I'm new to Godot / GDScript. Coming from C# and Typescript typing is something I hold in high regard, and find some frustrations in GDScripts handling of it. Just tried a small rewrite of this to remove the need for the as Callable casts, and here's what i came up with.

# honestly just using this class to get around 'no nested typed containers' error. 
# Curious if there's a cleaner / less bulky way to do this
class_name State
extends RefCounted

var normal: Callable
var enter: Callable
var leave: Callable

func _init(normal: Callable, enter: Callable, leave: Callable):
	self.normal = normal
	self.enter = normal
	self.leave = normal

class_name CallableStateMachine

var state_dictionary: Dictionary[String, State] = {}
var current_state: String

func add_state(
	normal_state_callable: Callable,
	enter_state_callable: Callable,
	leave_state_callable: Callable
):
	state_dictionary[normal_state_callable.get_method()] = \
			State.new(normal_state_callable, enter_state_callable, leave_state_callable)

func set_initial_state(state_callable: Callable):
	var state_name = state_callable.get_method()
	if state_dictionary.has(state_name):
		_set_state(state_name)
	else:
		push_warning("No state with name " + state_name)


func update():
	if current_state != "":  #null here was a bug. Strings default to "", not null
		state_dictionary[current_state].normal.call()


func change_state(state_callable: Callable):
	var state_name = state_callable.get_method()
	if state_dictionary.has(state_name):
		_set_state.call_deferred(state_name)
	else:
		push_warning("No state with name " + state_name)


func _set_state(state_name: String):
	if current_state:
		var leave_callable = state_dictionary[current_state].leave
		if leave_callable.is_valid():
			leave_callable.call()
	
	current_state = state_name
	var enter_callable = state_dictionary[current_state].enter
	if enter_callable.is_valid():
		enter_callable.call()

Heya, if you're still working with this I'm running into an issue where the "enter" and "leave" callables are still being called when using await timer randf_range to determine when to transition. Any suggestions? Found the problem, I had another part of the script executing functions that shouldn't have been executed.

@mbnewsom All your code can be in a single script. I've updated it below, also fixed the typo in the State._init declaration.

I'll probably update this code further once I test it with the new variadic arguments, so we will be allowed to declare this as a single external Node, extend its script to declare the states and give arguments to .update() and .state_change(), allowing transient variables, like delta and controller inputs, being passed into, instead of holding global references, to access these values inside the state functions.

# callable_state_machine.gd
class_name CallableStateMachine


class_name State

    var normal: Callable
    var enter: Callable
    var leave: Callable

    func _init(normal: Callable, enter: Callable, leave: Callable):
        self.normal = normal
        self.enter = enter
        self.leave = leave


var state_dictionary: Dictionary[String, State] = {}
var current_state: String

func add_state(
    normal_state_callable: Callable,
    enter_state_callable: Callable,
    leave_state_callable: Callable
):
    state_dictionary[normal_state_callable.get_method()] = \
        State.new(normal_state_callable, enter_state_callable, leave_state_callable)

func set_initial_state(state_callable: Callable):
    var state_name = state_callable.get_method()
    if state_dictionary.has(state_name):
        _set_state(state_name)
    else:
        push_warning("No state with name " + state_name)


func update():
    if current_state != "":  #null here was a bug. Strings default to "", not null
        state_dictionary[current_state].normal.call()


func change_state(state_callable: Callable):
    var state_name = state_callable.get_method()
    if state_dictionary.has(state_name):
        _set_state.call_deferred(state_name)
    else:
        push_warning("No state with name " + state_name)


func _set_state(state_name: String):
    if current_state:
        var leave_callable = state_dictionary[current_state].leave
        if leave_callable.is_valid():
	    leave_callable.call()
	
    current_state = state_name
    var enter_callable = state_dictionary[current_state].enter
    if enter_callable.is_valid():
        enter_callable.call()