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Generic functions

Tell me your size, I'll tell you which type you are

Generic functions are functions that take arguments that have generic types.

Generic types are types we don't know the size at compile-time.

We don't know their size at compile time, because they could be any type.

Because they could be any type, and thus any size (e.g. a 32bits int, a 64bits pointer, or a larger type such as a struct), we can't pass them directly as an argument. Instead, we pass a pointer to them.

C allows pointers to pretty much anything. When, at runtime, we know the type of the generic variable, we can cast the pointer and then dereference it.

Which allows thing like:

print: func <T> (value: T) {
    if(T == Int) {
        printf("%d\n", value as Int)
    } else {
        printf("Unknown type %s\n", T name)
    }
}

to work.

If we call print(42), it'll print "42", and if we call for example print(3.14) it'll print "Unknown type Float"

There are several things going on here.

  • We declare a generic type, e.g. 'T'. It means that the real signature of the function is:

    print: func (T: Class, value: Pointer)
    
  • We compare T and Int. How it is possible? As pointed just above, 'T' is in fact a class. And Int is the class of an Int, too so we can just go ahead and compare them

  • We cast value to Int. How is it done in the C? Well, value is a void, so we just cast it to int and dereference it. We'd write it like this in ooc:

    (value as Int*)@
    

What about each?

If you've read all of the above, you now probably know why a naive each like this:

[1, 2, 3, 4] as ArrayList each(|x| x toString() println())

Won't work.

each() except a function whose signature is:

func <T> (value: T)

ie whose real signature is:

func (T: Class, value: Pointer)

And we pass it a function whose signature is:

func (value: Int)

That simply doesn't work.

What needs to be done here, is we need to generate an intermediate function that does:

func <T> (value: T) { otherFunction(value as Int) }

The 'value as Int' part is just a Cast AST node. 'Int' is a BaseType.

Constructing a generic function is just adding typeArgs to it. TypeArgs are usually VariableAccesses.

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