they can be per-type (e.g., ‘
<ival>’), per-symbol (e.g., ‘exp’, ‘NUM’, ‘«float»’), typed per-default (i.e., ‘<*>’, or untyped
per-default (i.e., ‘
<>’).

For example:

%union { char *string; }
%token <string> STRING1 STRING2
%type  <string> string1 string2
%union { char character; }
%token <character> CHR
%type  <character> chr
%token TAGLESS

%printer { fprintf (yyoutput, "'%c'", $$); } <character>
%printer { fprintf (yyoutput, "&%p", $$); } <*>
%printer { fprintf (yyoutput, ""%s"", $$); } STRING1 string1
%printer { fprintf (yyoutput, "<>"); } <>

guarantees that, when the parser print any symbol that has a semantic type tag other than <character>, it display the address of the semantic value by default. However, when the parser displays a STRING1 or a
string1, it formats it as a string in double quotes. It performs only the second %printer in this case, so it prints only once. Finally, the parser print ‘<>’ for any symbol, such as
TAGLESS, that has no semantic type tag. See Enabling Debug Traces
for mfcalc, for a complete example.

3.7.8 Suppressing Conflict Warnings

Bison normally warns if there are any conflicts in the grammar (see Shift/Reduce Conflicts), but most real grammars have harmless shift/reduce conflicts which are resolved in a predictable way and would be difficult
to eliminate. It is desirable to suppress the warning about these conflicts unless the number of conflicts changes. You can do this with the %expect declaration.

The declaration looks like this:

Here n is a decimal integer. The declaration says there should be n shift/reduce conflicts and no reduce/reduce conflicts. Bison reports an error if the number of shift/reduce conflicts differs from n, or if there
are any reduce/reduce conflicts.

For deterministic parsers, reduce/reduce conflicts are more serious, and should be eliminated entirely. Bison will always report reduce/reduce conflicts for these parsers. With GLR parsers, however, both kinds of conflicts are routine; otherwise,
there would be no need to use GLR parsing. Therefore, it is also possible to specify an expected number of reduce/reduce conflicts in GLR parsers, using the declaration:

In general, using %expect involves these steps:

• Compile your grammar without %expect. Use the ‘-v’ option to get a verbose list of where the conflicts occur. Bison will also print the number of conflicts.
• Check each of the conflicts to make sure that Bison’s default resolution is what you really want. If not, rewrite the grammar and go back to the beginning.
• Add an %expect declaration, copying the number n from the number which Bison printed. With GLR parsers, add an
  %expect-rr declaration as well.

Now Bison will report an error if you introduce an unexpected conflict, but will keep silent otherwise.

3.7.9 The Start-Symbol

Bison assumes by default that the start symbol for the grammar is the first nonterminal specified in the grammar specification section. The programmer may override this restriction with the %start declaration as follows:

3.7.10 A Pure (Reentrant) Parser

A reentrant program is one which does not alter in the course of execution; in other words, it consists entirely of pure (read-only) code. Reentrancy is important whenever asynchronous execution is possible; for example, a nonreentrant
program may not be safe to call from a signal handler. In systems with multiple threads of control, a nonreentrant program must be called only within interlocks.

Normally, Bison generates a parser which is not reentrant. This is suitable for most uses, and it permits compatibility with Yacc. (The standard Yacc interfaces are inherently nonreentrant, because they use statically allocated variables for communication
with yylex, including yylval and yylloc.)

Alternatively, you can generate a pure, reentrant parser. The Bison declaration ‘%define api.pure’ says that you want the parser to be reentrant. It looks like this:

The result is that the communication variables yylval and
yylloc become local variables in yyparse, and a different calling convention is used for the lexical analyzer function
yylex. See Calling Conventions for Pure
Parsers, for the details of this. The variable yynerrs becomes local in yyparse in pull mode but it becomes a member of yypstate in push mode. (see The Error
Reporting Function yyerror). The convention for calling
yyparse itself is unchanged.

Whether the parser is pure has nothing to do with the grammar rules. You can generate either a pure parser or a nonreentrant parser from any valid grammar.

3.7.11 A Push Parser

(The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)

A pull parser is called once and it takes control until all its input is completely parsed. A push parser, on the other hand, is called each time a new token is made available.

A push parser is typically useful when the parser is part of a main event loop in the client’s application. This is typically a requirement of a GUI, when the main event loop needs to be triggered within a certain time period.

Normally, Bison generates a pull parser. The following Bison declaration says that you want the parser to be a push parser (see api.push-pull):

%define api.push-pull push

In almost all cases, you want to ensure that your push parser is also a pure parser (see A Pure (Reentrant) Parser). The only time you should create an impure push parser is to have backwards compatibility with the impure
Yacc pull mode interface. Unless you know what you are doing, your declarations should look like this:

%define api.pure full
%define api.push-pull push

There is a major notable functional difference between the pure push parser and the impure push parser. It is acceptable for a pure push parser to have many parser instances, of the same type of parser, in memory at the same time. An impure push
parser should only use one parser at a time.

When a push parser is selected, Bison will generate some new symbols in the generated parser. yypstate is a structure that the generated parser uses to store the parser’s state. yypstate_new is the function that
will create a new parser instance. yypstate_delete will free the resources associated with the corresponding parser instance. Finally, yypush_parse is the function that should be called whenever a token is available
to provide the parser. A trivial example of using a pure push parser would look like this:

int status;
yypstate *ps = yypstate_new ();
do {
  status = yypush_parse (ps, yylex (), NULL);
} while (status == YYPUSH_MORE);
yypstate_delete (ps);

If the user decided to use an impure push parser, a few things about the generated parser will change. The yychar variable becomes a global variable instead of a variable in the yypush_parse function. For this reason,
the signature of the yypush_parse function is changed to remove the token as a parameter. A nonreentrant push parser example would thus look like this:

extern int yychar;
int status;
yypstate *ps = yypstate_new ();
do {
  yychar = yylex ();
  status = yypush_parse (ps);
} while (status == YYPUSH_MORE);
yypstate_delete (ps);

That’s it. Notice the next token is put into the global variable yychar for use by the next invocation of the yypush_parse function.

Bison also supports both the push parser interface along with the pull parser interface in the same generated parser. In order to get this functionality, you should replace the ‘%define api.push-pull push’ declaration
with the ‘
%define api.push-pull both’ declaration. Doing this will create all of the symbols mentioned earlier along with the two extra symbols, yyparse and yypull_parse. yyparse can be used
exactly as it normally would be used. However, the user should note that it is implemented in the generated parser by calling yypull_parse. This makes the yyparse function that is generated with the ‘
%define api.push-pull both’ declaration slower than the normal
yyparse function. If the user calls the yypull_parse function it will parse the rest of the input stream. It is possible to yypush_parse tokens to select a subgrammar and then yypull_parse the rest of the input stream. If you would like to switch back and forth between between parsing styles, you would have to write your own yypull_parse function that knows when to quit looking for input. An example of using the
yypull_parse function would look like this:

yypstate *ps = yypstate_new ();
yypull_parse (ps); /* Will call the lexer */
yypstate_delete (ps);

Adding the ‘%define api.pure’ declaration does exactly the same thing to the generated parser with ‘%define api.push-pull both’ as it did for ‘
%define api.push-pull push’.

3.7.12 Bison Declaration Summary

Here is a summary of the declarations used to define a grammar:

Directive: %union

Declare the collection of data types that semantic values may have (see The Union Declaration).

Directive: %token

Declare a terminal symbol (token type name) with no precedence or associativity specified (see Token Type Names).

Directive: %right

Declare a terminal symbol (token type name) that is right-associative (see Operator Precedence).

Directive: %left

Declare a terminal symbol (token type name) that is left-associative (see Operator Precedence).

Directive: %nonassoc

Declare a terminal symbol (token type name) that is nonassociative (see Operator Precedence). Using it in a way that would be associative is a syntax error.

Directive: %type

Declare the type of semantic values for a nonterminal symbol (see Nonterminal Symbols).

Directive: %start

Specify the grammar’s start symbol (see The
Start-Symbol).

Directive: %expect

Declare the expected number of shift-reduce conflicts (see Suppressing Conflict Warnings).

In order to change the behavior of bison, use the following directives:

Directive: %code {code}

Directive: %code qualifier {code}

Insert code verbatim into the output parser source at the default location or at the location specified by qualifier. See %code Summary.

Directive: %debug

Instrument the parser for traces. Obsoleted by ‘%define
parse.trace’. See Tracing Your Parser.

Directive: %define variable

Directive: %define variable value

Directive: %define variable {value}

Directive: %define variable «value«

Define a variable to adjust Bison’s behavior. See %define Summary.

Directive: %defines

Write a parser header file containing macro definitions for the token type names defined in the grammar as well as a few other declarations. If the parser implementation file is named name.c then the parser header
file is named name.h.

For C parsers, the parser header file declares YYSTYPE unless
YYSTYPE is already defined as a macro or you have used a
<type> tag without using %union. Therefore, if you are using a %union (see More Than One
Value Type) with components that require other definitions, or if you have defined a YYSTYPE macro or type definition (see Data Types of Semantic Values), you need to arrange for these definitions to be propagated
to all modules, e.g., by putting them in a prerequisite header that is included both by your parser and by any other module that needs YYSTYPE.

Unless your parser is pure, the parser header file declares
yylval as an external variable. See A Pure
(Reentrant) Parser.

If you have also used locations, the parser header file declares
YYLTYPE and yylloc using a protocol similar to that of the
YYSTYPE macro and yylval. See Tracking Locations.

This parser header file is normally essential if you wish to put the definition of yylex in a separate source file, because
yylex typically needs to be able to refer to the above-mentioned declarations and to the token type codes. See Semantic Values of Tokens.

If you have declared %code requires or %code provides, the output header also contains their code. See %code Summary.

The generated header is protected against multiple inclusions with a C preprocessor guard: ‘YY_PREFIX_FILE_INCLUDED’, where
PREFIX and FILE are the prefix (see Multiple Parsers in the Same Program) and generated file name turned uppercase, with each series of non alphanumerical characters converted to
a single underscore.

For instance with ‘%define api.prefix {calc}’ and ‘%defines
«lib/parse.h»’, the header will be guarded as follows.

#ifndef YY_CALC_LIB_PARSE_H_INCLUDED
# define YY_CALC_LIB_PARSE_H_INCLUDED
...
#endif /* ! YY_CALC_LIB_PARSE_H_INCLUDED */

Directive: %defines defines-file

Same as above, but save in the file defines-file.

Directive: %destructor

Specify how the parser should reclaim the memory associated to discarded symbols. See Freeing Discarded Symbols.

Directive: %file-prefix «prefix«

Specify a prefix to use for all Bison output file names. The names are chosen as if the grammar file were named prefix.y.

Directive: %language «language«

Specify the programming language for the generated parser. Currently supported languages include C, C++, and Java.
language is case-insensitive.

Directive: %locations

Generate the code processing the locations (see Special Features for Use in Actions). This mode is enabled as soon as the grammar uses the special ‘@n’ tokens, but if your
grammar does not use it, using ‘%locations’ allows for more accurate syntax error messages.

Directive: %name-prefix «prefix«

Rename the external symbols used in the parser so that they start with
prefix instead of ‘yy’. The precise list of symbols renamed in C parsers is yyparse, yylex, yyerror, yynerrs,
yylval, yychar, yydebug, and (if locations are used) yylloc. If you use a push parser,
yypush_parse, yypull_parse, yypstate,
yypstate_new and yypstate_delete will also be renamed. For example, if you use ‘%name-prefix «c_»’, the names become c_parse, c_lex, and so on.
For C++ parsers, see the ‘%define api.namespace’ documentation in this section. See Multiple Parsers in the Same Program.

Directive: %no-lines

Don’t generate any #line preprocessor commands in the parser implementation file. Ordinarily Bison writes these commands in the parser implementation file so that the C compiler and debuggers will associate errors and
object code with your source file (the grammar file). This directive causes them to associate errors with the parser implementation file, treating it as an independent source file in its own right.

Directive: %output «file«

Generate the parser implementation in file.

Directive: %pure-parser

Deprecated version of ‘%define api.pure’ (see api.pure), for which Bison is more careful to warn about unreasonable usage.

Directive: %require «version«

Require version version or higher of Bison. See Require a Version of Bison.

Directive: %skeleton «file«

Specify the skeleton to use.

If file does not contain a /, file is the name of a skeleton file in the Bison installation directory. If it does, file is an absolute file name or a file name relative to the directory of
the grammar file. This is similar to how most shells resolve commands.

Directive: %token-table

Generate an array of token names in the parser implementation file. The name of the array is yytname; yytname[i] is the name of the token whose internal Bison token code number is
i. The first three elements of yytname correspond to the predefined tokens "$end", "error", and
"$undefined"; after these come the symbols defined in the grammar file.

The name in the table includes all the characters needed to represent the token in Bison. For single-character literals and literal strings, this includes the surrounding quoting characters and any escape sequences. For example, the Bison
single-character literal
'+' corresponds to a three-character name, represented in C as
"'+'"; and the Bison two-character literal string "\/" corresponds to a five-character name, represented in C as
""\\/"".

When you specify %token-table, Bison also generates macro definitions for macros YYNTOKENS, YYNNTS, and
YYNRULES, and YYNSTATES:

YYNTOKENS

The highest token number, plus one.

YYNNTS

The number of nonterminal symbols.

YYNRULES

The number of grammar rules,

YYNSTATES

The number of parser states (see Parser States).

Directive: %verbose

Write an extra output file containing verbose descriptions of the parser states and what is done for each type of lookahead token in that state. See Understanding Your Parser, for more information.

Directive: %yacc

Pretend the option —yacc was given, i.e., imitate Yacc, including its naming conventions. See Bison Options, for more.

3.7.13 %define Summary

There are many features of Bison’s behavior that can be controlled by assigning the feature a single value. For historical reasons, some such features are assigned values by dedicated directives, such as
%start, which assigns the start symbol. However, newer such features are associated with variables, which are assigned by the
%define directive:

Directive: %define variable

Directive: %define variable value

Directive: %define variable {value}

Directive: %define variable «value«

Define variable to value.

The type of the values depend on the syntax. Braces denote value in the target language (e.g., a namespace, a type, etc.). Keyword values (no delimiters) denote finite choice (e.g., a variation of a feature). String values denote remaining
cases (e.g., a file name).

It is an error if a variable is defined by %define multiple times, but see -D name[=value].

The rest of this section summarizes variables and values that
%define accepts.

Some variables take Boolean values. In this case, Bison will complain if the variable definition does not meet one of the following four conditions:

1. value is true
2. value is omitted (or "" is specified). This is equivalent to true.
3. value is false.
4. variable is never defined. In this case, Bison selects a default value.

What variables are accepted, as well as their meanings and default values, depend on the selected target language and/or the parser skeleton (see %language, see %skeleton). Unaccepted
variables produce an error. Some of the accepted variables are described below.

Directive: %define api.namespace {namespace}

• Languages(s): C++
• Purpose: Specify the namespace for the parser class. For example, if you specify:

  %define api.namespace {foo::bar}
  

  Bison uses foo::bar verbatim in references such as:

  foo::bar::parser::semantic_type
  

  However, to open a namespace, Bison removes any leading :: and then splits on any remaining occurrences:

  namespace foo { namespace bar {
    class position;
    class location;
  } }
  

• Accepted Values: Any absolute or relative C++ namespace reference without a trailing
  "::". For example, "foo" or "::foo::bar".

• Default Value: The value specified by %name-prefix, which defaults to yy. This usage of %name-prefix is for backward compatibility and can be confusing since %name-prefix also specifies
  the textual prefix for the lexical analyzer function. Thus, if you specify
  %name-prefix, it is best to also specify ‘%define
  api.namespace’ so that %name-prefix only affects the lexical analyzer function. For example, if you specify:

  %define api.namespace {foo}
  %name-prefix "bar::"
  

  The parser namespace is foo and yylex is referenced as
  bar::lex.

Directive: %define api.location.type {type}

• Language(s): C++, Java
• Purpose: Define the location type. See User Defined Location Type.
• Accepted Values: String
• Default Value: none
• History: Introduced in Bison 2.7 for C, C++ and Java. Introduced under the name
  location_type for C++ in Bison 2.5 and for Java in Bison 2.4.

Directive: %define api.prefix {prefix}

• Language(s): All
• Purpose: Rename exported symbols. See Multiple Parsers in the Same Program.
• Accepted Values: String
• Default Value: yy
• History: introduced in Bison 2.6

Directive: %define api.pure purity

• Language(s): C
• Purpose: Request a pure (reentrant) parser program. See A Pure (Reentrant) Parser.
• Accepted Values: true, false, full

  The value may be omitted: this is equivalent to specifying true, as is the case for Boolean values.

  When %define api.pure full is used, the parser is made reentrant. This changes the signature for yylex (see Pure Calling), and also that of
  yyerror when the tracking of locations has been activated, as shown below.

  The true value is very similar to the full value, the only difference is in the signature of yyerror on Yacc parsers without
  %parse-param, for historical reasons.

  I.e., if ‘%locations %define api.pure’ is passed then the prototypes for
  yyerror are:

  void yyerror (char const *msg);                 // Yacc parsers.
  void yyerror (YYLTYPE *locp, char const *msg);  // GLR parsers.
  

  But if ‘%locations %define api.pure %parse-param {int *nastiness}’ is used, then both parsers have the same signature:

  void yyerror (YYLTYPE *llocp, int *nastiness, char const *msg);
  

  (see The Error
  Reporting Function yyerror)

• Default Value: false
• History: the full value was introduced in Bison 2.7

Directive: %define api.push-pull kind

• Language(s): C (deterministic parsers only)
• Purpose: Request a pull parser, a push parser, or both. See A Push Parser. (The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)
• Accepted Values: pull, push, both
• Default Value: pull

Directive: %define api.token.constructor

• Language(s): C++
• Purpose: When variant-based semantic values are enabled (see C++ Variants), request that symbols be handled as a whole (type, value, and possibly location) in the scanner. See Complete Symbols,
  for details.

• Accepted Values: Boolean.
• Default Value:
  false
• History: introduced in Bison 3.0

Directive: %define api.token.prefix {prefix}

• Languages(s): all
• Purpose: Add a prefix to the token names when generating their definition in the target language. For instance

  %token FILE for ERROR
  %define api.token.prefix {TOK_}
  %%
  start: FILE for ERROR;
  

  generates the definition of the symbols TOK_FILE, TOK_for, and TOK_ERROR in the generated source files. In particular, the scanner must use these prefixed token names, while the grammar itself
  may still use the short names (as in the sample rule given above). The generated informational files (*.output, *.xml,
  *.dot) are not modified by this prefix.

  Bison also prefixes the generated member names of the semantic value union. See Generating the Semantic Value Type, for more details.

  See Calc++ Parser and Calc++ Scanner, for a complete example.

• Accepted Values: Any string. Should be a valid identifier prefix in the target language, in other words, it should typically be an identifier itself (sequence of letters, underscores, and —not at the beginning— digits).
• Default Value: empty
• History: introduced in Bison 3.0

Directive: %define api.value.type support

Directive: %define api.value.type {type}

• Language(s): all
• Purpose: The type for semantic values.
• Accepted Values:

  ‘{}’

  This grammar has no semantic value at all. This is not properly supported yet.

  ‘union-directive’ (C, C++)

  The type is defined thanks to the %union directive. You don’t have to define api.value.type in that case, using %union suffices. See The Union Declaration.
  For instance:

  %define api.value.type union-directive
  %union
  {
    int ival;
    char *sval;
  }
  %token <ival> INT "integer"
  %token <sval> STR "string"
  

  ‘union’ (C, C++)

  The symbols are defined with type names, from which Bison will generate a
  union. For instance:

  %define api.value.type union
  %token <int> INT "integer"
  %token <char *> STR "string"
  

  This feature needs user feedback to stabilize. Note that most C++ objects cannot be stored in a union.

