GDScript: Transparent funcref and _call

[poke1024]

On my 3.1 wish list for gdscript: make funcref transparent.

This PR is a proof of concept, enabling everything you'd expect:

Variant 1:

func my_print(text):
	print("my_print ", text)

func f2(f):
	f("ok!")
	
func _ready():
	f2(self.my_print)

>> my_print ok!

Variant 2:

class A:
	func f(text):
		print("my_print ", text)
		
class B:
	var work
	
func do_it(b):
	b.work("ok2!")
	
func _ready():
	var a = A.new()
	var b = B.new()
	b.work = a.f
	
	do_it(b)

>> my_print ok2!

It's still a funcref underneath, but the syntax let's you use them like first-class values.

[Zylann]

Wasn't this also adressed in a more general way by the lambda PR? (which sadly sunk into limbo twice^^")

[poke1024]

I found the lambda PR now you mentioned it. This PR here is a much smaller change. Lambdas could be added on top of this as a special kind of FuncRef.

[Geequlim]

Who is the self when you call b.work in your Variant 2 ?

[HummusSamurai]

@Geequlim it's the implicit "Class" of the file (in OOP terms, much like Python).

[poke1024]

@Geequlim Yes, it's just the current class instance itself. This could be amended to allow for writing just the name of any class method (i.e. f2(my_print)), but on the other hand I think writing self.some_method is actually not so bad for indicating that a function ref is taken here.

[poke1024]

I've revised the underlying implementation for this one. Variant 1 and variant 2 above are unchanged. In addition, GDScript users can now access the underlying mechanism that is used for funcref calling by implementing a _call method similar to Python's __call__ method:

class Addition:
	var x
	
	func _init(p_x):
		x = p_x	
	
	func _call(p_y):
		return x + p_y

func _ready():
	var x = Addition.new(7)
	print(x(3))

> 10

This new implementation is cleaner. While the old implementation had several core changes and an opcode change, the new implementation has only one very small change to funcref (adding _call).

[akien-mga]

As we discussed together in our IRC meeting on June 20 (http://godot.eska.me/irc-logs/devel/2018-06-20.log from 21:10), this should be added in Godot 3.2 following the GDScript roadmap that we agreed upon: godotengine/godot-roadmap#23.

I'll close this PR for now, but it will likely be used as a reference for implementing this feature which is planned on the roadmap (and gladly by you or with your help :)).