Lua throw error

Need for Error Handling

Error handling is quite critical since real-world operations often require the use of complex operations, which includes file operations, database transactions and web service calls.

In any programming, there is always a requirement for error handling. Errors can be of two types which includes,

• Syntax errors
• Run time errors

Syntax Errors

Syntax errors occur due to improper use of various program components like operators and expressions. A simple example for syntax error is shown below.

a == 2

As you know, there is a difference between the use of a single «equal to» and double «equal to». Using one instead of the other can lead to an error. One «equal to» refers to assignment while a double «equal to» refers to comparison. Similarly, we have expressions and functions having their predefined ways of implementation.

Another example for syntax error is shown below −

for a= 1,10
   print(a)
end

When we run the above program, we will get the following output −

lua: test2.lua:2: 'do' expected near 'print'

Syntax errors are much easier to handle than run time errors since, the Lua interpreter locates the error more clearly than in case of runtime error. From the above error, we can know easily that adding a do statement before print statement is required as per the Lua structure.

Run Time Errors

In case of runtime errors, the program executes successfully, but it can result in runtime errors due to mistakes in input or mishandled functions. A simple example to show run time error is shown below.

function add(a,b)
   return a+b
end

add(10)

When we build the program, it will build successfully and run. Once it runs, shows a run time error.

lua: test2.lua:2: attempt to perform arithmetic on local 'b' (a nil value)
stack traceback:
	test2.lua:2: in function 'add'
	test2.lua:5: in main chunk
	[C]: ?

This is a runtime error, which had occurred due to not passing two variables. The b parameter is expected and here it is nil and produces an error.

Assert and Error Functions

In order to handle errors, we often use two functions − assert and error. A simple example is shown below.

local function add(a,b)
   assert(type(a) == "number", "a is not a number")
   assert(type(b) == "number", "b is not a number")
   return a+b
end

add(10)

When we run the above program, we will get the following error output.

lua: test2.lua:3: b is not a number
stack traceback:
	[C]: in function 'assert'
	test2.lua:3: in function 'add'
	test2.lua:6: in main chunk
	[C]: ?

The error (message [, level]) terminates the last protected function called and returns message as the error message. This function error never returns. Usually, error adds some information about the error position at the beginning of the message. The level argument specifies how to get the error position. With level 1 (the default), the error position is where the error function was called. Level 2 points the error to where the function that called error was called; and so on. Passing a level 0 avoids the addition of error position information to the message.

pcall and xpcall

In Lua programming, in order to avoid throwing these errors and handling errors, we need to use the functions pcall or xpcall.

The pcall (f, arg1, …) function calls the requested function in protected mode. If some error occurs in function f, it does not throw an error. It just returns the status of error. A simple example using pcall is shown below.

function myfunction ()
   n = n/nil
end

if pcall(myfunction) then
   print("Success")
else
	print("Failure")
end

When we run the above program, we will get the following output.

Failure

The xpcall (f, err) function calls the requested function and also sets the error handler. Any error inside f is not propagated; instead, xpcall catches the error, calls the err function with the original error object, and returns a status code.

A simple example for xpcall is shown below.

function myfunction ()
   n = n/nil
end

function myerrorhandler( err )
   print( "ERROR:", err )
end

status = xpcall( myfunction, myerrorhandler )
print( status)

When we run the above program, we will get the following output.

ERROR:	test2.lua:2: attempt to perform arithmetic on global 'n' (a nil value)
false

As a programmer, it is most important to ensure that you take care of proper error handling in the programs you write. Using error handling can ensure that unexpected conditions beyond the boundary conditions are handled without disturbing the user of the program.

Using pcall

pcall stands for «protected call». It is used to add error handling to functions. pcall works similar as try-catch in other languages. The advantage of pcall is that the whole execution of the script is not being interrupted if errors occur in functions called with pcall. If an error inside a function called with pcall occurs an error is thrown and the rest of the code continues execution.

Syntax:

pcall( f , arg1,···)

Return Values:

Returns two values

1. status (boolean)

• Returns true if the function was executed with no errors.
• Returns false if an error occured inside the function.

1. return value of the function or error message if an error occurred inside the function block.

pcall may be used for various cases, however a common one is to catch errors from the function which has been given to your function. For instance, lets say we have this function:

local function executeFunction(funcArg, times) then
    for i = 1, times do
        local ran, errorMsg = pcall( funcArg )
        if not ran then
            error("Function errored on run " .. tostring(i) .. "n" .. errorMsg)
        end
    end
end

When the given function errors on run 3, the error message will be clear to the user that it is not coming from your function, but from the function which was given to our function. Also, with this in mind a fancy BSoD can be made notifying the user. However, that is up to the application which implements this function, as an API most likely won’t be doing that.

Example A — Execution without pcall

function square(a)
  return a * "a"    --This will stop the execution of the code and throws an error, because of the attempt to perform arithmetic on a string value
end

square(10);

print ("Hello World")    -- This is not being executed because the script was interrupted due to the error

Example B — Execution with pcall

function square(a)
  return a * "a"
end

local status, retval = pcall(square,10);

print ("Status: ", status)        -- will print "false" because an error was thrown.
print ("Return Value: ", retval)  -- will print "input:2: attempt to perform arithmetic on a string value"
print ("Hello World")    -- Prints "Hello World"

Example — Execution of flawless code

function square(a)
  return a * a
end

local status, retval = pcall(square,10);

print ("Status: ", status)        -- will print "true" because no errors were thrown 
print ("Return Value: ", retval)  -- will print "100"
print ("Hello World")    -- Prints "Hello World"

Handling errors in Lua

Assuming we have the following function:

function foo(tab)
  return tab.a
end -- Script execution errors out w/ a stacktrace when tab is not a table

Let’s improve it a bit

function foo(tab)
  if type(tab) ~= "table" then
    error("Argument 1 is not a table!", 2)
  end
  return tab.a
end -- This gives us more information, but script will still error out

If we don’t want a function to crash a program even in case of an error, it is standard in lua to do the following:

function foo(tab)
    if type(tab) ~= "table" then return nil, "Argument 1 is not a table!" end
    return tab.a
end -- This never crashes the program, but simply returns nil and an error message

Now we have a function that behaves like that, we can do things like this:

if foo(20) then print(foo(20)) end -- prints nothing
result, error = foo(20)
if result then print(result) else log(error) end

And if we DO want the program to crash if something goes wrong, we can still do this:

result, error = foo(20)
if not result then error(error) end

Fortunately we don’t even have to write all that every time; lua has a function that does exactly this

result = assert(foo(20))