  ‘variant’ (C++)

  This is similar to union, but special storage techniques are used to allow any kind of C++ object to be used. For instance:

  %define api.value.type variant
  %token <int> INT "integer"
  %token <std::string> STR "string"
  

  This feature needs user feedback to stabilize. See C++ Variants.

  ‘{type}’

  Use this type as semantic value.

  %code requires
  {
    struct my_value
    {
      enum
      {
        is_int, is_str
      } kind;
      union
      {
        int ival;
        char *sval;
      } u;
    };
  }
  %define api.value.type {struct my_value}
  %token <u.ival> INT "integer"
  %token <u.sval> STR "string"
  

• Default Value:
  • — union-directive if %union is used, otherwise …
  • — int if type tags are used (i.e., ‘%token <type>…’ or ‘
    %type <type>…’ is used), otherwise …
  • — undefined.
• History: introduced in Bison 3.0. Was introduced for Java only in 2.3b as
  stype.

Directive: %define api.value.union.name name

• Language(s): C
• Purpose: The tag of the generated union (not the name of the
  typedef). This variable is set to id when ‘%union
  id’ is used. There is no clear reason to give this union a name.

• Accepted Values: Any valid identifier.
• Default Value:
  YYSTYPE.

• History: Introduced in Bison 3.0.3.

Directive: %define location_type

Obsoleted by api.location.type since Bison 2.7.

Directive: %define lr.default-reduction when

• Language(s): all
• Purpose: Specify the kind of states that are permitted to contain default reductions. See Default Reductions. (The ability to specify where default reductions should be used is experimental. More user
  feedback will help to stabilize it.)

• Accepted Values: most, consistent, accepting
• Default Value:
  • accepting if lr.type is canonical-lr.
  • most otherwise.
• History: introduced as lr.default-reductions in 2.5, renamed as
  lr.default-reduction in 3.0.

Directive: %define lr.keep-unreachable-state

• Language(s): all
• Purpose: Request that Bison allow unreachable parser states to remain in the parser tables. See Unreachable States.
• Accepted Values: Boolean
• Default Value: false
• History: introduced as lr.keep_unreachable_states in 2.3b, renamed as
  lr.keep-unreachable-states in 2.5, and as
  lr.keep-unreachable-state in 3.0.

Directive: %define lr.type type

• Language(s): all
• Purpose: Specify the type of parser tables within the LR(1) family. See LR Table Construction. (This feature is experimental. More user feedback will help to stabilize it.)
• Accepted Values: lalr, ielr, canonical-lr
• Default Value: lalr

Directive: %define namespace {namespace}

Obsoleted by api.namespace

Directive: %define parse.assert

• Languages(s): C++
• Purpose: Issue runtime assertions to catch invalid uses. In C++, when variants are used (see C++ Variants), symbols must be constructed and destroyed properly. This option checks these constraints.
• Accepted Values: Boolean
• Default Value: false

Directive: %define parse.error verbosity

• Languages(s): all
• Purpose: Control the kind of error messages passed to the error reporting function. See The Error Reporting Function
  yyerror.
• Accepted Values:
  • simple Error messages passed to yyerror are simply "syntax error"
    .
  • verbose Error messages report the unexpected token, and possibly the expected ones. However, this report can often be incorrect when LAC is not enabled (see LAC).
• Default Value:
  simple

Directive: %define parse.lac when

• Languages(s): C (deterministic parsers only)
• Purpose: Enable LAC (lookahead correction) to improve syntax error handling. See LAC.
• Accepted Values: none, full
• Default Value: none

Directive: %define parse.trace

• Languages(s): C, C++, Java
• Purpose: Require parser instrumentation for tracing. See Tracing Your Parser.

  In C/C++, define the macro YYDEBUG (or prefixDEBUG with ‘
  %define api.prefix {prefix}’), see Multiple Parsers in the Same Program) to 1 in the parser implementation file if it is not already defined, so that the debugging
  facilities are compiled.

• Accepted Values: Boolean
• Default Value: false

3.7.14 %code Summary

The %code directive inserts code verbatim into the output parser source at any of a predefined set of locations. It thus serves as a flexible and user-friendly alternative to the traditional Yacc prologue, %{code%}.
This section summarizes the functionality of %code for the various target languages supported by Bison. For a detailed discussion of how to use
%code in place of %{code%} for C/C++ and why it is advantageous to do so, see Prologue Alternatives.

Directive: %code {code}

This is the unqualified form of the %code directive. It inserts code verbatim at a language-dependent default location in the parser implementation.

For C/C++, the default location is the parser implementation file after the usual contents of the parser header file. Thus, the unqualified form replaces %{code%} for most purposes.

For Java, the default location is inside the parser class.

Directive: %code qualifier {code}

This is the qualified form of the %code directive.
qualifier identifies the purpose of code and thus the location(s) where Bison should insert it. That is, if you need to specify location-sensitive code that does not belong at the default location selected
by the unqualified %code form, use this form instead.

For any particular qualifier or for the unqualified form, if there are multiple occurrences of the %code directive, Bison concatenates the specified code in the order in which it appears in the grammar file.

Not all qualifiers are accepted for all target languages. Unaccepted qualifiers produce an error. Some of the accepted qualifiers are:

requires

• Language(s): C, C++
• Purpose: This is the best place to write dependency code required for
  YYSTYPE and YYLTYPE. In other words, it’s the best place to define types referenced in %union directives. If you use
  #define to override Bison’s default YYSTYPE and YYLTYPE definitions, then it is also the best place. However you should rather
  %define api.value.type and api.location.type.

• Location(s): The parser header file and the parser implementation file before the Bison-generated YYSTYPE and YYLTYPE definitions.

provides

• Language(s): C, C++
• Purpose: This is the best place to write additional definitions and declarations that should be provided to other modules.
• Location(s): The parser header file and the parser implementation file after the Bison-generated YYSTYPE, YYLTYPE, and token definitions.

top

• Language(s): C, C++
• Purpose: The unqualified %code or %code requires should usually be more appropriate than %code top. However, occasionally it is necessary to insert code much nearer the top of the parser implementation
  file. For example:

  %code top {
    #define _GNU_SOURCE
    #include <stdio.h>
  }
  

• Location(s): Near the top of the parser implementation file.

imports

• Language(s): Java
• Purpose: This is the best place to write Java import directives.
• Location(s): The parser Java file after any Java package directive and before any class definitions.

Though we say the insertion locations are language-dependent, they are technically skeleton-dependent. Writers of non-standard skeletons however should choose their locations consistently with the behavior of the standard Bison skeletons.

3.8 Multiple Parsers in the Same Program

Most programs that use Bison parse only one language and therefore contain only one Bison parser. But what if you want to parse more than one language with the same program? Then you need to avoid name conflicts between different definitions of
functions and variables such as yyparse,
yylval. To use different parsers from the same compilation unit, you also need to avoid conflicts on types and macros (e.g., YYSTYPE) exported in the generated header.

The easy way to do this is to define the %define variable
api.prefix. With different api.prefixs it is guaranteed that headers do not conflict when included together, and that compiled objects can be linked together too. Specifying ‘%define api.prefix
{prefix}’ (or passing the option ‘-Dapi.prefix={prefix}’, see
Invoking Bison) renames the interface functions and variables of the Bison parser to start with prefix instead of ‘
yy’, and all the macros to start by PREFIX (i.e., prefix upper-cased) instead of ‘YY’.

The renamed symbols include yyparse, yylex, yyerror,
yynerrs, yylval, yylloc, yychar and
yydebug. If you use a push parser, yypush_parse,
yypull_parse, yypstate, yypstate_new and
yypstate_delete will also be renamed. The renamed macros include
YYSTYPE, YYLTYPE, and YYDEBUG, which is treated specifically — more about this below.

For example, if you use ‘%define api.prefix {c}’, the names become
cparse, clex, …, CSTYPE, CLTYPE, and so on.

The %define variable api.prefix works in two different ways. In the implementation file, it works by adding macro definitions to the beginning of the parser implementation file, defining yyparse as
prefixparse, and so on:

#define YYSTYPE CTYPE
#define yyparse cparse
#define yylval  clval
...
YYSTYPE yylval;
int yyparse (void);

This effectively substitutes one name for the other in the entire parser implementation file, thus the “original” names (yylex,
YYSTYPE, …) are also usable in the parser implementation file.

However, in the parser header file, the symbols are defined renamed, for instance:

extern CSTYPE clval;
int cparse (void);

The macro YYDEBUG is commonly used to enable the tracing support in parsers. To comply with this tradition, when api.prefix is used,
YYDEBUG (not renamed) is used as a default value:

/* Debug traces.  */
#ifndef CDEBUG
# if defined YYDEBUG
#  if YYDEBUG
#   define CDEBUG 1
#  else
#   define CDEBUG 0
#  endif
# else
#  define CDEBUG 0
# endif
#endif
#if CDEBUG
extern int cdebug;
#endif

Prior to Bison 2.6, a feature similar to api.prefix was provided by the obsolete directive %name-prefix (see Bison
Symbols) and the option --name-prefix (see Bison Options).

4 Parser C-Language Interface

The Bison parser is actually a C function named yyparse. Here we describe the interface conventions of yyparse and the other functions that it needs to use.

Keep in mind that the parser uses many C identifiers starting with ‘
yy’ and ‘YY’ for internal purposes. If you use such an identifier (aside from those in this manual) in an action or in epilogue in the grammar file, you are likely to run into trouble.

4.1 The Parser Function yyparse

You call the function yyparse to cause parsing to occur. This function reads tokens, executes actions, and ultimately returns when it encounters end-of-input or an unrecoverable syntax error. You can also write an action which directs
yyparse to return immediately without reading further.

Function: int yyparse (void)

The value returned by yyparse is 0 if parsing was successful (return is due to end-of-input).

The value is 1 if parsing failed because of invalid input, i.e., input that contains a syntax error or that causes YYABORT to be invoked.

The value is 2 if parsing failed due to memory exhaustion.

In an action, you can cause immediate return from yyparse by using these macros:

Macro: YYACCEPT

Return immediately with value 0 (to report success).

Macro: YYABORT

Return immediately with value 1 (to report failure).

If you use a reentrant parser, you can optionally pass additional parameter information to it in a reentrant way. To do so, use the declaration %parse-param:

Directive: %parse-param {argument-declaration} …

Declare that one or more
argument-declaration are additional yyparse arguments. The argument-declaration is used when declaring functions or prototypes. The last identifier in
argument-declaration must be the argument name.

Here’s an example. Write this in the parser:

%parse-param {int *nastiness} {int *randomness}

Then call the parser like this:

{
  int nastiness, randomness;
  …  /* Store proper data in nastiness and randomness.  */
  value = yyparse (&nastiness, &randomness);
  …
}

In the grammar actions, use expressions like this to refer to the data:

exp: …    { …; *randomness += 1; … }

Using the following:

%parse-param {int *randomness}

Results in these signatures:

void yyerror (int *randomness, const char *msg);
int  yyparse (int *randomness);

Or, if both %define api.pure full (or just %define api.pure) and %locations are used:

void yyerror (YYLTYPE *llocp, int *randomness, const char *msg);
int  yyparse (int *randomness);

4.2 The Push Parser Function yypush_parse

(The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)

You call the function yypush_parse to parse a single token. This function is available if either the ‘%define api.push-pull push’ or ‘
%define api.push-pull both’ declaration is used. See A Push Parser.

Function: int yypush_parse (yypstate *yyps)

The value returned by yypush_parse is the same as for yyparse with the following exception: it returns YYPUSH_MORE if more input is required to finish parsing the grammar.

4.3 The Pull Parser Function yypull_parse

(The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)

You call the function yypull_parse to parse the rest of the input stream. This function is available if the ‘%define api.push-pull both’ declaration is used. See A Push Parser.

Function: int yypull_parse (yypstate *yyps)

The value returned by yypull_parse is the same as for yyparse.

4.4 The Parser Create Function yystate_new

(The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)

You call the function yypstate_new to create a new parser instance. This function is available if either the ‘%define api.push-pull push’ or ‘
%define api.push-pull both’ declaration is used. See A Push Parser.

Function: yypstate* yypstate_new (void)

The function will return a valid parser instance if there was memory available or 0 if no memory was available. In impure mode, it will also return 0 if a parser instance is currently allocated.

4.5 The Parser Delete Function yystate_delete

(The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)

You call the function yypstate_delete to delete a parser instance. function is available if either the ‘%define api.push-pull push’ or ‘
%define api.push-pull both’ declaration is used. See A Push Parser.

Function: void yypstate_delete (yypstate *yyps)

This function will reclaim the memory associated with a parser instance. After this call, you should no longer attempt to use the parser instance.

4.6 The Lexical Analyzer Function yylex

The lexical analyzer function, yylex, recognizes tokens from the input stream and returns them to the parser. Bison does not create this function automatically; you must write it so that yyparse can call it.
The function is sometimes referred to as a lexical scanner.

In simple programs, yylex is often defined at the end of the Bison grammar file. If yylex is defined in a separate source file, you need to arrange for the token-type macro definitions to be available there. To do this,
use the ‘-d’ option when you run Bison, so that it will write these macro definitions into the separate parser header file, name.tab.h, which you can include in the other source files that need
it. See Invoking
Bison.

4.6.1 Calling Convention for yylex

The value that yylex returns must be the positive numeric code for the type of token it has just found; a zero or negative value signifies end-of-input.

When a token is referred to in the grammar rules by a name, that name in the parser implementation file becomes a C macro whose definition is the proper numeric code for that token type. So yylex can use the name to indicate that
type. See Symbols.

When a token is referred to in the grammar rules by a character literal, the numeric code for that character is also the code for the token type. So yylex can simply return that character code, possibly converted to unsigned char to avoid sign-extension. The null character must not be used this way, because its code is zero and that signifies end-of-input.

Here is an example showing these things:

int
yylex (void)
{
  …
  if (c == EOF)    /* Detect end-of-input.  */
    return 0;
  …
  if (c == '+' || c == '-')
    return c;      /* Assume token type for '+' is '+'.  */
  …
  return INT;      /* Return the type of the token.  */
  …
}

This interface has been designed so that the output from the lex utility can be used without change as the definition of yylex.

If the grammar uses literal string tokens, there are two ways that
yylex can determine the token type codes for them:

• If the grammar defines symbolic token names as aliases for the literal string tokens, yylex can use these symbolic names like all others. In this case, the use of the literal string tokens in the grammar file has no effect on
  yylex.

• yylex can find the multicharacter token in the yytname table. The index of the token in the table is the token type’s code. The name of a multicharacter token is recorded in yytname with a double-quote,
  the token’s characters, and another double-quote. The token’s characters are escaped as necessary to be suitable as input to Bison.

  Here’s code for looking up a multicharacter token in yytname, assuming that the characters of the token are stored in
  token_buffer, and assuming that the token does not contain any characters like ‘«’ that require escaping.

  for (i = 0; i < YYNTOKENS; i++)
    {
      if (yytname[i] != 0
          && yytname[i][0] == '"'
          && ! strncmp (yytname[i] + 1, token_buffer,
                        strlen (token_buffer))
          && yytname[i][strlen (token_buffer) + 1] == '"'
          && yytname[i][strlen (token_buffer) + 2] == 0)
        break;
    }
  

  The yytname table is generated only if you use the
  %token-table declaration. See Decl Summary.

4.6.2 Semantic Values of Tokens

In an ordinary (nonreentrant) parser, the semantic value of the token must be stored into the global variable yylval. When you are using just one data type for semantic values, yylval has that type. Thus, if the type
is int (the default), you might write this in
yylex:

  …
  yylval = value;  /* Put value onto Bison stack.  */
  return INT;      /* Return the type of the token.  */
  …

When you are using multiple data types, yylval’s type is a union made from the %union declaration (see The
Union Declaration). So when you store a token’s value, you must use the proper member of the union. If the %union declaration looks like this:

%union {
  int intval;
  double val;
  symrec *tptr;
}

then the code in yylex might look like this:

  …
  yylval.intval = value; /* Put value onto Bison stack.  */
  return INT;            /* Return the type of the token.  */
  …

4.6.3 Textual Locations of Tokens

If you are using the ‘@n’-feature (see Tracking Locations) in actions to keep track of the textual locations of tokens and groupings, then you must provide this information
in yylex. The function
yyparse expects to find the textual location of a token just parsed in the global variable yylloc. So yylex must store the proper data in that variable.

By default, the value of yylloc is a structure and you need only initialize the members that are going to be used by the actions. The four members are called first_line, first_column,
last_line and last_column. Note that the use of this feature makes the parser noticeably slower.

The data type of yylloc has the name YYLTYPE.

4.6.4 Calling Conventions for Pure Parsers

When you use the Bison declaration %define api.pure full to request a pure, reentrant parser, the global communication variables yylval and yylloc cannot be used. (See A Pure (Reentrant)
Parser.) In such parsers the two global variables are replaced by pointers passed as arguments to yylex. You must declare them as shown here, and pass the information back by storing it through those pointers.

int
yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
{
  …
  *lvalp = value;  /* Put value onto Bison stack.  */
  return INT;      /* Return the type of the token.  */
  …
}

If the grammar file does not use the ‘@’ constructs to refer to textual locations, then the type YYLTYPE will not be defined. In this case, omit the second argument; yylex will be called with
only one argument.

If you wish to pass additional arguments to yylex, use
%lex-param just like %parse-param (see Parser Function). To pass additional arguments to both yylex and
yyparse, use %param.

Directive: %lex-param {argument-declaration} …

Specify that argument-declaration are additional yylex argument declarations. You may pass one or more such declarations, which is equivalent to repeating %lex-param.

Directive: %param {argument-declaration} …

Specify that argument-declaration are additional
yylex/yyparse argument declaration. This is equivalent to ‘
%lex-param {argument-declaration} … %parse-param
{argument-declaration} …’. You may pass one or more declarations, which is equivalent to repeating %param.

For instance:

%lex-param   {scanner_mode *mode}
%parse-param {parser_mode *mode}
%param       {environment_type *env}

results in the following signatures:

int yylex   (scanner_mode *mode, environment_type *env);
int yyparse (parser_mode *mode, environment_type *env);

If ‘%define api.pure full’ is added:

int yylex   (YYSTYPE *lvalp, scanner_mode *mode, environment_type *env);
int yyparse (parser_mode *mode, environment_type *env);

and finally, if both ‘%define api.pure full’ and %locations are used:

int yylex   (YYSTYPE *lvalp, YYLTYPE *llocp,
             scanner_mode *mode, environment_type *env);
int yyparse (parser_mode *mode, environment_type *env);

4.7 The Error Reporting Function yyerror

The Bison parser detects a syntax error (or parse error) whenever it reads a token which cannot satisfy any syntax rule. An action in the grammar can also explicitly proclaim an error, using the macro YYERROR (see
Special Features for Use in Actions).

The Bison parser expects to report the error by calling an error reporting function named yyerror, which you must supply. It is called by yyparse whenever a syntax error is found, and it receives one argument. For a syntax
error, the string is normally
"syntax error"
.

If you invoke ‘%define parse.error verbose’ in the Bison declarations section (see The Bison Declarations Section), then Bison provides a more verbose and specific error message string
instead of just plain "syntax error"
. However, that message sometimes contains incorrect information if LAC is not enabled (see LAC).

The parser can detect one other kind of error: memory exhaustion. This can happen when the input contains constructions that are very deeply nested. It isn’t likely you will encounter this, since the Bison parser normally extends its stack
automatically up to a very large limit. But if memory is exhausted, yyparse calls yyerror in the usual fashion, except that the argument string is "memory exhausted"
.

In some cases diagnostics like "syntax error"
are translated automatically from English to some other language before they are passed to yyerror. See Internationalization.

The following definition suffices in simple programs:

void
yyerror (char const *s)
{

  fprintf (stderr, "%sn", s);
}

After yyerror returns to yyparse, the latter will attempt error recovery if you have written suitable error recovery grammar rules (see Error Recovery). If recovery is impossible, yyparse will immediately return 1.

Obviously, in location tracking pure parsers, yyerror should have an access to the current location. With %define api.pure, this is indeed the case for the GLR parsers, but not for the Yacc parser, for historical reasons,
and this is the why %define api.pure full should be prefered over %define api.pure.

