CODEWITHSUNDEEP
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Patrick Bassut
Patrick Bassut

Reputation: 3338

setting function prototype in Lua

A simple question: How do I set the prototype for a function that has not been implemented yet?

I just want to do this, cause I'm referring to a function that does not exist(yet). In C, we would do something like this:

int foo(int bar);

int myint = foo(1);

int foo(int bar)
{
     return bar;
}

How do I do this in Lua (with corona)?

Upvotes: 5

Views: 4543

Answers (1)

Mud
Mud

Reputation: 29000

You can't. Amber's comment is correct.

Lua doesn't have a concept of type signatures or function prototypes.

The type of foo is that of the object it contains, which is dynamic, changing at runtime. It could be function in one instant, and string or integer or something else in the next.

Conceptually Lua doesn't have a compilation step like C. When you say "run this code" it starts starts executing instructions at the top and works it's way down. In practice, Lua first compiles your code into bytecode before executing it, but the compiler won't balk at something like this:

greet()

function greet()
    print('Hello.')
end

Because the value contained in greet is determined at runtime. It's only when you actually try to call (i.e. invoke like a function) the value in greet, at runtime, that Lua will discover that it doesn't contain a callable value (a function or a table/userdata with a metatable containing a __call member) and you'll get a runtime error: "attempt to call global 'greet' (a nil value)". Where "nil value" is whatever value greet contained at the time the call was attempted. In our case, it was nil.

So you will have to make sure that that the code that creates a function and assigns it to foo is called before you attempt to call foo.

It might help if you recognize that this:

local myint = foo(1)

function foo(bar)
     return bar
end

Is syntax sugar for this:

local myint = foo(1)

foo = function(bar)
     return bar
end

foo is being assigned a function value. That has to happen before you attempt to call that function.

The most common solution to this problem is to treat the file's function as the "compilation time", that is: declare all of your constant data and functions when the file is executed, ready to be used during "execution time". Then, call a main function to begin the "execution time".

For example:

function main()
    greet()
end

function greet()
    print('Hello.')
end

main()

As greet has been declared in _G, main can access it.

Upvotes: 8

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