GDScript: Add type hints on Callable

[dass2608]

I think most people agree that type hints can help to reduce runtime errors. GDScript type hints can uncover many errors early, before even running the code. Callables are also a useful feature, but sadly, they lack type hints.
I think it would be great, if Callables could have type annotations. As GDScript is inspired by Python, I would consider a syntax similar to Python:

var my_add_lambda: Callable[[int, int] int] = func(a, b):
    return a + b

This syntax could also be used in function parameters:

func my_function_with_callback(callback: Callable[[int, int], int]):
    ...

A annotation directly on the function definition might also make sense:

some_signal.connect(func(a:int, b:int) -> int: return a + b)

[azur-wolve]

using [] could lead to confusion with arrays
what about

var whatever: Callable((int, int) -> int)

[AThousandShips]

How would that be confusing? You can't use float[] in GDScript, so there's no confusion without the Array[float] part

[azur-wolve]

still can be confusing for users that come from other languages (C/C++, C#, Java, etc)
not showstopping though

using ()isnt good idea either, easy confusion with explicit precedence

[AThousandShips]

Fair enough, but those are entirely unrelated and different languages so there shouldn't be any more confusion than, for example, that we don't use { for blocks etc.

[dalexeev]

The problem is that GDScript supports dynamic typing, so in general this requires runtime checks, similar to how typed arrays are implemented. Also, this would affect not only GDScript, but also the core. Godot doesn't have a unified type system, if you expect something like:

class Array[T]

func filter(method: Callable[[T], bool]) -> Array[T]
func map[TOther](method: Callable[[T], TOther]) -> Array[TOther]
func reduce[TAccum](method: Callable[[TAccum, T], TAccum], accum: TAccum|Nil = null) -> TAccum