When %locations %define api.pure full is used, yyerror has the following signature:

void yyerror (YYLTYPE *locp, char const *msg);

The prototypes are only indications of how the code produced by Bison uses yyerror. Bison-generated code always ignores the returned value, so yyerror can return any type, including void. Also, yyerror can be a variadic function; that is why the message is always passed last.

Traditionally yyerror returns an int that is always ignored, but this is purely for historical reasons, and void is preferable since it more accurately describes the return type for
yyerror.

The variable yynerrs contains the number of syntax errors reported so far. Normally this variable is global; but if you request a pure parser (see A Pure (Reentrant) Parser) then it is a local variable which
only the actions can access.

4.8 Special Features for Use in Actions

Here is a table of Bison constructs, variables and macros that are useful in actions.

Variable: $$

Acts like a variable that contains the semantic value for the grouping made by the current rule. See Actions.

Variable: $ n

Acts like a variable that contains the semantic value for the
nth component of the current rule. See Actions.

Variable: $< typealt>$

Like $$ but specifies alternative typealt in the union specified by the %union declaration. See Data
Types of Values in Actions.

Variable: $< typealt>n

Like $n but specifies alternative typealt in the union specified by the %union declaration. See Data Types of Values in Actions.

Macro: YYABORT ;

Return immediately from yyparse, indicating failure. See The Parser Function yyparse.

Macro: YYACCEPT ;

Return immediately from yyparse, indicating success. See The Parser Function yyparse.

Macro: YYBACKUP (token, value);

Unshift a token. This macro is allowed only for rules that reduce a single value, and only when there is no lookahead token. It is also disallowed in GLR parsers. It installs a lookahead token with token type token and semantic
value value; then it discards the value that was going to be reduced by this rule.

If the macro is used when it is not valid, such as when there is a lookahead token already, then it reports a syntax error with a message ‘cannot back up’ and performs ordinary error recovery.

In either case, the rest of the action is not executed.

Macro: YYEMPTY

Value stored in yychar when there is no lookahead token.

Macro: YYEOF

Value stored in yychar when the lookahead is the end of the input stream.

Macro: YYERROR ;

Cause an immediate syntax error. This statement initiates error recovery just as if the parser itself had detected an error; however, it does not call yyerror, and does not print any message. If you want to print an error
message, call yyerror explicitly before the ‘YYERROR;’ statement. See Error Recovery.

Macro: YYRECOVERING

The expression YYRECOVERING () yields 1 when the parser is recovering from a syntax error, and 0 otherwise. See Error Recovery.

Variable: yychar

Variable containing either the lookahead token, or YYEOF when the lookahead is the end of the input stream, or YYEMPTY when no lookahead has been performed so the next token is not yet known. Do not modify yychar in a deferred semantic action (see GLR Semantic Actions). See Lookahead Tokens.

Macro: yyclearin ;

Discard the current lookahead token. This is useful primarily in error rules. Do not invoke yyclearin in a deferred semantic action (see GLR Semantic Actions). See Error Recovery.

Macro: yyerrok ;

Resume generating error messages immediately for subsequent syntax errors. This is useful primarily in error rules. See Error Recovery.

Variable: yylloc

Variable containing the lookahead token location when yychar is not set to YYEMPTY or YYEOF. Do not modify yylloc in a deferred semantic action (see GLR Semantic Actions).
See Actions and Locations.

Variable: yylval

Variable containing the lookahead token semantic value when yychar is not set to YYEMPTY or YYEOF. Do not modify yylval in a deferred semantic action (see GLR Semantic Actions).
See Actions.

Value: @ $

Acts like a structure variable containing information on the textual location of the grouping made by the current rule. See Tracking Locations.

Value: @ n

Acts like a structure variable containing information on the textual location of the nth component of the current rule. See Tracking Locations.

4.9 Parser Internationalization

A Bison-generated parser can print diagnostics, including error and tracing messages. By default, they appear in English. However, Bison also supports outputting diagnostics in the user’s native language. To make this work, the user should
set the usual environment variables. See The User’s View in GNU gettext utilities. For example, the shell command ‘export LC_ALL=fr_CA.UTF-8’
might set the user’s locale to French Canadian using the UTF-8 encoding. The exact set of available locales depends on the user’s installation.

The maintainer of a package that uses a Bison-generated parser enables the internationalization of the parser’s output through the following steps. Here we assume a package that uses GNU Autoconf and GNU Automake.

1. Into the directory containing the GNU Autoconf macros used by the package —often called m4— copy the
   bison-i18n.m4 file installed by Bison under ‘
   share/aclocal/bison-i18n.m4’ in Bison’s installation directory. For example:

   cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4
   

2. In the top-level configure.ac, after the AM_GNU_GETTEXT invocation, add an invocation of BISON_I18N. This macro is defined in the file bison-i18n.m4 that you copied earlier. It causes
   ‘
   configure’ to find the value of the
   BISON_LOCALEDIR variable, and it defines the source-language symbol YYENABLE_NLS to enable translations in the Bison-generated parser.

3. In the main function of your program, designate the directory containing Bison’s runtime message catalog, through a call to ‘
   bindtextdomain’ with domain name ‘bison-runtime’. For example:

   bindtextdomain ("bison-runtime", BISON_LOCALEDIR);
   

   Typically this appears after any other call bindtextdomain
(PACKAGE, LOCALEDIR) that your package already has. Here we rely on ‘
   BISON_LOCALEDIR’ to be defined as a string through the
   Makefile.

4. In the Makefile.am that controls the compilation of the main function, make ‘BISON_LOCALEDIR’ available as a C preprocessor macro, either in ‘DEFS’ or in ‘AM_CPPFLAGS’.
   For example:

   DEFS = @DEFS@ -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
   

   or:

   AM_CPPFLAGS = -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
   

5. Finally, invoke the command autoreconf to generate the build infrastructure.

5 The Bison Parser Algorithm

As Bison reads tokens, it pushes them onto a stack along with their semantic values. The stack is called the parser stack. Pushing a token is traditionally called shifting.

For example, suppose the infix calculator has read ‘1 + 5 *’, with a ‘
3’ to come. The stack will have four elements, one for each token that was shifted.

But the stack does not always have an element for each token read. When the last n tokens and groupings shifted match the components of a grammar rule, they can be combined according to that rule. This is called
reduction. Those tokens and groupings are replaced on the stack by a single grouping whose symbol is the result (left hand side) of that rule. Running the rule’s action is part of the process of reduction, because this is what
computes the semantic value of the resulting grouping.

For example, if the infix calculator’s parser stack contains this:

and the next input token is a newline character, then the last three elements can be reduced to 15 via the rule:

Then the stack contains just these three elements:

At this point, another reduction can be made, resulting in the single value 16. Then the newline token can be shifted.

The parser tries, by shifts and reductions, to reduce the entire input down to a single grouping whose symbol is the grammar’s start-symbol (see Languages and Context-Free Grammars).

This kind of parser is known in the literature as a bottom-up parser.

5.1 Lookahead Tokens

The Bison parser does not always reduce immediately as soon as the last n tokens and groupings match a rule. This is because such a simple strategy is inadequate to handle most languages. Instead, when a reduction is possible,
the parser sometimes “looks ahead” at the next token in order to decide what to do.

When a token is read, it is not immediately shifted; first it becomes the
lookahead token, which is not on the stack. Now the parser can perform one or more reductions of tokens and groupings on the stack, while the lookahead token remains off to the side. When no more reductions should take place, the
lookahead token is shifted onto the stack. This does not mean that all possible reductions have been done; depending on the token type of the lookahead token, some rules may choose to delay their application.

Here is a simple case where lookahead is needed. These three rules define expressions which contain binary addition operators and postfix unary factorial operators (‘!’), and allow parentheses for grouping.

expr:
  term '+' expr
| term
;

term:
  '(' expr ')'
| term '!'
| "number"
;

Suppose that the tokens ‘1 + 2’
have been read and shifted; what should be done? If the following token is ‘)’, then the first three tokens must be reduced to form an expr. This is the only valid course, because shifting the ‘)’
would produce a sequence of symbols
term ')'
, and no rule allows this.

If the following token is ‘!’, then it must be shifted immediately so that ‘2 !’
can be reduced to make a term. If instead the parser were to reduce before shifting, ‘1 + 2’
would become an
expr. It would then be impossible to shift the ‘!’ because doing so would produce on the stack the sequence of symbols expr
'!'. No rule allows that sequence.

The lookahead token is stored in the variable yychar. Its semantic value and location, if any, are stored in the variables
yylval and yylloc. See Special Features for Use in Actions.

5.2 Shift/Reduce Conflicts

Suppose we are parsing a language which has if-then and if-then-else statements, with a pair of rules like this:

if_stmt:
  "if" expr "then" stmt
| "if" expr "then" stmt "else" stmt
;

Here "if", "then" and "else" are terminal symbols for specific keyword tokens.

When the "else" token is read and becomes the lookahead token, the contents of the stack (assuming the input is valid) are just right for reduction by the first rule. But it is also legitimate to shift the
"else", because that would lead to eventual reduction by the second rule.

This situation, where either a shift or a reduction would be valid, is called a shift/reduce conflict. Bison is designed to resolve these conflicts by choosing to shift, unless otherwise directed by operator precedence declarations. To
see the reason for this, let’s contrast it with the other alternative.

Since the parser prefers to shift the "else", the result is to attach the else-clause to the innermost if-statement, making these two inputs equivalent:

if x then if y then win; else lose;

if x then do; if y then win; else lose; end;

But if the parser chose to reduce when possible rather than shift, the result would be to attach the else-clause to the outermost if-statement, making these two inputs equivalent:

if x then if y then win; else lose;

if x then do; if y then win; end; else lose;

The conflict exists because the grammar as written is ambiguous: either parsing of the simple nested if-statement is legitimate. The established convention is that these ambiguities are resolved by attaching the else-clause to the innermost if-statement;
this is what Bison accomplishes by choosing to shift rather than reduce. (It would ideally be cleaner to write an unambiguous grammar, but that is very hard to do in this case.) This particular ambiguity was first encountered in the specifications
of Algol 60 and is called the “dangling else” ambiguity.

To avoid warnings from Bison about predictable, legitimate shift/reduce conflicts, you can use the %expect n declaration. There will be no warning as long as the number of shift/reduce conflicts is exactly n,
and Bison will report an error if there is a different number. See Suppressing Conflict Warnings. However, we don’t recommend the use of %expect (except ‘%expect 0’!),
as an equal number of conflicts does not mean that they are the same. When possible, you should rather use precedence directives to fix the conflicts explicitly (see Using Precedence For Non
Operators).

The definition of if_stmt above is solely to blame for the conflict, but the conflict does not actually appear without additional rules. Here is a complete Bison grammar file that actually manifests the conflict:

%%

stmt:
  expr
| if_stmt
;

if_stmt:
  "if" expr "then" stmt
| "if" expr "then" stmt "else" stmt
;

expr:
  "identifier"
;

5.3 Operator Precedence

Another situation where shift/reduce conflicts appear is in arithmetic expressions. Here shifting is not always the preferred resolution; the Bison declarations for operator precedence allow you to specify when to shift and when to reduce.

5.3.1 When Precedence is Needed

Consider the following ambiguous grammar fragment (ambiguous because the input ‘1 — 2 * 3’
can be parsed in two different ways):

expr:
  expr '-' expr
| expr '*' expr
| expr '<' expr
| '(' expr ')'
…
;

Suppose the parser has seen the tokens ‘1’, ‘—’ and ‘2’; should it reduce them via the rule for the subtraction operator? It depends on the next token. Of course, if the
next token is ‘)’, we must reduce; shifting is invalid because no single rule can reduce the token sequence ‘— 2 )’
or anything starting with that. But if the next token is ‘*’ or ‘<’, we have a choice: either shifting or reduction would allow the parse to complete, but with different results.

To decide which one Bison should do, we must consider the results. If the next operator token op is shifted, then it must be reduced first in order to permit another opportunity to reduce the difference. The result is (in effect) ‘1 — (2 op 3)’
. On the other hand, if the subtraction is reduced before shifting op, the result is ‘(1 — 2) op 3’
. Clearly, then, the choice of shift or reduce should depend on the relative precedence of the operators ‘
—’ and op: ‘*’ should be shifted first, but not ‘
<’.

What about input such as ‘1 — 2 — 5’
; should this be ‘
(1 — 2) — 5’
or should it be ‘1 — (2 — 5)’
? For most operators we prefer the former, which is called left association. The latter alternative, right association, is desirable for assignment operators. The choice of left or right association is a matter of whether
the parser chooses to shift or reduce when the stack contains ‘1 — 2’
and the lookahead token is ‘—’: shifting makes right-associativity.

5.3.2 Specifying Operator Precedence

Bison allows you to specify these choices with the operator precedence declarations %left and %right. Each such declaration contains a list of tokens, which are operators whose precedence and associativity is being declared.
The %left declaration makes all those operators left-associative and the %right declaration makes them right-associative. A third alternative is %nonassoc, which declares that it is a syntax error to
find the same operator twice “in a row”. The last alternative, %precedence, allows to define only precedence and no associativity at all. As a result, any associativity-related conflict that remains will be reported
as an compile-time error. The directive %nonassoc creates run-time error: using the operator in a associative way is a syntax error. The directive %precedence creates compile-time errors: an operator
can be involved in an associativity-related conflict, contrary to what expected the grammar author.

The relative precedence of different operators is controlled by the order in which they are declared. The first precedence/associativity declaration in the file declares the operators whose precedence is lowest, the next such declaration declares
the operators whose precedence is a little higher, and so on.

5.3.3 Specifying Precedence Only

Since POSIX Yacc defines only %left, %right, and
%nonassoc, which all defines precedence and associativity, little attention is paid to the fact that precedence cannot be defined without defining associativity. Yet, sometimes, when trying to solve a conflict, precedence suffices.
In such a case, using %left,
%right, or %nonassoc might hide future (associativity related) conflicts that would remain hidden.

The dangling else ambiguity (see Shift/Reduce
Conflicts) can be solved explicitly. This shift/reduce conflicts occurs in the following situation, where the period denotes the current parsing state:

if e1 then if  e2 then s1 . else s2

The conflict involves the reduction of the rule ‘IF expr THEN
stmt’, which precedence is by default that of its last token (
THEN), and the shifting of the token ELSE. The usual disambiguation (attach the else to the closest if), shifting must be preferred, i.e., the precedence of ELSE must be higher
than that of THEN. But neither is expected to be involved in an associativity related conflict, which can be specified as follows.

%precedence THEN
%precedence ELSE

The unary-minus is another typical example where associativity is usually over-specified, see Infix Notation
Calculator — calc. The %left directive is traditionally used to declare the precedence of NEG, which is more than needed since it also defines its associativity. While this is harmless in the traditional example,
who knows how NEG might be used in future evolutions of the grammar…

5.3.4 Precedence Examples

In our example, we would want the following declarations:

%left '<'
%left '-'
%left '*'

In a more complete example, which supports other operators as well, we would declare them in groups of equal precedence. For example, '+' is declared with '-':

%left '<' '>' '=' "!=" "<=" ">="
%left '+' '-'
%left '*' '/'

5.3.5 How Precedence Works

The first effect of the precedence declarations is to assign precedence levels to the terminal symbols declared. The second effect is to assign precedence levels to certain rules: each rule gets its precedence from the last terminal symbol mentioned
in the components. (You can also specify explicitly the precedence of a rule. See Context-Dependent Precedence.)

Finally, the resolution of conflicts works by comparing the precedence of the rule being considered with that of the lookahead token. If the token’s precedence is higher, the choice is to shift. If the rule’s precedence is higher,
the choice is to reduce. If they have equal precedence, the choice is made based on the associativity of that precedence level. The verbose output file made by ‘-v’ (see Invoking Bison) says
how each conflict was resolved.

Not all rules and not all tokens have precedence. If either the rule or the lookahead token has no precedence, then the default is to shift.

5.3.6 Using Precedence For Non Operators

Using properly precedence and associativity directives can help fixing shift/reduce conflicts that do not involve arithmetics-like operators. For instance, the “dangling else” problem (see Shift/Reduce Conflicts)
can be solved elegantly in two different ways.

In the present case, the conflict is between the token "else" willing to be shifted, and the rule ‘if_stmt: «if» expr «then» stmt’, asking for reduction. By default, the precedence
of a rule is that of its last token, here "then", so the conflict will be solved appropriately by giving "else" a precedence higher than that of "then", for instance as follows:

%precedence "then"
%precedence "else"

Alternatively, you may give both tokens the same precedence, in which case associativity is used to solve the conflict. To preserve the shift action, use right associativity:

Neither solution is perfect however. Since Bison does not provide, so far, “scoped” precedence, both force you to declare the precedence of these keywords with respect to the other operators your grammar. Therefore, instead of being
warned about new conflicts you would be unaware of (e.g., a shift/reduce conflict due to ‘if test then 1 else 2 + 3’ being ambiguous: ‘if test then 1 else (2 + 3)’ or ‘(if test then 1
else 2) + 3’?), the conflict will be already “fixed”.

5.4 Context-Dependent Precedence

Often the precedence of an operator depends on the context. This sounds outlandish at first, but it is really very common. For example, a minus sign typically has a very high precedence as a unary operator, and a somewhat lower precedence (lower
than multiplication) as a binary operator.

The Bison precedence declarations can only be used once for a given token; so a token has only one precedence declared in this way. For context-dependent precedence, you need to use an additional mechanism: the %prec modifier for
rules.

The %prec modifier declares the precedence of a particular rule by specifying a terminal symbol whose precedence should be used for that rule. It’s not necessary for that symbol to appear otherwise in the rule. The modifier’s
syntax is:

and it is written after the components of the rule. Its effect is to assign the rule the precedence of terminal-symbol, overriding the precedence that would be deduced for it in the ordinary way. The altered rule precedence then affects
how conflicts involving that rule are resolved (see Operator Precedence).

Here is how %prec solves the problem of unary minus. First, declare a precedence for a fictitious terminal symbol named UMINUS. There are no tokens of this type, but the symbol serves to stand for its precedence:

…
%left '+' '-'
%left '*'
%left UMINUS

Now the precedence of UMINUS can be used in specific rules:

exp:
  …
| exp '-' exp
  …
| '-' exp %prec UMINUS

5.5 Parser States

The function yyparse is implemented using a finite-state machine. The values pushed on the parser stack are not simply token type codes; they represent the entire sequence of terminal and nonterminal symbols at or near the top of
the stack. The current state collects all the information about previous input which is relevant to deciding what to do next.

Each time a lookahead token is read, the current parser state together with the type of lookahead token are looked up in a table. This table entry can say, “Shift the lookahead token.” In this case, it also specifies the new parser
state, which is pushed onto the top of the parser stack. Or it can say, “Reduce using rule number n.” This means that a certain number of tokens or groupings are taken off the top of the stack, and replaced by one grouping.
In other words, that number of states are popped from the stack, and one new state is pushed.

There is one other alternative: the table can say that the lookahead token is erroneous in the current state. This causes error processing to begin (see Error Recovery).

5.6 Reduce/Reduce Conflicts

A reduce/reduce conflict occurs if there are two or more rules that apply to the same sequence of input. This usually indicates a serious error in the grammar.

For example, here is an erroneous attempt to define a sequence of zero or more word groupings.

sequence:
  %empty         { printf ("empty sequencen"); }
| maybeword
| sequence word  { printf ("added word %sn", $2); }
;

maybeword:
  %empty    { printf ("empty maybewordn"); }
| word      { printf ("single word %sn", $1); }
;

The error is an ambiguity: there is more than one way to parse a single
word into a sequence. It could be reduced to a
maybeword and then into a sequence via the second rule. Alternatively, nothing-at-all could be reduced into a sequence via the first rule, and this could be combined with the word using the
third rule for sequence.

There is also more than one way to reduce nothing-at-all into a
sequence. This can be done directly via the first rule, or indirectly via maybeword and then the second rule.

You might think that this is a distinction without a difference, because it does not change whether any particular input is valid or not. But it does affect which actions are run. One parsing order runs the second rule’s action; the other
runs the first rule’s action and the third rule’s action. In this example, the output of the program changes.

Bison resolves a reduce/reduce conflict by choosing to use the rule that appears first in the grammar, but it is very risky to rely on this. Every reduce/reduce conflict must be studied and usually eliminated. Here is the proper way to define
sequence:

sequence:
  %empty         { printf ("empty sequencen"); }
| sequence word  { printf ("added word %sn", $2); }
;

Here is another common error that yields a reduce/reduce conflict:

sequence:
  %empty
| sequence words
| sequence redirects
;

words:
  %empty
| words word
;

redirects:
  %empty
| redirects redirect
;

The intention here is to define a sequence which can contain either
word or redirect groupings. The individual definitions of
sequence, words and redirects are error-free, but the three together make a subtle ambiguity: even an empty input can be parsed in infinitely many ways!

Consider: nothing-at-all could be a words. Or it could be two
words in a row, or three, or any number. It could equally well be a
redirects, or two, or any number. Or it could be a words followed by three redirects and another words. And so on.

Here are two ways to correct these rules. First, to make it a single level of sequence:

sequence:
  %empty
| sequence word
| sequence redirect
;

Second, to prevent either a words or a redirects from being empty:

sequence:
  %empty
| sequence words
| sequence redirects
;

words:
  word
| words word
;

redirects:
  redirect
| redirects redirect
;

Yet this proposal introduces another kind of ambiguity! The input ‘
word word’ can be parsed as a single words composed of two ‘
word’s, or as two one-word words (and likewise for
redirect/redirects). However this ambiguity is now a shift/reduce conflict, and therefore it can now be addressed with precedence directives.

To simplify the matter, we will proceed with word and redirect being tokens: "word" and "redirect".

To prefer the longest words, the conflict between the token
"word" and the rule ‘sequence: sequence words’ must be resolved as a shift. To this end, we use the same techniques as exposed above, see
Using Precedence For Non Operators. One solution relies on precedences: use %prec to give a lower precedence to the rule:

%precedence "word"
%precedence "sequence"
%%

sequence:
  %empty
| sequence word      %prec "sequence"
| sequence redirect  %prec "sequence"
;

words:
  word
| words "word"
;

Another solution relies on associativity: provide both the token and the rule with the same precedence, but make them right-associative:

%right "word" "redirect"
%%

sequence:
  %empty
| sequence word      %prec "word"
| sequence redirect  %prec "redirect"
;

5.7 Mysterious Conflicts

Sometimes reduce/reduce conflicts can occur that don’t look warranted. Here is an example:

%%
def: param_spec return_spec ',';
param_spec:
  type
| name_list ':' type
;

return_spec:
  type
| name ':' type
;

type: "id";

name: "id";
name_list:
  name
| name ',' name_list
;

It would seem that this grammar can be parsed with only a single token of lookahead: when a param_spec is being read, an "id" is a
name if a comma or colon follows, or a type if another
"id" follows. In other words, this grammar is LR(1).

However, for historical reasons, Bison cannot by default handle all LR(1) grammars. In this grammar, two contexts, that after an "id" at the beginning of a param_spec and likewise at the beginning of a
return_spec, are similar enough that Bison assumes they are the same. They appear similar because the same set of rules would be active—the rule for reducing to a name and that for reducing to a type.
Bison is unable to determine at that stage of processing that the rules would require different lookahead tokens in the two contexts, so it makes a single parser state for them both. Combining the two contexts causes a conflict later. In parser
terminology, this occurrence means that the grammar is not LALR(1).

For many practical grammars (specifically those that fall into the non-LR(1) class), the limitations of LALR(1) result in difficulties beyond just mysterious reduce/reduce conflicts. The best way to fix all these problems is to select a different
parser table construction algorithm. Either IELR(1) or canonical LR(1) would suffice, but the former is more efficient and easier to debug during development. See LR Table Construction, for details. (Bison’s
IELR(1) and canonical LR(1) implementations are experimental. More user feedback will help to stabilize them.)

If you instead wish to work around LALR(1)’s limitations, you can often fix a mysterious conflict by identifying the two parser states that are being confused, and adding something to make them look distinct. In the above example, adding
one rule to
return_spec as follows makes the problem go away:

…
return_spec:
  type
| name ':' type
| "id" "bogus"       /* This rule is never used.  */
;

This corrects the problem because it introduces the possibility of an additional active rule in the context after the "id" at the beginning of
return_spec. This rule is not active in the corresponding context in a param_spec, so the two contexts receive distinct parser states. As long as the token "bogus" is never generated by yylex,
the added rule cannot alter the way actual input is parsed.

In this particular example, there is another way to solve the problem: rewrite the rule for return_spec to use "id" directly instead of via name. This also causes the two confusing contexts to have
different sets of active rules, because the one for
return_spec activates the altered rule for return_spec rather than the one for name.

param_spec:
  type
| name_list ':' type
;

return_spec:
  type
| "id" ':' type
;

For a more detailed exposition of LALR(1) parsers and parser generators, see DeRemer 1982.

5.8 Tuning LR

The default behavior of Bison’s LR-based parsers is chosen mostly for historical reasons, but that behavior is often not robust. For example, in the previous section, we discussed the mysterious conflicts that can be produced by LALR(1),
Bison’s default parser table construction algorithm. Another example is Bison’s %define parse.error verbose directive, which instructs the generated parser to produce verbose syntax error messages, which can sometimes
contain incorrect information.

In this section, we explore several modern features of Bison that allow you to tune fundamental aspects of the generated LR-based parsers. Some of these features easily eliminate shortcomings like those mentioned above. Others can be helpful purely
for understanding your parser.

Most of the features discussed in this section are still experimental. More user feedback will help to stabilize them.

5.8.1 LR Table Construction

For historical reasons, Bison constructs LALR(1) parser tables by default. However, LALR does not possess the full language-recognition power of LR. As a result, the behavior of parsers employing LALR parser tables is often mysterious. We presented
a simple example of this effect in Mysterious Conflicts.

As we also demonstrated in that example, the traditional approach to eliminating such mysterious behavior is to restructure the grammar. Unfortunately, doing so correctly is often difficult. Moreover, merely discovering that LALR causes mysterious
behavior in your parser can be difficult as well.

Fortunately, Bison provides an easy way to eliminate the possibility of such mysterious behavior altogether. You simply need to activate a more powerful parser table construction algorithm by using the %define lr.type directive.

Directive: %define lr.type type

Specify the type of parser tables within the LR(1) family. The accepted values for type are:

• lalr (default)
• ielr
• canonical-lr

(This feature is experimental. More user feedback will help to stabilize it.)

For example, to activate IELR, you might add the following directive to you grammar file:

For the example in Mysterious Conflicts, the mysterious conflict is then eliminated, so there is no need to invest time in comprehending the conflict or restructuring the grammar to fix it. If, during future
development, the grammar evolves such that all mysterious behavior would have disappeared using just LALR, you need not fear that continuing to use IELR will result in unnecessarily large parser tables. That is, IELR generates LALR tables
when LALR (using a deterministic parsing algorithm) is sufficient to support the full language-recognition power of LR. Thus, by enabling IELR at the start of grammar development, you can safely and completely eliminate the need to consider
LALR’s shortcomings.

While IELR is almost always preferable, there are circumstances where LALR or the canonical LR parser tables described by Knuth (see Knuth 1965) can be useful. Here we summarize the relative advantages of each parser
table construction algorithm within Bison:

• LALR

  There are at least two scenarios where LALR can be worthwhile:

  • GLR without static conflict resolution.

    When employing GLR parsers (see GLR Parsers), if you do not resolve any conflicts statically (for example, with %left or %precedence), then the parser explores all potential
    parses of any given input. In this case, the choice of parser table construction algorithm is guaranteed not to alter the language accepted by the parser. LALR parser tables are the smallest parser tables Bison can currently
    construct, so they may then be preferable. Nevertheless, once you begin to resolve conflicts statically, GLR behaves more like a deterministic parser in the syntactic contexts where those conflicts appear, and so either IELR
    or canonical LR can then be helpful to avoid LALR’s mysterious behavior.

  • Malformed grammars.

    Occasionally during development, an especially malformed grammar with a major recurring flaw may severely impede the IELR or canonical LR parser table construction algorithm. LALR can be a quick way to construct parser tables in
    order to investigate such problems while ignoring the more subtle differences from IELR and canonical LR.

• IELR

  IELR (Inadequacy Elimination LR) is a minimal LR algorithm. That is, given any grammar (LR or non-LR), parsers using IELR or canonical LR parser tables always accept exactly the same set of sentences. However, like LALR, IELR merges parser
  states during parser table construction so that the number of parser states is often an order of magnitude less than for canonical LR. More importantly, because canonical LR’s extra parser states may contain duplicate conflicts
  in the case of non-LR grammars, the number of conflicts for IELR is often an order of magnitude less as well. This effect can significantly reduce the complexity of developing a grammar.

• Canonical LR

  While inefficient, canonical LR parser tables can be an interesting means to explore a grammar because they possess a property that IELR and LALR tables do not. That is, if %nonassoc is not used and default reductions are
  left disabled (see Default Reductions), then, for every left context of every canonical LR state, the set of tokens accepted by that state is guaranteed to be the exact set of tokens that is syntactically
  acceptable in that left context. It might then seem that an advantage of canonical LR parsers in production is that, under the above constraints, they are guaranteed to detect a syntax error as soon as possible without performing any
  unnecessary reductions. However, IELR parsers that use LAC are also able to achieve this behavior without sacrificing %nonassoc or default reductions. For details and a few caveats of LAC, see LAC.

For a more detailed exposition of the mysterious behavior in LALR parsers and the benefits of IELR, see Denny 2008 March, and
Denny 2010 November.

5.8.2 Default Reductions

After parser table construction, Bison identifies the reduction with the largest lookahead set in each parser state. To reduce the size of the parser state, traditional Bison behavior is to remove that lookahead set and to assign that reduction
to be the default parser action. Such a reduction is known as a default reduction.

Default reductions affect more than the size of the parser tables. They also affect the behavior of the parser:

• Delayed yylex invocations.

  A consistent state is a state that has only one possible parser action. If that action is a reduction and is encoded as a default reduction, then that consistent state is called a defaulted state. Upon reaching a defaulted
  state, a Bison-generated parser does not bother to invoke yylex to fetch the next token before performing the reduction. In other words, whether default reductions are enabled in consistent states determines how soon a
  Bison-generated parser invokes yylex for a token: immediately when it reaches that token in the input or when it eventually needs that token as a lookahead to determine the next parser action. Traditionally,
  default reductions are enabled, and so the parser exhibits the latter behavior.

  The presence of defaulted states is an important consideration when designing yylex and the grammar file. That is, if the behavior of
  yylex can influence or be influenced by the semantic actions associated with the reductions in defaulted states, then the delay of the next yylex invocation until after those reductions is significant. For
  example, the semantic actions might pop a scope stack that yylex uses to determine what token to return. Thus, the delay might be necessary to ensure that yylex does not look up the next token in a scope that
  should already be considered closed.

• Delayed syntax error detection.

  When the parser fetches a new token by invoking yylex, it checks whether there is an action for that token in the current parser state. The parser detects a syntax error if and only if either (1) there is no action for that
  token or (2) the action for that token is the error action (due to the use of %nonassoc). However, if there is a default reduction in that state (which might or might not be a defaulted state), then it is impossible for
  condition 1 to exist. That is, all tokens have an action. Thus, the parser sometimes fails to detect the syntax error until it reaches a later state.

  While default reductions never cause the parser to accept syntactically incorrect sentences, the delay of syntax error detection can have unexpected effects on the behavior of the parser. However, the delay can be caused anyway by parser
  state merging and the use of %nonassoc, and it can be fixed by another Bison feature, LAC. We discuss the effects of delayed syntax error detection and LAC more in the next section (see LAC).

For canonical LR, the only default reduction that Bison enables by default is the accept action, which appears only in the accepting state, which has no other action and is thus a defaulted state. However, the default accept action does not delay
any yylex invocation or syntax error detection because the accept action ends the parse.

For LALR and IELR, Bison enables default reductions in nearly all states by default. There are only two exceptions. First, states that have a shift action on the error token do not have default reductions because delayed syntax error
detection could then prevent the error token from ever being shifted in that state. However, parser state merging can cause the same effect anyway, and LAC fixes it in both cases, so future versions of Bison might drop this exception
when LAC is activated. Second, GLR parsers do not record the default reduction as the action on a lookahead token for which there is a conflict. The correct action in this case is to split the parse instead.

To adjust which states have default reductions enabled, use the
%define lr.default-reduction directive.

Directive: %define lr.default-reduction where

Specify the kind of states that are permitted to contain default reductions. The accepted values of where are:

• most (default for LALR and IELR)
• consistent
• accepting (default for canonical LR)

(The ability to specify where default reductions are permitted is experimental. More user feedback will help to stabilize it.)

5.8.3 LAC

Canonical LR, IELR, and LALR can suffer from a couple of problems upon encountering a syntax error. First, the parser might perform additional parser stack reductions before discovering the syntax error. Such reductions can perform user semantic
actions that are unexpected because they are based on an invalid token, and they cause error recovery to begin in a different syntactic context than the one in which the invalid token was encountered. Second, when verbose error messages are
enabled (see
Error Reporting), the expected token list in the syntax error message can both contain invalid tokens and omit valid tokens.

The culprits for the above problems are %nonassoc, default reductions in inconsistent states (see Default Reductions), and parser state merging. Because IELR and LALR merge parser states, they suffer
the most. Canonical LR can suffer only if %nonassoc is used or if default reductions are enabled for inconsistent states.

LAC (Lookahead Correction) is a new mechanism within the parsing algorithm that solves these problems for canonical LR, IELR, and LALR without sacrificing %nonassoc, default reductions, or state merging. You can enable LAC with the
%define parse.lac directive.

Directive: %define parse.lac value

Enable LAC to improve syntax error handling.

• none (default)
• full

(This feature is experimental. More user feedback will help to stabilize it. Moreover, it is currently only available for deterministic parsers in C.)

Conceptually, the LAC mechanism is straight-forward. Whenever the parser fetches a new token from the scanner so that it can determine the next parser action, it immediately suspends normal parsing and performs an exploratory parse using a temporary
copy of the normal parser state stack. During this exploratory parse, the parser does not perform user semantic actions. If the exploratory parse reaches a shift action, normal parsing then resumes on the normal parser stacks. If the exploratory
parse reaches an error instead, the parser reports a syntax error. If verbose syntax error messages are enabled, the parser must then discover the list of expected tokens, so it performs a separate exploratory parse for each token in the grammar.

There is one subtlety about the use of LAC. That is, when in a consistent parser state with a default reduction, the parser will not attempt to fetch a token from the scanner because no lookahead is needed to determine the next parser action.
Thus, whether default reductions are enabled in consistent states (see Default Reductions) affects how soon the parser detects a syntax error: immediately when it reaches an erroneous token or when
it eventually
needs that token as a lookahead to determine the next parser action. The latter behavior is probably more intuitive, so Bison currently provides no way to achieve the former behavior while default reductions are enabled in consistent
states.

Thus, when LAC is in use, for some fixed decision of whether to enable default reductions in consistent states, canonical LR and IELR behave almost exactly the same for both syntactically acceptable and syntactically unacceptable input. While
LALR still does not support the full language-recognition power of canonical LR and IELR, LAC at least enables LALR’s syntax error handling to correctly reflect LALR’s language-recognition power.

There are a few caveats to consider when using LAC:

• Infinite parsing loops.

  IELR plus LAC does have one shortcoming relative to canonical LR. Some parsers generated by Bison can loop infinitely. LAC does not fix infinite parsing loops that occur between encountering a syntax error and detecting it, but enabling
  canonical LR or disabling default reductions sometimes does.

• Verbose error message limitations.

  Because of internationalization considerations, Bison-generated parsers limit the size of the expected token list they are willing to report in a verbose syntax error message. If the number of expected tokens exceeds that limit, the list
  is simply dropped from the message. Enabling LAC can increase the size of the list and thus cause the parser to drop it. Of course, dropping the list is better than reporting an incorrect list.

• Performance.

  Because LAC requires many parse actions to be performed twice, it can have a performance penalty. However, not all parse actions must be performed twice. Specifically, during a series of default reductions in consistent states and shift
  actions, the parser never has to initiate an exploratory parse. Moreover, the most time-consuming tasks in a parse are often the file I/O, the lexical analysis performed by the scanner, and the user’s semantic actions, but none
  of these are performed during the exploratory parse. Finally, the base of the temporary stack used during an exploratory parse is a pointer into the normal parser state stack so that the stack is never physically copied. In our experience,
  the performance penalty of LAC has proved insignificant for practical grammars.

While the LAC algorithm shares techniques that have been recognized in the parser community for years, for the publication that introduces LAC, see Denny 2010 May.

5.8.4 Unreachable States

If there exists no sequence of transitions from the parser’s start state to some state s, then Bison considers s to be an unreachable
state. A state can become unreachable during conflict resolution if Bison disables a shift action leading to it from a predecessor state.

By default, Bison removes unreachable states from the parser after conflict resolution because they are useless in the generated parser. However, keeping unreachable states is sometimes useful when trying to understand the relationship between
the parser and the grammar.

Directive: %define lr.keep-unreachable-state value

Request that Bison allow unreachable states to remain in the parser tables.
value must be a Boolean. The default is false.

There are a few caveats to consider:

• Missing or extraneous warnings.

  Unreachable states may contain conflicts and may use rules not used in any other state. Thus, keeping unreachable states may induce warnings that are irrelevant to your parser’s behavior, and it may eliminate warnings that are relevant.
  Of course, the change in warnings may actually be relevant to a parser table analysis that wants to keep unreachable states, so this behavior will likely remain in future Bison releases.

• Other useless states.

  While Bison is able to remove unreachable states, it is not guaranteed to remove other kinds of useless states. Specifically, when Bison disables reduce actions during conflict resolution, some goto actions may become useless, and thus
  some additional states may become useless. If Bison were to compute which goto actions were useless and then disable those actions, it could identify such states as unreachable and then remove those states. However, Bison does not
  compute which goto actions are useless.

5.9 Generalized LR (GLR) Parsing

Bison produces deterministic parsers that choose uniquely when to reduce and which reduction to apply based on a summary of the preceding input and on one extra token of lookahead. As a result, normal Bison handles a proper subset of
the family of context-free languages. Ambiguous grammars, since they have strings with more than one possible sequence of reductions cannot have deterministic parsers in this sense. The same is true of languages that require more than one
symbol of lookahead, since the parser lacks the information necessary to make a decision at the point it must be made in a shift-reduce parser. Finally, as previously mentioned (see Mysterious Conflicts),
there are languages where Bison’s default choice of how to summarize the input seen so far loses necessary information.

When you use the ‘%glr-parser’ declaration in your grammar file, Bison generates a parser that uses a different algorithm, called Generalized LR (or GLR). A Bison GLR parser uses the same basic algorithm for parsing as
an ordinary Bison parser, but behaves differently in cases where there is a shift-reduce conflict that has not been resolved by precedence rules (see Precedence) or a reduce-reduce conflict. When a GLR parser encounters
such a situation, it effectively splits into a several parsers, one for each possible shift or reduction. These parsers then proceed as usual, consuming tokens in lock-step. Some of the stacks may encounter other conflicts and split
further, with the result that instead of a sequence of states, a Bison GLR parsing stack is what is in effect a tree of states.

In effect, each stack represents a guess as to what the proper parse is. Additional input may indicate that a guess was wrong, in which case the appropriate stack silently disappears. Otherwise, the semantics actions generated in each stack are
saved, rather than being executed immediately. When a stack disappears, its saved semantic actions never get executed. When a reduction causes two stacks to become equivalent, their sets of semantic actions are both saved with the state that
results from the reduction. We say that two stacks are equivalent when they both represent the same sequence of states, and each pair of corresponding states represents a grammar symbol that produces the same segment of the input token stream.

Whenever the parser makes a transition from having multiple states to having one, it reverts to the normal deterministic parsing algorithm, after resolving and executing the saved-up actions. At this transition, some of the states on the stack
will have semantic values that are sets (actually multisets) of possible actions. The parser tries to pick one of the actions by first finding one whose rule has the highest dynamic precedence, as set by the ‘%dprec’
declaration. Otherwise, if the alternative actions are not ordered by precedence, but there the same merging function is declared for both rules by the ‘%merge’ declaration, Bison resolves and evaluates both and then
calls the merge function on the result. Otherwise, it reports an ambiguity.

It is possible to use a data structure for the GLR parsing tree that permits the processing of any LR(1) grammar in linear time (in the size of the input), any unambiguous (not necessarily LR(1)) grammar in quadratic worst-case time, and any general
(possibly ambiguous) context-free grammar in cubic worst-case time. However, Bison currently uses a simpler data structure that requires time proportional to the length of the input times the maximum number of stacks required for any prefix
of the input. Thus, really ambiguous or nondeterministic grammars can require exponential time and space to process. Such badly behaving examples, however, are not generally of practical interest. Usually, nondeterminism in a grammar is local—the
parser is “in doubt” only for a few tokens at a time. Therefore, the current data structure should generally be adequate. On LR(1) portions of a grammar, in particular, it is only slightly slower than with the deterministic LR(1)
Bison parser.

For a more detailed exposition of GLR parsers, see Scott
2000.

5.10 Memory Management, and How to Avoid Memory Exhaustion

The Bison parser stack can run out of memory if too many tokens are shifted and not reduced. When this happens, the parser function yyparse calls yyerror and then returns 2.

Because Bison parsers have growing stacks, hitting the upper limit usually results from using a right recursion instead of a left recursion, see Recursive Rules.

By defining the macro YYMAXDEPTH, you can control how deep the parser stack can become before memory is exhausted. Define the macro with a value that is an integer. This value is the maximum number of tokens that can be shifted (and
not reduced) before overflow.

The stack space allowed is not necessarily allocated. If you specify a large value for YYMAXDEPTH, the parser normally allocates a small stack at first, and then makes it bigger by stages as needed. This increasing allocation happens
automatically and silently. Therefore, you do not need to make YYMAXDEPTH painfully small merely to save space for ordinary inputs that do not need much stack.

However, do not allow YYMAXDEPTH to be a value so large that arithmetic overflow could occur when calculating the size of the stack space. Also, do not allow YYMAXDEPTH to be less than
YYINITDEPTH.

The default value of YYMAXDEPTH, if you do not define it, is 10000.

You can control how much stack is allocated initially by defining the macro YYINITDEPTH to a positive integer. For the deterministic parser in C, this value must be a compile-time constant unless you are assuming C99 or some other
target language or compiler that allows variable-length arrays. The default is 200.

Do not allow YYINITDEPTH to be greater than YYMAXDEPTH.

You can generate a deterministic parser containing C++ user code from the default (C) skeleton, as well as from the C++ skeleton (see C++ Parsers). However, if you do use the default skeleton and want to allow
the parsing stack to grow, be careful not to use semantic types or location types that require non-trivial copy constructors. The C skeleton bypasses these constructors when copying data to new, larger stacks.

6 Error Recovery

It is not usually acceptable to have a program terminate on a syntax error. For example, a compiler should recover sufficiently to parse the rest of the input file and check it for errors; a calculator should accept another expression.

In a simple interactive command parser where each input is one line, it may be sufficient to allow yyparse to return 1 on error and have the caller ignore the rest of the input line when that happens (and then call
yyparse again). But this is inadequate for a compiler, because it forgets all the syntactic context leading up to the error. A syntax error deep within a function in the compiler input should not cause the compiler to treat the
following line like the beginning of a source file.

You can define how to recover from a syntax error by writing rules to recognize the special token error. This is a terminal symbol that is always defined (you need not declare it) and reserved for error handling. The Bison parser
generates an error token whenever a syntax error happens; if you have provided a rule to recognize this token in the current context, the parse can continue.

For example:

stmts:
  %empty
| stmts 'n'
| stmts exp 'n'
| stmts error 'n'

The fourth rule in this example says that an error followed by a newline makes a valid addition to any stmts.

What happens if a syntax error occurs in the middle of an exp? The error recovery rule, interpreted strictly, applies to the precise sequence of a stmts, an error and a newline. If an error occurs in the
middle of an exp, there will probably be some additional tokens and subexpressions on the stack after the last stmts, and there will be tokens to read before the next newline. So the rule is not applicable in the
ordinary way.

But Bison can force the situation to fit the rule, by discarding part of the semantic context and part of the input. First it discards states and objects from the stack until it gets back to a state in which the
error token is acceptable. (This means that the subexpressions already parsed are discarded, back to the last complete stmts.) At this point the error token can be shifted. Then, if the old lookahead
token is not acceptable to be shifted next, the parser reads tokens and discards them until it finds a token which is acceptable. In this example, Bison reads and discards input until the next newline so that the fourth rule can apply. Note
that discarded symbols are possible sources of memory leaks, see Freeing
Discarded Symbols, for a means to reclaim this memory.

The choice of error rules in the grammar is a choice of strategies for error recovery. A simple and useful strategy is simply to skip the rest of the current input line or current statement if an error is detected:

stmt: error ';'  /* On error, skip until ';' is read.  */

It is also useful to recover to the matching close-delimiter of an opening-delimiter that has already been parsed. Otherwise the close-delimiter will probably appear to be unmatched, and generate another, spurious error message:

primary:
  '(' expr ')'
| '(' error ')'
…
;

Error recovery strategies are necessarily guesses. When they guess wrong, one syntax error often leads to another. In the above example, the error recovery rule guesses that an error is due to bad input within one
stmt. Suppose that instead a spurious semicolon is inserted in the middle of a valid stmt. After the error recovery rule recovers from the first error, another syntax error will be found straightaway, since the text
following the spurious semicolon is also an invalid
stmt.

To prevent an outpouring of error messages, the parser will output no error message for another syntax error that happens shortly after the first; only after three consecutive input tokens have been successfully shifted will error messages resume.

Note that rules which accept the error token may have actions, just as any other rules can.

You can make error messages resume immediately by using the macro
yyerrok in an action. If you do this in the error rule’s action, no error messages will be suppressed. This macro requires no arguments; ‘
yyerrok;’ is a valid C statement.

The previous lookahead token is reanalyzed immediately after an error. If this is unacceptable, then the macro yyclearin may be used to clear this token. Write the statement ‘yyclearin;’ in the error rule’s
action. See Special Features for Use in Actions.

For example, suppose that on a syntax error, an error handling routine is called that advances the input stream to some point where parsing should once again commence. The next symbol returned by the lexical scanner is probably correct. The previous
lookahead token ought to be discarded with ‘yyclearin;’.

The expression YYRECOVERING () yields 1 when the parser is recovering from a syntax error, and 0 otherwise. Syntax error diagnostics are suppressed while recovering from a syntax error.

7 Handling Context Dependencies

The Bison paradigm is to parse tokens first, then group them into larger syntactic units. In many languages, the meaning of a token is affected by its context. Although this violates the Bison paradigm, certain techniques (known as kludges)
may enable you to write Bison parsers for such languages.

(Actually, “kludge” means any technique that gets its job done but is neither clean nor robust.)

7.1 Semantic Info in Token Types

The C language has a context dependency: the way an identifier is used depends on what its current meaning is. For example, consider this:

This looks like a function call statement, but if foo is a typedef name, then this is actually a declaration of x. How can a Bison parser for C decide how to parse this input?

The method used in GNU C is to have two different token types,
IDENTIFIER and TYPENAME. When yylex finds an identifier, it looks up the current declaration of the identifier in order to decide which token type to return: TYPENAME if the identifier is
declared as a typedef, IDENTIFIER otherwise.

The grammar rules can then express the context dependency by the choice of token type to recognize. IDENTIFIER is accepted as an expression, but TYPENAME is not. TYPENAME can start a declaration, but
IDENTIFIER cannot. In contexts where the meaning of the identifier is not significant, such as in declarations that can shadow a typedef name, either TYPENAME or IDENTIFIER is accepted—there
is one rule for each of the two token types.

This technique is simple to use if the decision of which kinds of identifiers to allow is made at a place close to where the identifier is parsed. But in C this is not always so: C allows a declaration to redeclare a typedef name provided an explicit
type has been specified earlier:

typedef int foo, bar;
int baz (void)

{
  static bar (bar);      /* redeclare bar as static variable */
  extern foo foo (foo);  /* redeclare foo as function */
  return foo (bar);
}

Unfortunately, the name being declared is separated from the declaration construct itself by a complicated syntactic structure—the “declarator”.

As a result, part of the Bison parser for C needs to be duplicated, with all the nonterminal names changed: once for parsing a declaration in which a typedef name can be redefined, and once for parsing a declaration in which that can’t be
done. Here is a part of the duplication, with actions omitted for brevity:

initdcl:
  declarator maybeasm '=' init
| declarator maybeasm
;

notype_initdcl:
  notype_declarator maybeasm '=' init
| notype_declarator maybeasm
;

Here initdcl can redeclare a typedef name, but notype_initdcl cannot. The distinction between declarator and
notype_declarator is the same sort of thing.

There is some similarity between this technique and a lexical tie-in (described next), in that information which alters the lexical analysis is changed during parsing by other parts of the program. The difference is here the information is global,
and is used for other purposes in the program. A true lexical tie-in has a special-purpose flag controlled by the syntactic context.

7.2 Lexical Tie-ins

One way to handle context-dependency is the lexical tie-in: a flag which is set by Bison actions, whose purpose is to alter the way tokens are parsed.

For example, suppose we have a language vaguely like C, but with a special construct ‘hex (hex-expr)’. After the keyword hex comes an expression in parentheses in which all integers are hexadecimal.
In particular, the token ‘a1b’ must be treated as an integer rather than as an identifier if it appears in that context. Here is how you can do it:

%{
  int hexflag;
  int yylex (void);
  void yyerror (char const *);
%}
%%
…

expr:
  IDENTIFIER
| constant
| HEX '('        { hexflag = 1; }
    expr ')'     { hexflag = 0; $$ = $4; }
| expr '+' expr  { $$ = make_sum ($1, $3); }
…
;

constant:
  INTEGER
| STRING
;

Here we assume that yylex looks at the value of hexflag; when it is nonzero, all integers are parsed in hexadecimal, and tokens starting with letters are parsed as integers if possible.

The declaration of hexflag shown in the prologue of the grammar file is needed to make it accessible to the actions (see The Prologue). You must also write the code in yylex to obey the flag.

7.3 Lexical Tie-ins and Error Recovery

Lexical tie-ins make strict demands on any error recovery rules you have. See Error Recovery.

The reason for this is that the purpose of an error recovery rule is to abort the parsing of one construct and resume in some larger construct. For example, in C-like languages, a typical error recovery rule is to skip tokens until the next semicolon,
and then start a new statement, like this:

stmt:
  expr ';'
| IF '(' expr ')' stmt { … }
…
| error ';'  { hexflag = 0; }
;

If there is a syntax error in the middle of a ‘hex (expr)’ construct, this error rule will apply, and then the action for the completed ‘hex (expr)’ will never run. So hexflag would remain set for the entire rest of the input, or until the next hex keyword, causing identifiers to be misinterpreted as integers.

To avoid this problem the error recovery rule itself clears hexflag.

There may also be an error recovery rule that works within expressions. For example, there could be a rule which applies within parentheses and skips to the close-parenthesis:

expr:
  …
| '(' expr ')'   { $$ = $2; }
| '(' error ')'
…

If this rule acts within the hex construct, it is not going to abort that construct (since it applies to an inner level of parentheses within the construct). Therefore, it should not clear the flag: the rest of the hex construct should be parsed with the flag still in effect.

What if there is an error recovery rule which might abort out of the
hex construct or might not, depending on circumstances? There is no way you can write the action to determine whether a hex construct is being aborted or not. So if you are using a lexical tie-in, you had better make
sure your error recovery rules are not of this kind. Each rule must be such that you can be sure that it always will, or always won’t, have to clear the flag.

8 Debugging Your Parser

Developing a parser can be a challenge, especially if you don’t understand the algorithm (see The Bison Parser Algorithm). This chapter explains how understand and debug a parser.

The first sections focus on the static part of the parser: its structure. They explain how to generate and read the detailed description of the automaton. There are several formats available:

• — as text, see Understanding Your Parser;
• — as a graph, see Visualizing Your Parser;
• — or as a markup report that can be turned, for instance, into HTML, see
  Visualizing your parser in multiple formats.

The last section focuses on the dynamic part of the parser: how to enable and understand the parser run-time traces (see Tracing Your
Parser).

8.1 Understanding Your Parser

As documented elsewhere (see The Bison Parser Algorithm) Bison parsers are shift/reduce automata. In some cases (much more frequent than one would hope), looking at this automaton is required to tune or simply
fix a parser.

The textual file is generated when the options —report or
—verbose are specified, see Invoking
Bison. Its name is made by removing ‘.tab.c’ or ‘.c’ from the parser implementation file name, and adding ‘.output’ instead. Therefore, if the grammar file is foo.y,
then the parser implementation file is called foo.tab.c by default. As a consequence, the verbose output file is called foo.output.

The following grammar file, calc.y, will be used in the sequel:

%token NUM STR

%left '+' '-'
%left '*'

%%

exp:
  exp '+' exp
| exp '-' exp
| exp '*' exp
| exp '/' exp
| NUM
;

useless: STR;
%%

bison reports:

calc.y: warning: 1 nonterminal useless in grammar
calc.y: warning: 1 rule useless in grammar
calc.y:12.1-7: warning: nonterminal useless in grammar: useless
calc.y:12.10-12: warning: rule useless in grammar: useless: STR
calc.y: conflicts: 7 shift/reduce

When given —report=state, in addition to calc.tab.c, it creates a file calc.output with contents detailed below. The order of the output and the exact presentation might vary, but the interpretation is the
same.

The first section reports useless tokens, nonterminals and rules. Useless nonterminals and rules are removed in order to produce a smaller parser, but useless tokens are preserved, since they might be used by the scanner (note the difference between
“useless” and “unused” below):

Nonterminals useless in grammar
   useless

Terminals unused in grammar
   STR

Rules useless in grammar
    6 useless: STR

The next section lists states that still have conflicts.

State 8 conflicts: 1 shift/reduce
State 9 conflicts: 1 shift/reduce
State 10 conflicts: 1 shift/reduce
State 11 conflicts: 4 shift/reduce

Then Bison reproduces the exact grammar it used:

Grammar

    0 $accept: exp $end

    1 exp: exp '+' exp
    2    | exp '-' exp
    3    | exp '*' exp
    4    | exp '/' exp
    5    | NUM

and reports the uses of the symbols:

Terminals, with rules where they appear

$end (0) 0
'*' (42) 3
'+' (43) 1
'-' (45) 2
'/' (47) 4
error (256)
NUM (258) 5
STR (259)

Nonterminals, with rules where they appear

$accept (9)
    on left: 0
exp (10)
    on left: 1 2 3 4 5, on right: 0 1 2 3 4

Bison then proceeds onto the automaton itself, describing each state with its set of items, also known as pointed rules. Each item is a production rule together with a point (‘.’) marking the location
of the input cursor.

State 0

    0 $accept: . exp $end

    NUM  shift, and go to state 1

    exp  go to state 2

This reads as follows: “state 0 corresponds to being at the very beginning of the parsing, in the initial rule, right before the start symbol (here, exp). When the parser returns to this state right after having reduced a rule
that produced an exp, the control flow jumps to state 2. If there is no such transition on a nonterminal symbol, and the lookahead is a NUM, then this token is shifted onto the parse stack, and the control flow jumps
to state 1. Any other lookahead triggers a syntax error.”

Even though the only active rule in state 0 seems to be rule 0, the report lists NUM as a lookahead token because NUM can be at the beginning of any rule deriving an exp. By default Bison reports the so-called
core or kernel of the item set, but if you want to see more detail you can invoke bison with
—report=itemset to list the derived items as well:

State 0

    0 $accept: . exp $end
    1 exp: . exp '+' exp
    2    | . exp '-' exp
    3    | . exp '*' exp
    4    | . exp '/' exp
    5    | . NUM

    NUM  shift, and go to state 1

    exp  go to state 2

In the state 1…

State 1

    5 exp: NUM .

    $default  reduce using rule 5 (exp)

the rule 5, ‘exp: NUM;’, is completed. Whatever the lookahead token (‘
$default’), the parser will reduce it. If it was coming from State 0, then, after this reduction it will return to state 0, and will jump to state 2 (‘exp: go to state 2’).

State 2

    0 $accept: exp . $end
    1 exp: exp . '+' exp
    2    | exp . '-' exp
    3    | exp . '*' exp
    4    | exp . '/' exp

    $end  shift, and go to state 3
    '+'   shift, and go to state 4
    '-'   shift, and go to state 5
    '*'   shift, and go to state 6
    '/'   shift, and go to state 7

In state 2, the automaton can only shift a symbol. For instance, because of the item ‘exp: exp . ‘+’ exp’, if the lookahead is ‘
+’ it is shifted onto the parse stack, and the automaton jumps to state 4, corresponding to the item ‘exp: exp ‘+’ . exp’. Since there is no default action, any lookahead not listed triggers a syntax
error.

The state 3 is named the final state, or the accepting
state:

State 3

    0 $accept: exp $end .

    $default  accept

the initial rule is completed (the start symbol and the end-of-input were read), the parsing exits successfully.

The interpretation of states 4 to 7 is straightforward, and is left to the reader.

State 4

    1 exp: exp '+' . exp

    NUM  shift, and go to state 1

    exp  go to state 8


State 5

    2 exp: exp '-' . exp

    NUM  shift, and go to state 1

    exp  go to state 9


State 6

    3 exp: exp '*' . exp

    NUM  shift, and go to state 1

    exp  go to state 10


State 7

    4 exp: exp '/' . exp

    NUM  shift, and go to state 1

    exp  go to state 11

As was announced in beginning of the report, ‘State 8 conflicts:
1 shift/reduce’:

State 8

    1 exp: exp . '+' exp
    1    | exp '+' exp .
    2    | exp . '-' exp
    3    | exp . '*' exp
    4    | exp . '/' exp

    '*'  shift, and go to state 6
    '/'  shift, and go to state 7

    '/'       [reduce using rule 1 (exp)]
    $default  reduce using rule 1 (exp)

Indeed, there are two actions associated to the lookahead ‘/’: either shifting (and going to state 7), or reducing rule 1. The conflict means that either the grammar is ambiguous, or the parser lacks information to make
the right decision. Indeed the grammar is ambiguous, as, since we did not specify the precedence of ‘/’, the sentence ‘NUM + NUM / NUM’ can be parsed as ‘NUM + (NUM /
NUM)’, which corresponds to shifting ‘/’, or as ‘(NUM + NUM) /
NUM’, which corresponds to reducing rule 1.

Because in deterministic parsing a single decision can be made, Bison arbitrarily chose to disable the reduction, see Shift/Reduce Conflicts. Discarded actions are reported between square brackets.

Note that all the previous states had a single possible action: either shifting the next token and going to the corresponding state, or reducing a single rule. In the other cases, i.e., when shifting
and reducing is possible or when several reductions are possible, the lookahead is required to select the action. State 8 is one such state: if the lookahead is ‘*’ or ‘/’
then the action is shifting, otherwise the action is reducing rule 1. In other words, the first two items, corresponding to rule 1, are not eligible when the lookahead token is ‘*’, since we specified that ‘*’
has higher precedence than ‘+’. More generally, some items are eligible only with some set of possible lookahead tokens. When run with
—report=lookahead, Bison specifies these lookahead tokens:

State 8

    1 exp: exp . '+' exp
    1    | exp '+' exp .  [$end, '+', '-', '/']
    2    | exp . '-' exp
    3    | exp . '*' exp
    4    | exp . '/' exp

    '*'  shift, and go to state 6
    '/'  shift, and go to state 7

    '/'       [reduce using rule 1 (exp)]
    $default  reduce using rule 1 (exp)

Note however that while ‘NUM + NUM / NUM’ is ambiguous (which results in the conflicts on ‘/’), ‘NUM + NUM * NUM’ is not: the conflict was solved thanks to associativity
and precedence directives. If invoked with
—report=solved, Bison includes information about the solved conflicts in the report:

Conflict between rule 1 and token '+' resolved as reduce (%left '+').
Conflict between rule 1 and token '-' resolved as reduce (%left '-').
Conflict between rule 1 and token '*' resolved as shift ('+' < '*').

The remaining states are similar:

State 9

    1 exp: exp . '+' exp
    2    | exp . '-' exp
    2    | exp '-' exp .
    3    | exp . '*' exp
    4    | exp . '/' exp

    '*'  shift, and go to state 6
    '/'  shift, and go to state 7

    '/'       [reduce using rule 2 (exp)]
    $default  reduce using rule 2 (exp)

State 10

    1 exp: exp . '+' exp
    2    | exp . '-' exp
    3    | exp . '*' exp
    3    | exp '*' exp .
    4    | exp . '/' exp

    '/'  shift, and go to state 7

    '/'       [reduce using rule 3 (exp)]
    $default  reduce using rule 3 (exp)

State 11

    1 exp: exp . '+' exp
    2    | exp . '-' exp
    3    | exp . '*' exp
    4    | exp . '/' exp
    4    | exp '/' exp .

    '+'  shift, and go to state 4
    '-'  shift, and go to state 5
    '*'  shift, and go to state 6
    '/'  shift, and go to state 7

    '+'       [reduce using rule 4 (exp)]
    '-'       [reduce using rule 4 (exp)]
    '*'       [reduce using rule 4 (exp)]
    '/'       [reduce using rule 4 (exp)]
    $default  reduce using rule 4 (exp)

Observe that state 11 contains conflicts not only due to the lack of precedence of ‘/’ with respect to ‘+’, ‘—’, and ‘*’, but also because the
associativity of ‘/’ is not specified.

Bison may also produce an HTML version of this output, via an XML file and XSLT processing (see Visualizing your parser in multiple formats).

8.2 Visualizing Your Parser

As another means to gain better understanding of the shift/reduce automaton corresponding to the Bison parser, a DOT file can be generated. Note that debugging a real grammar with this is tedious at best, and impractical most of the times, because
the generated files are huge (the generation of a PDF or PNG file from it will take very long, and more often than not it will fail due to memory exhaustion). This option was rather designed for beginners, to help them understand LR parsers.

This file is generated when the —graph option is specified (see Invoking Bison). Its name is made by removing ‘
.tab.c’ or ‘.c’ from the parser implementation file name, and adding ‘.dot’ instead. If the grammar file is foo.y, the Graphviz output file is called foo.dot.
A DOT file may also be produced via an XML file and XSLT processing (see Visualizing your
parser in multiple formats).

The following grammar file, rr.y, will be used in the sequel:

%%

exp: a ";" | b ".";
a: "0";
b: "0";

The graphical output (see Figure 8.1) is very similar to the textual one, and as such it is easier understood by making direct comparisons between them. See Debugging Your
Parser, for a detailled analysis of the textual report.

Graphical Representation of States

The items (pointed rules) for each state are grouped together in graph nodes. Their numbering is the same as in the verbose file. See the following points, about transitions, for examples

When invoked with —report=lookaheads, the lookahead tokens, when needed, are shown next to the relevant rule between square brackets as a comma separated list. This is the case in the figure for the representation of reductions,
below.

The transitions are represented as directed edges between the current and the target states.

Graphical Representation of Shifts

Shifts are shown as solid arrows, labelled with the lookahead token for that shift. The following describes a reduction in the rr.output file:

State 3

    1 exp: a . ";"

    ";"  shift, and go to state 6

A Graphviz rendering of this portion of the graph could be:

Graphical Representation of Reductions

Reductions are shown as solid arrows, leading to a diamond-shaped node bearing the number of the reduction rule. The arrow is labelled with the appropriate comma separated lookahead tokens. If the reduction is the default action for the given
state, there is no such label.

This is how reductions are represented in the verbose file rr.output:

State 1

    3 a: "0" .  [";"]
    4 b: "0" .  ["."]

    "."       reduce using rule 4 (b)
    $default  reduce using rule 3 (a)

A Graphviz rendering of this portion of the graph could be:

When unresolved conflicts are present, because in deterministic parsing a single decision can be made, Bison can arbitrarily choose to disable a reduction, see Shift/Reduce Conflicts. Discarded actions are distinguished
by a red filling color on these nodes, just like how they are reported between square brackets in the verbose file.

The reduction corresponding to the rule number 0 is the acceptation state. It is shown as a blue diamond, labelled “Acc”.

Graphical representation of go tos

The ‘go to’ jump transitions are represented as dotted lines bearing the name of the rule being jumped to.

8.3 Visualizing your parser in multiple formats

Bison supports two major report formats: textual output (see Understanding Your Parser) when invoked with option —verbose, and DOT (see Visualizing Your Parser) when invoked with
option
—graph. However, another alternative is to output an XML file that may then be, with
xsltproc, rendered as either a raw text format equivalent to the verbose file, or as an HTML version of the same file, with clickable transitions, or even as a DOT. The .output and DOT files obtained via XSLT have
no difference whatsoever with those obtained by invoking
bison with options —verbose or —graph.

The XML file is generated when the options -x or
—xml[=FILE] are specified, see Invoking Bison. If not specified, its name is made by removing ‘.tab.c’ or ‘.c’ from the parser implementation file name,
and adding ‘.xml’ instead. For instance, if the grammar file is foo.y, the default XML output file is foo.xml.

Bison ships with a data/xslt directory, containing XSL Transformation files to apply to the XML file. Their names are non-ambiguous:

xml2dot.xsl

Used to output a copy of the DOT visualization of the automaton.

xml2text.xsl

Used to output a copy of the ‘.output’ file.

xml2xhtml.xsl

Used to output an xhtml enhancement of the ‘.output’ file.

Sample usage (requires xsltproc):

$ bison -x gr.y

$ bison --print-datadir
/usr/local/share/bison

$ xsltproc /usr/local/share/bison/xslt/xml2xhtml.xsl gr.xml >gr.html

8.4 Tracing Your Parser

When a Bison grammar compiles properly but parses “incorrectly”, the
yydebug parser-trace feature helps figuring out why.

8.4.1 Enabling Traces

There are several means to enable compilation of trace facilities:

the macro YYDEBUG

Define the macro YYDEBUG to a nonzero value when you compile the parser. This is compliant with POSIX Yacc. You could use ‘
-DYYDEBUG=1’ as a compiler option or you could put ‘#define
YYDEBUG 1’ in the prologue of the grammar file (see The
Prologue).

If the %define variable api.prefix is used (see Multiple Parsers in the Same Program), for instance ‘%define
api.prefix x’, then if CDEBUG is defined, its value controls the tracing feature (enabled if and only if nonzero); otherwise tracing is enabled if and only if YYDEBUG is nonzero.

the option -t (POSIX Yacc compliant)

the option —debug (Bison extension)

Use the ‘-t’ option when you run Bison (see Invoking
Bison). With ‘%define api.prefix {c}’, it defines CDEBUG to 1, otherwise it defines YYDEBUG to 1.

the directive ‘%debug’

Add the %debug directive (see Bison Declaration
Summary). This Bison extension is maintained for backward compatibility with previous versions of Bison.

the variable ‘parse.trace’

Add the ‘%define parse.trace’ directive (see parse.trace), or pass the -Dparse.trace option (see Bison Options). This is a Bison extension,
which is especially useful for languages that don’t use a preprocessor. Unless POSIX and Yacc portability matter to you, this is the preferred solution.

We suggest that you always enable the trace option so that debugging is always possible.

The trace facility outputs messages with macro calls of the form
YYFPRINTF (stderr, format, args) where
format and args are the usual printf format and variadic arguments. If you define YYDEBUG to a nonzero value but do not define YYFPRINTF, <stdio.h> is automatically
included and YYFPRINTF is defined to fprintf.

Once you have compiled the program with trace facilities, the way to request a trace is to store a nonzero value in the variable yydebug. You can do this by making the C code do it (in main, perhaps), or you can alter
the value with a C debugger.

Each step taken by the parser when yydebug is nonzero produces a line or two of trace information, written on stderr. The trace messages tell you these things:

• Each time the parser calls yylex, what kind of token was read.
• Each time a token is shifted, the depth and complete contents of the state stack (see Parser States).
• Each time a rule is reduced, which rule it is, and the complete contents of the state stack afterward.

To make sense of this information, it helps to refer to the automaton description file (see Understanding Your Parser). This file shows the meaning of each state in terms of positions in various rules, and also what
each state will do with each possible input token. As you read the successive trace messages, you can see that the parser is functioning according to its specification in the listing file. Eventually you will arrive at the place where something
undesirable happens, and you will see which parts of the grammar are to blame.

The parser implementation file is a C/C++/Java program and you can use debuggers on it, but it’s not easy to interpret what it is doing. The parser function is a finite-state machine interpreter, and aside from the actions it executes the
same code over and over. Only the values of variables show where in the grammar it is working.

8.4.2 Enabling Debug Traces for mfcalc

The debugging information normally gives the token type of each token read, but not its semantic value. The %printer directive allows specify how semantic values are reported, see Printing
Semantic Values. For backward compatibility, Yacc like C parsers may also use the YYPRINT (see The YYPRINT
Macro), but its use is discouraged.

As a demonstration of %printer, consider the multi-function calculator, mfcalc (see Multi-function Calc). To enable run-time traces, and semantic value reports, insert the following
directives in its prologue:

/* Generate the parser description file.  */
%verbose
/* Enable run-time traces (yydebug).  */
%define parse.trace

/* Formatting semantic values.  */
%printer { fprintf (yyoutput, "%s", $$->name); } VAR;
%printer { fprintf (yyoutput, "%s()", $$->name); } FNCT;
%printer { fprintf (yyoutput, "%g", $$); } <double>;

The %define directive instructs Bison to generate run-time trace support. Then, activation of these traces is controlled at run-time by the
yydebug variable, which is disabled by default. Because these traces will refer to the “states” of the parser, it is helpful to ask for the creation of a description of that parser; this is the purpose of (admittedly
ill-named)
%verbose directive.

The set of %printer directives demonstrates how to format the semantic value in the traces. Note that the specification can be done either on the symbol type (e.g., VAR or FNCT), or on the type tag: since
<double> is the type for both NUM and exp, this printer will be used for them.

Here is a sample of the information provided by run-time traces. The traces are sent onto standard error.

$ echo 'sin(1-1)' | ./mfcalc -p
Starting parse
Entering state 0
Reducing stack by rule 1 (line 34):
-> $$ = nterm input ()
Stack now 0
Entering state 1

This first batch shows a specific feature of this grammar: the first rule (which is in line 34 of mfcalc.y can be reduced without even having to look for the first token. The resulting left-hand symbol ($$) is a valueless
(‘
()’) input non terminal (nterm).

Then the parser calls the scanner.

Reading a token: Next token is token FNCT (sin())
Shifting token FNCT (sin())
Entering state 6

That token (token) is a function (FNCT) whose value is ‘
sin’ as formatted per our %printer specification: ‘sin()’. The parser stores (Shifting) that token, and others, until it can do something about it.

Reading a token: Next token is token '(' ()
Shifting token '(' ()
Entering state 14
Reading a token: Next token is token NUM (1.000000)
Shifting token NUM (1.000000)
Entering state 4
Reducing stack by rule 6 (line 44):
   $1 = token NUM (1.000000)
-> $$ = nterm exp (1.000000)
Stack now 0 1 6 14
Entering state 24

The previous reduction demonstrates the %printer directive for
<double>: both the token NUM and the resulting nonterminal
exp have ‘1’ as value.

Reading a token: Next token is token '-' ()
Shifting token '-' ()
Entering state 17
Reading a token: Next token is token NUM (1.000000)
Shifting token NUM (1.000000)
Entering state 4
Reducing stack by rule 6 (line 44):
   $1 = token NUM (1.000000)
-> $$ = nterm exp (1.000000)
Stack now 0 1 6 14 24 17
Entering state 26
Reading a token: Next token is token ')' ()
Reducing stack by rule 11 (line 49):
   $1 = nterm exp (1.000000)
   $2 = token '-' ()
   $3 = nterm exp (1.000000)
-> $$ = nterm exp (0.000000)
Stack now 0 1 6 14
Entering state 24

The rule for the subtraction was just reduced. The parser is about to discover the end of the call to sin.

Next token is token ')' ()
Shifting token ')' ()
Entering state 31
Reducing stack by rule 9 (line 47):
   $1 = token FNCT (sin())
   $2 = token '(' ()
   $3 = nterm exp (0.000000)
   $4 = token ')' ()
-> $$ = nterm exp (0.000000)
Stack now 0 1
Entering state 11

Finally, the end-of-line allow the parser to complete the computation, and display its result.

Reading a token: Next token is token 'n' ()
Shifting token 'n' ()
Entering state 22
Reducing stack by rule 4 (line 40):
   $1 = nterm exp (0.000000)
   $2 = token 'n' ()
⇒ 0
-> $$ = nterm line ()
Stack now 0 1
Entering state 10
Reducing stack by rule 2 (line 35):
   $1 = nterm input ()
   $2 = nterm line ()
-> $$ = nterm input ()
Stack now 0
Entering state 1

The parser has returned into state 1, in which it is waiting for the next expression to evaluate, or for the end-of-file token, which causes the completion of the parsing.

Reading a token: Now at end of input.
Shifting token $end ()
Entering state 2
Stack now 0 1 2
Cleanup: popping token $end ()
Cleanup: popping nterm input ()

8.4.3 The YYPRINT Macro

Before %printer support, semantic values could be displayed using the
YYPRINT macro, which works only for terminal symbols and only with the yacc.c skeleton.

Macro: YYPRINT (stream, token, value);

If you define YYPRINT, it should take three arguments. The parser will pass a standard I/O stream, the numeric code for the token type, and the token value (from yylval).

For yacc.c only. Obsoleted by %printer.

Here is an example of YYPRINT suitable for the multi-function calculator (see Declarations for mfcalc):

%{
  static void print_token_value (FILE *, int, YYSTYPE);
  #define YYPRINT(File, Type, Value)            
    print_token_value (File, Type, Value)
%}

… %% … %% …

static void
print_token_value (FILE *file, int type, YYSTYPE value)
{
  if (type == VAR)
    fprintf (file, "%s", value.tptr->name);
  else if (type == NUM)
    fprintf (file, "%d", value.val);
}

9 Invoking Bison

The usual way to invoke Bison is as follows:

Here infile is the grammar file name, which usually ends in ‘
.y’. The parser implementation file’s name is made by replacing the ‘.y’ with ‘.tab.c’ and removing any leading directory. Thus, the ‘bison foo.y’
file name yields foo.tab.c, and the ‘bison hack/foo.y’ file name yields foo.tab.c. It’s also possible, in case you are writing C++ code instead of C in your grammar file, to name it
foo.ypp or foo.y++. Then, the output files will take an extension like the given one as input (respectively foo.tab.cpp and foo.tab.c++). This feature takes effect with all options that manipulate
file names like ‘
-o’ or ‘-d’.

For example :

will produce infile.tab.cxx and infile.tab.hxx, and

bison -d -o output.c++ infile.y

will produce output.c++ and outfile.h++.

For compatibility with POSIX, the standard Bison distribution also contains a shell script called yacc that invokes Bison with the -y option.

9.1 Bison Options

Bison supports both traditional single-letter options and mnemonic long option names. Long option names are indicated with ‘—’ instead of ‘
—’. Abbreviations for option names are allowed as long as they are unique. When a long option takes an argument, like ‘
—file-prefix’, connect the option name and the argument with ‘
=’.

Here is a list of options that can be used with Bison, alphabetized by short option. It is followed by a cross key alphabetized by long option.

Operations modes:

-h

—help

Print a summary of the command-line options to Bison and exit.

-V

—version

Print the version number of Bison and exit.

—print-localedir

Print the name of the directory containing locale-dependent data.

—print-datadir

Print the name of the directory containing skeletons and XSLT.

-y

—yacc

Act more like the traditional Yacc command. This can cause different diagnostics to be generated, and may change behavior in other minor ways. Most importantly, imitate Yacc’s output file name conventions, so that the parser implementation
file is called y.tab.c, and the other outputs are called y.output and y.tab.h. Also, if generating a deterministic parser in C, generate
#define statements in addition to an enum to associate token numbers with token names. Thus, the following shell script can substitute for Yacc, and the Bison distribution contains such a script for compatibility
with POSIX:

The -y/—yacc option is intended for use with traditional Yacc grammars. If your grammar uses a Bison extension like ‘%glr-parser’, Bison might not be Yacc-compatible even if this option
is specified.

-W [category]

—warnings[=category]

Output warnings falling in category. category can be one of:

midrule-values

Warn about mid-rule values that are set but not used within any of the actions of the parent rule. For example, warn about unused $2 in:

exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };

Also warn about mid-rule values that are used but not set. For example, warn about unset $$ in the mid-rule action in:

exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };

These warnings are not enabled by default since they sometimes prove to be false alarms in existing grammars employing the Yacc constructs
$0 or $-n (where n is some positive integer).

yacc

Incompatibilities with POSIX Yacc.

conflicts-sr

conflicts-rr

S/R and R/R conflicts. These warnings are enabled by default. However, if the %expect or %expect-rr directive is specified, an unexpected number of conflicts is an error, and an expected number of conflicts
is not reported, so -W and —warning then have no effect on the conflict report.

deprecated

Deprecated constructs whose support will be removed in future versions of Bison.

empty-rule

Empty rules without %empty. See Empty Rules. Disabled by default, but enabled by uses of %empty, unless
-Wno-empty-rule was specified.

precedence

Useless precedence and associativity directives. Disabled by default.

Consider for instance the following grammar:

%nonassoc "="
%left "+"
%left "*"
%precedence "("

%%

stmt:
  exp
| "var" "=" exp
;

exp:
  exp "+" exp
| exp "*" "num"
| "(" exp ")"
| "num"
;

Bison reports:

warning: useless precedence and associativity for "="
 %nonassoc "="
           ^^^

warning: useless associativity for "*", use %precedence
 %left "*"
       ^^^

warning: useless precedence for "("
 %precedence "("
             ^^^

One would get the exact same parser with the following directives instead:

%left "+"
%precedence "*"

other

All warnings not categorized above. These warnings are enabled by default.

This category is provided merely for the sake of completeness. Future releases of Bison may move warnings from this category to new, more specific categories.

all

All the warnings except yacc.

none

Turn off all the warnings.

error

See -Werror, below.

A category can be turned off by prefixing its name with ‘no-’. For instance, -Wno-yacc will hide the warnings about POSIX Yacc incompatibilities.

-Werror

Turn enabled warnings for every category into errors, unless they are explicitly disabled by -Wno-error=category.

-Werror=category

Enable warnings falling in category, and treat them as errors.

category is the same as for —warnings, with the exception that it may not be prefixed with ‘no-’ (see above).

Note that the precedence of the ‘=’ and ‘,’ operators is such that the following commands are not equivalent, as the first will not treat S/R conflicts as errors.

$ bison -Werror=yacc,conflicts-sr input.y
$ bison -Werror=yacc,error=conflicts-sr input.y

-Wno-error

Do not turn enabled warnings for every category into errors, unless they are explicitly enabled by -Werror=category.

-Wno-error=category

Deactivate the error treatment for this category. However, the warning itself won’t be disabled, or enabled, by this option.

-f [feature]

—feature[=feature]

Activate miscellaneous feature. feature can be one of:

caret

diagnostics-show-caret

Show caret errors, in a manner similar to GCC’s
-fdiagnostics-show-caret, or Clang’s -fcaret-diagnotics. The location provided with the message is used to quote the corresponding line of the source file, underlining the important part of it
with carets (^). Here is an example, using the following file in.y:

%type <ival> exp
%%
exp: exp '+' exp { $exp = $1 + $2; };

When invoked with -fcaret (or nothing), Bison will report:

in.y:3.20-23: error: ambiguous reference: '$exp'
 exp: exp '+' exp { $exp = $1 + $2; };
                    ^^^^

in.y:3.1-3:       refers to: $exp at $$
 exp: exp '+' exp { $exp = $1 + $2; };
 ^^^

in.y:3.6-8:       refers to: $exp at $1
 exp: exp '+' exp { $exp = $1 + $2; };
      ^^^

in.y:3.14-16:     refers to: $exp at $3
 exp: exp '+' exp { $exp = $1 + $2; };
              ^^^

in.y:3.32-33: error: $2 of 'exp' has no declared type
 exp: exp '+' exp { $exp = $1 + $2; };
                                ^^

Whereas, when invoked with -fno-caret, Bison will only report:

in.y:3.20-23: error: ambiguous reference: ‘$exp’
in.y:3.1-3:       refers to: $exp at $$
in.y:3.6-8:       refers to: $exp at $1
in.y:3.14-16:     refers to: $exp at $3
in.y:3.32-33: error: $2 of ‘exp’ has no declared type

This option is activated by default.

Tuning the parser:

-t

—debug

In the parser implementation file, define the macro YYDEBUG to 1 if it is not already defined, so that the debugging facilities are compiled. See Tracing Your Parser.

-D name[=value]

—define=name[=value]

-F name[=value]

—force-define=name[=value]

Each of these is equivalent to ‘%define name «value«’ (see %define Summary) except that Bison processes multiple definitions for the same name as follows:

• Bison quietly ignores all command-line definitions for name except the last.
• If that command-line definition is specified by a -D or
  --define, Bison reports an error for any %define definition for name.
• If that command-line definition is specified by a -F or
  --force-define instead, Bison quietly ignores all %define definitions for name.
• Otherwise, Bison reports an error if there are multiple %define definitions for name.

You should avoid using -F and --force-define in your make files unless you are confident that it is safe to quietly ignore any conflicting %define that may be added to the grammar file.

-L language

—language=language

Specify the programming language for the generated parser, as if
%language was specified (see Bison Declaration
Summary). Currently supported languages include C, C++, and Java.
language is case-insensitive.

—locations

Pretend that %locations was specified. See Decl Summary.

-p prefix

—name-prefix=prefix

Pretend that %name-prefix "prefix" was specified (see Decl Summary). Obsoleted by -Dapi.prefix=prefix. See Multiple Parsers in the Same Program.

-l

—no-lines

Don’t put any #line preprocessor commands in the parser implementation file. Ordinarily Bison puts them in the parser implementation file so that the C compiler and debuggers will associate errors with your source file,
the grammar file. This option causes them to associate errors with the parser implementation file, treating it as an independent source file in its own right.

-S file

—skeleton=file

Specify the skeleton to use, similar to %skeleton (see Bison Declaration Summary).

If file does not contain a /, file is the name of a skeleton file in the Bison installation directory. If it does, file is an absolute file name or a file name relative to the current working
directory. This is similar to how most shells resolve commands.

-k

—token-table

Pretend that %token-table was specified. See Decl Summary.

Adjust the output:

—defines[=file]

Pretend that %defines was specified, i.e., write an extra output file containing macro definitions for the token type names defined in the grammar, as well as a few other declarations. See Decl Summary.

-d

This is the same as --defines except -d does not accept a
file argument since POSIX Yacc requires that -d can be bundled with other short options.

-b file-prefix

—file-prefix=prefix

Pretend that %file-prefix was specified, i.e., specify prefix to use for all Bison output file names. See Decl Summary.

-r things

—report=things

Write an extra output file containing verbose description of the comma separated list of things among:

state

Description of the grammar, conflicts (resolved and unresolved), and parser’s automaton.

itemset

Implies state and augments the description of the automaton with the full set of items for each state, instead of its core only.

lookahead

Implies state and augments the description of the automaton with each rule’s lookahead set.

solved

Implies state. Explain how conflicts were solved thanks to precedence and associativity directives.

all

Enable all the items.

none

Do not generate the report.

—report-file=file

Specify the file for the verbose description.

-v

—verbose

Pretend that %verbose was specified, i.e., write an extra output file containing verbose descriptions of the grammar and parser. See Decl Summary.

-o file

—output=file

Specify the file for the parser implementation file.

The other output files’ names are constructed from file as described under the ‘-v’ and ‘-d’ options.

-g [file]

—graph[=file]

Output a graphical representation of the parser’s automaton computed by Bison, in Graphviz
DOT format.
file is optional. If omitted and the grammar file is foo.y, the output file will be
foo.dot.

-x [file]

—xml[=file]

Output an XML report of the parser’s automaton computed by Bison.
file is optional. If omitted and the grammar file is foo.y, the output file will be
foo.xml. (The current XML schema is experimental and may evolve. More user feedback will help to stabilize it.)

9.2 Option Cross Key

Here is a list of options, alphabetized by long option, to help you find the corresponding short option and directive.

9.3 Yacc Library

The Yacc library contains default implementations of the
yyerror and main functions. These default implementations are normally not useful, but POSIX requires them. To use the Yacc library, link your program with the
-ly option. Note that Bison’s implementation of the Yacc library is distributed under the terms of the GNU General Public License (see Copying).

If you use the Yacc library’s yyerror function, you should declare yyerror as follows:

int yyerror (char const *);

The int value returned by this yyerror is ignored.

The implementation of Yacc library’s main function is:

int main (void)
{
  setlocale (LC_ALL, "");
  return yyparse ();
}

so if you use it, the internationalization support is enabled (e.g., error messages are translated), and your yyparse function should have the following type signature:

10 Parsers Written In Other Languages

10.1 C++ Parsers

10.1.1 C++ Bison Interface

The C++ deterministic parser is selected using the skeleton directive, ‘
%skeleton «lalr1.cc»’, or the synonymous command-line option
—skeleton=lalr1.cc. See Decl Summary.

When run, bison will create several entities in the ‘yy’ namespace.

Use the ‘%define api.namespace’ directive to change the namespace name, see api.namespace. The various classes are generated in the following files:

position.hh

location.hh

The definition of the classes position and location, used for location tracking when enabled. These files are not generated if the
%define variable api.location.type is defined. See C++ Location Values.

stack.hh

An auxiliary class stack used by the parser.

file.hh

file.cc

(Assuming the extension of the grammar file was ‘.yy’.) The declaration and implementation of the C++ parser class. The basename and extension of these two files follow the same rules as with regular C parsers
(see
Invocation).

The header is mandatory; you must either pass
-d/—defines to bison, or use the ‘
%defines’ directive.

All these files are documented using Doxygen; run doxygen for a complete and accurate documentation.

10.1.2 C++ Semantic Values

Bison supports two different means to handle semantic values in C++. One is alike the C interface, and relies on unions (see C++ Unions). As C++ practitioners know, unions are inconvenient in C++, therefore another
approach is provided, based on variants (see C++ Variants).

10.1.2.1 C++ Unions

The %union directive works as for C, see The
Union Declaration. In particular it produces a genuine
union, which have a few specific features in C++.

• — The type YYSTYPE is defined but its use is discouraged: rather you should refer to the parser’s encapsulated type
  yy::parser::semantic_type.
• — Non POD (Plain Old Data) types cannot be used. C++ forbids any instance of classes with constructors in unions: only pointers to such objects are allowed.

Because objects have to be stored via pointers, memory is not reclaimed automatically: using the %destructor directive is the only means to avoid leaks. See Freeing Discarded
Symbols.

10.1.2.2 C++ Variants

Bison provides a variant based implementation of semantic values for C++. This alleviates all the limitations reported in the previous section, and in particular, object types can be used without pointers.

To enable variant-based semantic values, set %define variable
variant (see variant). Once this defined,
%union is ignored, and instead of using the name of the fields of the
%union to “type” the symbols, use genuine types.

For instance, instead of

%union
{
  int ival;
  std::string* sval;
}
%token <ival> NUMBER;
%token <sval> STRING;

write

%token <int> NUMBER;
%token <std::string> STRING;

STRING is no longer a pointer, which should fairly simplify the user actions in the grammar and in the scanner (in particular the memory management).

Since C++ features destructors, and since it is customary to specialize
operator<< to support uniform printing of values, variants also typically simplify Bison printers and destructors.

Variants are stricter than unions. When based on unions, you may play any dirty game with yylval, say storing an int, reading a
char*, and then storing a double in it. This is no longer possible with variants: they must be initialized, then assigned to, and eventually, destroyed.

Method on semantic_type: T& build<T> ()

Initialize, but leave empty. Returns the address where the actual value may be stored. Requires that the variant was not initialized yet.

Method on semantic_type: T& build<T> (const T& t)

Initialize, and copy-construct from t.

Warning: We do not use Boost.Variant, for two reasons. First, it appeared unacceptable to require Boost on the user’s machine (i.e., the machine on which the generated parser will be compiled, not the machine on which bison was run). Second, for each possible semantic value, Boost.Variant not only stores the value, but also a tag specifying its type. But the parser already “knows” the type of the semantic value, so that would be duplicating the information.

Therefore we developed light-weight variants whose type tag is external (so they are really like unions for C++ actually). But our code is much less mature that Boost.Variant. So there is a number of limitations in (the current implementation
of) variants:

• Alignment must be enforced: values should be aligned in memory according to the most demanding type. Computing the smallest alignment possible requires meta-programming techniques that are not currently implemented in Bison, and therefore,
  since, as far as we know, double is the most demanding type on all platforms, alignments are enforced for double whatever types are actually used. This may waste space in some cases.

• There might be portability issues we are not aware of.

As far as we know, these limitations can be alleviated. All it takes is some time and/or some talented C++ hacker willing to contribute to Bison.

10.1.3 C++ Location Values

When the directive %locations is used, the C++ parser supports location tracking, see Tracking Locations.

By default, two auxiliary classes define a position, a single point in a file, and a location, a range composed of a pair of
positions (possibly spanning several files). But if the
%define variable api.location.type is defined, then these classes will not be generated, and the user defined type will be used.

In this section uint is an abbreviation for unsigned int: in genuine code only the latter is used.

10.1.3.1 C++ position

Constructor on position: position (std::string* file = 0, uint line = 1, uint col = 1)

Create a position denoting a given point. Note that file is not reclaimed when the position is destroyed: memory managed must be handled elsewhere.

Method on position: void initialize (std::string* file = 0, uint line = 1, uint col = 1)

Reset the position to the given values.

Instance Variable of position: std::string* file

The name of the file. It will always be handled as a pointer, the parser will never duplicate nor deallocate it. As an experimental feature you may change it to ‘type*’ using ‘%define
filename_type «type«’.

Instance Variable of position: uint line

The line, starting at 1.

Method on position: void lines (int height = 1)

If height is not null, advance by height lines, resetting the column number. The resulting line number cannot be less than 1.

Instance Variable of position: uint column

The column, starting at 1.

Method on position: void columns (int width = 1)

Advance by width columns, without changing the line number. The resulting column number cannot be less than 1.

Method on position: position& operator+= (int width)

Method on position: position operator+ (int width)

Method on position: position& operator-= (int width)

Method on position: position operator- (int width)

Various forms of syntactic sugar for columns.

Method on position: bool operator== (const position& that)

Method on position: bool operator!= (const position& that)

Whether *this and that denote equal/different positions.

Function: std::ostream& operator<< (std::ostream& o, const position& p)

Report p on o like this: ‘
file:line.column’, or ‘
line.column’ if file is null.

10.1.3.2 C++ location

Constructor on location: location (const position& begin, const position& end)

Create a Location from the endpoints of the range.

Constructor on location: location (const position& pos = position())

Constructor on location: location (std::string* file, uint line, uint col)

Create a Location denoting an empty range located at a given point.

Method on location: void initialize (std::string* file = 0, uint line = 1, uint col = 1)

Reset the location to an empty range at the given values.

Instance Variable of location: position begin

Instance Variable of location: position end

The first, inclusive, position of the range, and the first beyond.

Method on location: void columns (int width = 1)

Method on location: void lines (int height = 1)

Forwarded to the end position.

Method on location: location operator+ (int width)

Method on location: location operator+= (int width)

Method on location: location operator- (int width)

Method on location: location operator-= (int width)

Various forms of syntactic sugar for columns.

Method on location: location operator+ (const location& end)

Method on location: location operator+= (const location& end)

Join two locations: starts at the position of the first one, and ends at the position of the second.

Method on location: void step ()

Move begin onto end.

Method on location: bool operator== (const location& that)

Method on location: bool operator!= (const location& that)

Whether *this and that denote equal/different ranges of positions.

Function: std::ostream& operator<< (std::ostream& o, const location& p)

Report p on o, taking care of special cases such as: no
filename defined, or equal filename/line or column.

10.1.3.3 User Defined Location Type

Instead of using the built-in types you may use the %define variable
api.location.type to specify your own type:

%define api.location.type {LocationType}

The requirements over your LocationType are:

• it must be copyable;
• in order to compute the (default) value of @$ in a reduction, the parser basically runs

  @$.begin = @1.begin;
  @$.end   = @N.end; // The location of last right-hand side symbol.
  

  so there must be copyable begin and end members;

• alternatively you may redefine the computation of the default location, in which case these members are not required (see Location Default Action);
• if traces are enabled, then there must exist an ‘std::ostream&
  operator<< (std::ostream& o, const LocationType& s)’ function.

In programs with several C++ parsers, you may also use the %define variable api.location.type to share a common set of built-in definitions for position and location. For instance, one parser
master/parser.yy might use:

%defines
%locations
%define api.namespace {master::}

to generate the master/position.hh and master/location.hh files, reused by other parsers as follows:

%define api.location.type {master::location}
%code requires { #include <master/location.hh> }

10.1.4 C++ Parser Interface

The output files output.hh and output.cc declare and define the parser class in the namespace yy. The class name defaults to parser, but may be changed using ‘
%define parser_class_name {name}’. The interface of this class is detailed below. It can be extended using the
%parse-param feature: its semantics is slightly changed since it describes an additional member of the parser class, and an additional argument for its constructor.

Type of parser: semantic_type

Type of parser: location_type

The types for semantic values and locations (if enabled).

Type of parser: token

A structure that contains (only) the yytokentype enumeration, which defines the tokens. To refer to the token FOO, use yy::parser::token::FOO. The scanner can use ‘
typedef yy::parser::token token;’ to “import” the token enumeration (see Calc++ Scanner).

Type of parser: syntax_error

This class derives from std::runtime_error. Throw instances of it from the scanner or from the user actions to raise parse errors. This is equivalent with first invoking error to report the location and message
of the syntax error, and then to invoke YYERROR to enter the error-recovery mode. But contrary to YYERROR which can only be invoked from user actions (i.e., written in the action itself), the exception can
be thrown from function invoked from the user action.

Method on parser: parser (type1 arg1, …)

Build a new parser object. There are no arguments by default, unless ‘
%parse-param {type1 arg1}’ was used.

Method on syntax_error: syntax_error (const location_type& l, const std::string& m)

Method on syntax_error: syntax_error (const std::string& m)

Instantiate a syntax-error exception.

Method on parser: int parse ()

Run the syntactic analysis, and return 0 on success, 1 otherwise.

The whole function is wrapped in a try/catch block, so that when an exception is thrown, the %destructors are called to release the lookahead symbol, and the symbols pushed on the stack.

Method on parser: std::ostream& debug_stream ()

Method on parser: void set_debug_stream (std::ostream& o)

Get or set the stream used for tracing the parsing. It defaults to
std::cerr.

Method on parser: debug_level_type debug_level ()

Method on parser: void set_debug_level (debug_level l)

Get or set the tracing level. Currently its value is either 0, no trace, or nonzero, full tracing.

Method on parser: void error (const location_type& l, const std::string& m)

Method on parser: void error (const std::string& m)

The definition for this member function must be supplied by the user: the parser uses it to report a parser error occurring at l, described by m. If location tracking is not enabled, the second signature is used.

10.1.5 C++ Scanner Interface

The parser invokes the scanner by calling yylex. Contrary to C parsers, C++ parsers are always pure: there is no point in using the ‘
%define api.pure’ directive. The actual interface with yylex depends whether you use unions, or variants.

10.1.5.1 Split Symbols

The interface is as follows.

Method on parser: int yylex (semantic_type* yylval, location_type* yylloc, type1 arg1, …)

Method on parser: int yylex (semantic_type* yylval, type1 arg1, …)

Return the next token. Its type is the return value, its semantic value and location (if enabled) being yylval and yylloc. Invocations of ‘
%lex-param {type1 arg1}’ yield additional arguments.

Note that when using variants, the interface for yylex is the same, but yylval is handled differently.

Regular union-based code in Lex scanner typically look like:

[0-9]+   {
           yylval.ival = text_to_int (yytext);
           return yy::parser::INTEGER;
         }
[a-z]+   {
           yylval.sval = new std::string (yytext);
           return yy::parser::IDENTIFIER;
         }

Using variants, yylval is already constructed, but it is not initialized. So the code would look like:

[0-9]+   {
           yylval.build<int>() = text_to_int (yytext);
           return yy::parser::INTEGER;
         }
[a-z]+   {
           yylval.build<std::string> = yytext;
           return yy::parser::IDENTIFIER;
         }

or

[0-9]+   {
           yylval.build(text_to_int (yytext));
           return yy::parser::INTEGER;
         }
[a-z]+   {
           yylval.build(yytext);
           return yy::parser::IDENTIFIER;
         }

10.1.5.2 Complete Symbols

If you specified both %define api.value.type variant and
%define api.token.constructor, the parser class also defines the class parser::symbol_type which defines a complete symbol, aggregating its type (i.e., the traditional value returned by yylex),
its semantic value (i.e., the value passed in yylval, and possibly its location (yylloc).

Method on symbol_type: symbol_type (token_type type, const semantic_type& value, const location_type& location)

Build a complete terminal symbol which token type is type, and which semantic value is value. If location tracking is enabled, also pass the location.

This interface is low-level and should not be used for two reasons. First, it is inconvenient, as you still have to build the semantic value, which is a variant, and second, because consistency is not enforced: as with unions, it is still possible
to give an integer as semantic value for a string.

So for each token type, Bison generates named constructors as follows.

Method on symbol_type: make_ token (const value_type& value, const location_type& location)

Method on symbol_type: make_ token (const location_type& location)

Build a complete terminal symbol for the token type token (not including the api.token.prefix) whose possible semantic value is
value of adequate value_type. If location tracking is enabled, also pass the location.

For instance, given the following declarations:

%define api.token.prefix {TOK_}
%token <std::string> IDENTIFIER;
%token <int> INTEGER;
%token COLON;

Bison generates the following functions:

symbol_type make_IDENTIFIER(const std::string& v,
                            const location_type& l);
symbol_type make_INTEGER(const int& v,
                         const location_type& loc);
symbol_type make_COLON(const location_type& loc);

which should be used in a Lex-scanner as follows.

[0-9]+   return yy::parser::make_INTEGER(text_to_int (yytext), loc);
[a-z]+   return yy::parser::make_IDENTIFIER(yytext, loc);
":"      return yy::parser::make_COLON(loc);

Tokens that do not have an identifier are not accessible: you cannot simply use characters such as ':', they must be declared with %token.

10.1.6 A Complete C++ Example

This section demonstrates the use of a C++ parser with a simple but complete example. This example should be available on your system, ready to compile, in the directory …/bison/examples/calc++. It focuses on the use of Bison, therefore
the design of the various C++ classes is very naive: no accessors, no encapsulation of members etc. We will use a Lex scanner, and more precisely, a Flex scanner, to demonstrate the various interactions. A hand-written scanner is actually
easier to interface with.

10.1.6.1 Calc++ — C++ Calculator

Of course the grammar is dedicated to arithmetics, a single expression, possibly preceded by variable assignments. An environment containing possibly predefined variables such as
one and two, is exchanged with the parser. An example of valid input follows.

three := 3
seven := one + two * three
seven * seven

10.1.6.2 Calc++ Parsing Driver

To support a pure interface with the parser (and the scanner) the technique of the “parsing context” is convenient: a structure containing all the data to exchange. Since, in addition to simply launch the parsing, there are several
auxiliary tasks to execute (open the file for parsing, instantiate the parser etc.), we recommend transforming the simple parsing context structure into a fully blown
parsing driver class.

The declaration of this driver class, calc++-driver.hh, is as follows. The first part includes the CPP guard and imports the required standard library components, and the declaration of the parser class.

#ifndef CALCXX_DRIVER_HH
# define CALCXX_DRIVER_HH
# include <string>
# include <map>
# include "calc++-parser.hh"

Then comes the declaration of the scanning function. Flex expects the signature of yylex to be defined in the macro
YY_DECL, and the C++ parser expects it to be declared. We can factor both as follows.

// Tell Flex the lexer's prototype ...
# define YY_DECL 
  yy::calcxx_parser::symbol_type yylex (calcxx_driver& driver)
// ... and declare it for the parser's sake.
YY_DECL;

The calcxx_driver class is then declared with its most obvious members.

// Conducting the whole scanning and parsing of Calc++.
class calcxx_driver
{
public:
  calcxx_driver ();
  virtual ~calcxx_driver ();

  std::map<std::string, int> variables;

  int result;

To encapsulate the coordination with the Flex scanner, it is useful to have member functions to open and close the scanning phase.

  // Handling the scanner.
  void scan_begin ();
  void scan_end ();
  bool trace_scanning;

Similarly for the parser itself.

  // Run the parser on file F.
  // Return 0 on success.
  int parse (const std::string& f);
  // The name of the file being parsed.
  // Used later to pass the file name to the location tracker.
  std::string file;
  // Whether parser traces should be generated.
  bool trace_parsing;

To demonstrate pure handling of parse errors, instead of simply dumping them on the standard error output, we will pass them to the compiler driver using the following two member functions. Finally, we close the class declaration and CPP guard.

  // Error handling.
  void error (const yy::location& l, const std::string& m);
  void error (const std::string& m);
};
#endif // ! CALCXX_DRIVER_HH

The implementation of the driver is straightforward. The parse member function deserves some attention. The error functions are simple stubs, they should actually register the located error messages and set error state.

#include "calc++-driver.hh"
#include "calc++-parser.hh"

calcxx_driver::calcxx_driver ()
  : trace_scanning (false), trace_parsing (false)
{
  variables["one"] = 1;
  variables["two"] = 2;
}

calcxx_driver::~calcxx_driver ()
{
}

int
calcxx_driver::parse (const std::string &f)
{
  file = f;
  scan_begin ();
  yy::calcxx_parser parser (*this);
  parser.set_debug_level (trace_parsing);
  int res = parser.parse ();
  scan_end ();
  return res;
}

void
calcxx_driver::error (const yy::location& l, const std::string& m)
{
  std::cerr << l << ": " << m << std::endl;
}

void
calcxx_driver::error (const std::string& m)
{
  std::cerr << m << std::endl;
}

10.1.6.3 Calc++ Parser

The grammar file calc++-parser.yy starts by asking for the C++ deterministic parser skeleton, the creation of the parser header file, and specifies the name of the parser class. Because the C++ skeleton changed several times, it is
safer to require the version you designed the grammar for.

%skeleton "lalr1.cc" /* -*- C++ -*- */
%require "3.0.4"
%defines
%define parser_class_name {calcxx_parser}

This example will use genuine C++ objects as semantic values, therefore, we require the variant-based interface. To make sure we properly use it, we enable assertions. To fully benefit from type-safety and more natural definition of “symbol”,
we enable api.token.constructor.

%define api.token.constructor
%define api.value.type variant
%define parse.assert

Then come the declarations/inclusions needed by the semantic values. Because the parser uses the parsing driver and reciprocally, both would like to include the header of the other, which is, of course, insane. This mutual dependency will be broken
using forward declarations. Because the driver’s header needs detailed knowledge about the parser class (in particular its inner types), it is the parser’s header which will use a forward declaration of the driver. See %code Summary.

%code requires
{
# include <string>
class calcxx_driver;
}

The driver is passed by reference to the parser and to the scanner. This provides a simple but effective pure interface, not relying on global variables.

// The parsing context.
%param { calcxx_driver& driver }

Then we request location tracking, and initialize the first location’s file name. Afterward new locations are computed relatively to the previous locations: the file name will be propagated.

%locations
%initial-action
{
  // Initialize the initial location.
  @$.begin.filename = @$.end.filename = &driver.file;
};

Use the following two directives to enable parser tracing and verbose error messages. However, verbose error messages can contain incorrect information (see LAC).

%define parse.trace
%define parse.error verbose

The code between ‘%code {’ and ‘}’ is output in the
*.cc file; it needs detailed knowledge about the driver.

%code
{
# include "calc++-driver.hh"
}

The token numbered as 0 corresponds to end of file; the following line allows for nicer error messages referring to “end of file” instead of “$end”. Similarly user friendly names are provided for each symbol. To avoid name
clashes in the generated files (see Calc++ Scanner), prefix tokens with TOK_ (see api.token.prefix).

%define api.token.prefix {TOK_}
%token
  END  0  "end of file"
  ASSIGN  ":="
  MINUS   "-"
  PLUS    "+"
  STAR    "*"
  SLASH   "/"
  LPAREN  "("
  RPAREN  ")"
;

Since we use variant-based semantic values, %union is not used, and both %type and %token expect genuine types, as opposed to type tags.

%token <std::string> IDENTIFIER "identifier"
%token <int> NUMBER "number"
%type  <int> exp

No %destructor is needed to enable memory deallocation during error recovery; the memory, for strings for instance, will be reclaimed by the regular destructors. All the values are printed using their
operator<< (see Printing Semantic Values).

%printer { yyoutput << $$; } <*>;

The grammar itself is straightforward (see Location Tracking Calculator — ltcalc).

%%
%start unit;
unit: assignments exp  { driver.result = $2; };

assignments:
  %empty                 {}
| assignments assignment {};

assignment:
  "identifier" ":=" exp { driver.variables[$1] = $3; };

%left "+" "-";
%left "*" "/";
exp:
  exp "+" exp   { $$ = $1 + $3; }
| exp "-" exp   { $$ = $1 - $3; }
| exp "*" exp   { $$ = $1 * $3; }
| exp "/" exp   { $$ = $1 / $3; }
| "(" exp ")"   { std::swap ($$, $2); }
| "identifier"  { $$ = driver.variables[$1]; }
| "number"      { std::swap ($$, $1); };
%%

Finally the error member function registers the errors to the driver.

void
yy::calcxx_parser::error (const location_type& l,
                          const std::string& m)
{
  driver.error (l, m);
}

10.1.6.4 Calc++ Scanner

The Flex scanner first includes the driver declaration, then the parser’s to get the set of defined tokens.

%{ /* -*- C++ -*- */
# include <cerrno>
# include <climits>
# include <cstdlib>
# include <string>
# include "calc++-driver.hh"
# include "calc++-parser.hh"

// Work around an incompatibility in flex (at least versions
// 2.5.31 through 2.5.33): it generates code that does
// not conform to C89.  See Debian bug 333231
// <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=333231>.
# undef yywrap
# define yywrap() 1

// The location of the current token.
static yy::location loc;
%}

Because there is no #include-like feature we don’t need
yywrap, we don’t need unput either, and we parse an actual file, this is not an interactive session with the user. Finally, we enable scanner tracing.

%option noyywrap nounput batch debug noinput

Abbreviations allow for more readable rules.

id    [a-zA-Z][a-zA-Z_0-9]*
int   [0-9]+
blank [ t]

The following paragraph suffices to track locations accurately. Each time yylex is invoked, the begin position is moved onto the end position. Then when a pattern is matched, its width is added to the end column. When matching ends
of lines, the end cursor is adjusted, and each time blanks are matched, the begin cursor is moved onto the end cursor to effectively ignore the blanks preceding tokens. Comments would be treated equally.

%{
  // Code run each time a pattern is matched.
  # define YY_USER_ACTION  loc.columns (yyleng);
%}

%%

%{
  // Code run each time yylex is called.
  loc.step ();
%}

{blank}+   loc.step ();
[n]+      loc.lines (yyleng); loc.step ();

The rules are simple. The driver is used to report errors.

"-"      return yy::calcxx_parser::make_MINUS(loc);
"+"      return yy::calcxx_parser::make_PLUS(loc);
"*"      return yy::calcxx_parser::make_STAR(loc);
"/"      return yy::calcxx_parser::make_SLASH(loc);
"("      return yy::calcxx_parser::make_LPAREN(loc);
")"      return yy::calcxx_parser::make_RPAREN(loc);
":="     return yy::calcxx_parser::make_ASSIGN(loc);

{int}      {
  errno = 0;
  long n = strtol (yytext, NULL, 10);
  if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
    driver.error (loc, "integer is out of range");
  return yy::calcxx_parser::make_NUMBER(n, loc);
}

{id}       return yy::calcxx_parser::make_IDENTIFIER(yytext, loc);
.          driver.error (loc, "invalid character");
<<EOF>>    return yy::calcxx_parser::make_END(loc);
%%

Finally, because the scanner-related driver’s member-functions depend on the scanner’s data, it is simpler to implement them in this file.

void
calcxx_driver::scan_begin ()
{
  yy_flex_debug = trace_scanning;
  if (file.empty () || file == "-")
    yyin = stdin;
  else if (!(yyin = fopen (file.c_str (), "r")))
    {
      error ("cannot open " + file + ": " + strerror(errno));
      exit (EXIT_FAILURE);
    }
}

void
calcxx_driver::scan_end ()
{
  fclose (yyin);
}

10.1.6.5 Calc++ Top Level

The top level file, calc++.cc, poses no problem.

#include <iostream>
#include "calc++-driver.hh"

int
main (int argc, char *argv[])
{
  int res = 0;
  calcxx_driver driver;
  for (int i = 1; i < argc; ++i)
    if (argv[i] == std::string ("-p"))
      driver.trace_parsing = true;
    else if (argv[i] == std::string ("-s"))
      driver.trace_scanning = true;
    else if (!driver.parse (argv[i]))
      std::cout << driver.result << std::endl;
    else
      res = 1;
  return res;
}

10.2 Java Parsers

10.2.1 Java Bison Interface

(The current Java interface is experimental and may evolve. More user feedback will help to stabilize it.)

The Java parser skeletons are selected using the %language "Java" directive or the -L java/—language=java option.

When generating a Java parser, bison basename.y will create a single Java source file named basename.java containing the parser implementation. Using a grammar file without a
.y suffix is currently broken. The basename of the parser implementation file can be changed by the %file-prefix directive or the -p/—name-prefix option. The entire parser implementation
file name can be changed by the
%output directive or the -o/—output option. The parser implementation file contains a single class for the parser.

You can create documentation for generated parsers using Javadoc.

Contrary to C parsers, Java parsers do not use global variables; the state of the parser is always local to an instance of the parser class. Therefore, all Java parsers are “pure”, and the %pure-parser and %define api.pure directives do nothing when used in Java.

Push parsers are currently unsupported in Java and %define
api.push-pull have no effect.

GLR parsers are currently unsupported in Java. Do not use the
glr-parser directive.

No header file can be generated for Java parsers. Do not use the
%defines directive or the -d/—defines options.

Currently, support for tracing is always compiled in. Thus the ‘%define parse.trace’ and ‘%token-table’ directives and the
-t/—debug and -k/—token-table options have no effect. This may change in the future to eliminate unused code in the generated parser, so use ‘%define parse.trace’
explicitly if needed. Also, in the future the
%token-table directive might enable a public interface to access the token names and codes.

Getting a “code too large” error from the Java compiler means the code hit the 64KB bytecode per method limitation of the Java class file. Try reducing the amount of code in actions and static initializers; otherwise, report a bug
so that the parser skeleton will be improved.

10.2.2 Java Semantic Values

There is no %union directive in Java parsers. Instead, the semantic values’ types (class names) should be specified in the
%type or %token directive:

%type <Expression> expr assignment_expr term factor
%type <Integer> number

By default, the semantic stack is declared to have Object members, which means that the class types you specify can be of any class. To improve the type safety of the parser, you can declare the common superclass of all the semantic
values using the ‘%define api.value.type’ directive. For example, after the following declaration:

%define api.value.type {ASTNode}

any %type or %token specifying a semantic type which is not a subclass of ASTNode, will cause a compile-time error.

Types used in the directives may be qualified with a package name. Primitive data types are accepted for Java version 1.5 or later. Note that in this case the autoboxing feature of Java 1.5 will be used. Generic types may not be used; this is
due to a limitation in the implementation of Bison, and may change in future releases.

Java parsers do not support %destructor, since the language adopts garbage collection. The parser will try to hold references to semantic values for as little time as needed.

Java parsers do not support %printer, as toString() can be used to print the semantic values. This however may change (in a backwards-compatible way) in future versions of Bison.

10.2.3 Java Location Values

When the directive %locations is used, the Java parser supports location tracking, see Tracking Locations. An auxiliary user-defined class defines a position, a single point in a file; Bison
itself defines a class representing a location, a range composed of a pair of positions (possibly spanning several files). The location class is an inner class of the parser; the name is Location by default, and may also
be renamed using %define api.location.type {class-name}.

The location class treats the position as a completely opaque value. By default, the class name is Position, but this can be changed with %define api.position.type {class-name}. This class must be supplied
by the user.

Instance Variable of Location: Position begin

Instance Variable of Location: Position end

The first, inclusive, position of the range, and the first beyond.

Constructor on Location: Location (Position loc)

Create a Location denoting an empty range located at a given point.

Constructor on Location: Location (Position begin, Position end)

Create a Location from the endpoints of the range.

Method on Location: String toString ()

Prints the range represented by the location. For this to work properly, the position class should override the equals and
toString methods appropriately.

10.2.4 Java Parser Interface

The name of the generated parser class defaults to YYParser. The
YY prefix may be changed using the %name-prefix directive or the -p/—name-prefix option. Alternatively, use ‘
%define parser_class_name {name}’ to give a custom name to the class. The interface of this class is detailed below.

By default, the parser class has package visibility. A declaration ‘
%define public’ will change to public visibility. Remember that, according to the Java language specification, the name of the .java file should match the name of the class in this case. Similarly, you can use
abstract,
final and strictfp with the
%define declaration to add other modifiers to the parser class. A single ‘%define annotations {annotations}’ directive can be used to add any number of annotations to the parser class.

The Java package name of the parser class can be specified using the ‘
%define package’ directive. The superclass and the implemented interfaces of the parser class can be specified with the %define
extends and ‘%define implements’ directives.

The parser class defines an inner class, Location, that is used for location tracking (see Java Location Values), and a inner interface, Lexer (see Java Scanner Interface).
Other than these inner class/interface, and the members described in the interface below, all the other members and fields are preceded with a yy or
YY prefix to avoid clashes with user code.

The parser class can be extended using the %parse-param directive. Each occurrence of the directive will add a protected
final field to the parser class, and an argument to its constructor, which initialize them automatically.

Constructor on YYParser: YYParser (lex_param, …, parse_param, …)

Build a new parser object with embedded %code lexer. There are no parameters, unless %params and/or %parse-params and/or
%lex-params are used.

Use %code init for code added to the start of the constructor body. This is especially useful to initialize superclasses. Use ‘
%define init_throws’ to specify any uncaught exceptions.

Constructor on YYParser: YYParser (Lexer lexer, parse_param, …)

Build a new parser object using the specified scanner. There are no additional parameters unless %params and/or %parse-params are used.

If the scanner is defined by %code lexer, this constructor is declared protected and is called automatically with a scanner created with the correct %params and/or %lex-params.

Use %code init for code added to the start of the constructor body. This is especially useful to initialize superclasses. Use ‘
%define init_throws’ to specify any uncaught exceptions.

Method on YYParser: boolean parse ()

Run the syntactic analysis, and return true on success,
false otherwise.

Method on YYParser: boolean getErrorVerbose ()

Method on YYParser: void setErrorVerbose (boolean verbose)

Get or set the option to produce verbose error messages. These are only available with ‘%define parse.error verbose’, which also turns on verbose error messages.

Method on YYParser: void yyerror (String msg)

Method on YYParser: void yyerror (Position pos, String msg)

Method on YYParser: void yyerror (Location loc, String msg)

Print an error message using the yyerror method of the scanner instance in use. The Location and Position parameters are available only if location tracking is active.

Method on YYParser: boolean recovering ()

During the syntactic analysis, return true if recovering from a syntax error. See Error Recovery.

Method on YYParser: java.io.PrintStream getDebugStream ()

Method on YYParser: void setDebugStream (java.io.printStream o)

Get or set the stream used for tracing the parsing. It defaults to
System.err.

Method on YYParser: int getDebugLevel ()

Method on YYParser: void setDebugLevel (int l)

Get or set the tracing level. Currently its value is either 0, no trace, or nonzero, full tracing.

Constant of YYParser: String bisonVersion

Constant of YYParser: String bisonSkeleton

Identify the Bison version and skeleton used to generate this parser.

10.2.5 Java Scanner Interface

There are two possible ways to interface a Bison-generated Java parser with a scanner: the scanner may be defined by %code lexer, or defined elsewhere. In either case, the scanner has to implement the
Lexer inner interface of the parser class. This interface also contain constants for all user-defined token names and the predefined
EOF token.

In the first case, the body of the scanner class is placed in
%code lexer blocks. If you want to pass parameters from the parser constructor to the scanner constructor, specify them with
%lex-param; they are passed before %parse-params to the constructor.

In the second case, the scanner has to implement the Lexer interface, which is defined within the parser class (e.g., YYParser.Lexer). The constructor of the parser object will then accept an object implementing the interface;
%lex-param is not used in this case.

In both cases, the scanner has to implement the following methods.

Method on Lexer: void yyerror (Location loc, String msg)

This method is defined by the user to emit an error message. The first parameter is omitted if location tracking is not active. Its type can be changed using %define api.location.type {class-name}.

Method on Lexer: int yylex ()

Return the next token. Its type is the return value, its semantic value and location are saved and returned by the their methods in the interface.

Use ‘%define lex_throws’ to specify any uncaught exceptions. Default is java.io.IOException.

Method on Lexer: Position getStartPos ()

Method on Lexer: Position getEndPos ()

Return respectively the first position of the last token that
yylex returned, and the first position beyond it. These methods are not needed unless location tracking is active.

The return type can be changed using %define api.position.type
{class-name}.

Method on Lexer: Object getLVal ()

Return the semantic value of the last token that yylex returned.

The return type can be changed using ‘%define api.value.type
{class-name}’.

10.2.6 Special Features for Use in Java Actions

The following special constructs can be uses in Java actions. Other analogous C action features are currently unavailable for Java.

Use ‘%define throws’ to specify any uncaught exceptions from parser actions, and initial actions specified by %initial-action.

Variable: $ n

The semantic value for the nth component of the current rule. This may not be assigned to. See Java Semantic Values.

Variable: $< typealt>n

Like $n but specifies a alternative type typealt. See Java Semantic Values.

Variable: $$

The semantic value for the grouping made by the current rule. As a value, this is in the base type (Object or as specified by ‘
%define api.value.type’) as in not cast to the declared subtype because casts are not allowed on the left-hand side of Java assignments. Use an explicit Java cast if the correct subtype is needed. See Java Semantic Values.

Variable: $< typealt>$

Same as $$ since Java always allow assigning to the base type. Perhaps we should use this and $<>$ for the value and $$ for setting the value but there is currently no easy way to distinguish
these constructs. See Java Semantic Values.

Variable: @ n

The location information of the nth component of the current rule. This may not be assigned to. See Java Location Values.

Variable: @ $

The location information of the grouping made by the current rule. See Java Location Values.

Statement: return YYABORT ;

Return immediately from the parser, indicating failure. See Java Parser Interface.

Statement: return YYACCEPT ;

Return immediately from the parser, indicating success. See Java Parser Interface.