Functional programming concepts. And applications on GDScript

Transcription

1 Functional programming concepts And applications on GDScript

2 George Marques Godot contributor since 2015 Member of the project s leadership since 2017 Worked at Javary Games and IMVU (using Godot) Hired by Godot since November 2019 Co-author of the book Godot Engine Game Development in 24 Hours, published by Pearson Responsible for the UWP port and typed GDScript

3 Topics Introduction to functional programming Monad Functional programming characteristics Functions Applications (in general and on GDScript) Other resources

4 What is functional programming? Everything is a function Declarative instead of imperative Application of mathematical concepts Avoids changing external state (pure functions) Functions are also values (higher-order functions)

5 Let s talk about monad? A monad is a monoid in an endofunctor category - Rúnar Bjarnason

6 Or will we? Okay, let s not talk about monad

7 Disadvantages Declarative code is not always the most readable It s hard to combine pure functions and intended side-effects Immutability and recursion can lead to performance penalties The functional paradigm avoids loops in favor of recursion

8 Vantages Implementation of pure functions are easier to understand Looking at a function signature may be enough to know what it does Easier to run things in parallel (no need for locks) Testing functions in isolation is more straightforward Also easier to debug

9 Side effects By definition: changes in objects outside the function scope Change of global state Input and output Yes, this includes keyboard input and video output Changes in the object that contains the function (instance members)

10 Immutability Variables are treated as constants Forbidden to re-assign a value to a variable Forbidden to change properties of an object (Okay to change a local object for the sake of initialization) Instead of changes, make copies func nope(obj_to_change: Object) -> void func yep(obj_to_update: Object) -> Object

11 Pure function Don t have side effects All parameters are immutable A call with the same arguments return the same value every time Similar to the mathematical definition

12 First-class functions Function is also a value It can be assigned to a variable It can be passed as arguments to other functions It can be the return value of a function Everything that can be done with a value can also be done with a function Including putting them in arrays and dictionaries

13 First-class functions in GDScript Well, we re not there yet, but will soon

14 FuncRef to the rescue Exclusive Godot type that allows a function reference Then you can treat the reference as a value And call it later (with arguments) Making it in GDScript: Calling: var my_func = funcref(self, "my_method") my_func.call_func("an argument")

15 Higher-order functions Functions are first-order by default They re higher-order if they receive a function as an argument Or return a function as a result It s not mandatory that they are pure But we like them more if they are

16 Higher-order functions Examples map filter reduce Still not available in GDScript for now, but not hard to implement in script

17 Map Take an array and a unary function, returns a copy of the array with the function applied to every element Signature: func map(input: Array, function: FuncRef) -> Array Equivalent to: var my_arr := [1, 2, 3] var mapped := [] for element in my_arr: mapped.append(some_func(element))

18 Map Example 1 func half(num: int) -> int: return num / 2 func half_array(numbers: Array) -> Array: return map(numbers, funcref(self, "half")) func _ready() -> void: var numbers := [2, 4, 6] var halves := half_array(numbers) # [1, 2, 3]

19 Map Example 2 func get_id(user: User) -> int: return user.id func list_user_ids() -> Array: var users: Array = fetch_users() return map(users, funcref(self, "get_id"))

20 Map Possible implementation func map(input: Array, function: FuncRef) -> Array: var result := [] result.resize(input.size()) for i in range(input.size()): result[i] = function.call_func(input[i])

21 Filter Takes an array and a unary function, returns a new array with only the elements to which the function returns true Signature: func filter(input: Array, function: FuncRef) -> Array Equivalent to: var my_arr := [1, 2, 3] var filtered := [] for element in my_arr: if some_func(element): filtered.append(element)

22 Filter Example 1 func is_even(num: int) -> bool: return num % 2 == 0 func get_evens(numbers: Array) -> Array: return filter(numbers, funcref(self, "is_even")) func _ready() -> void: var numbers := [1, 2, 3, 4] var evens := get_evens(numbers) # [2, 4]

23 Filter Example 2 func is_active(user: User) -> bool: return user.active func list_active_users() -> Array var users: Array = fetch_users() return filter(users, funcref(self, "is_active"))

24 Filter Possible implementation func filter(input: Array, function: FuncRef) -> Array: var result := [] for element in input: if function.call_func(element): result.append(element) return result

25 Reduce Takes an array, a binary function, and a value, returns a single value with the function applied to each element and the previous result Signature: func reduce(input: Array, function: FuncRef, base) Equivalent to: var my_arr := [1, 2, 3] var reduced = base for element in my_arr: reduced = some_func(reduced, element)

26 Reduce Example 1 func sum(num1: int, num2: int) -> int: return num1 + num2 func array_sum(numbers: Array) -> int: return reduce(numbers, funcref(self, "sum"), 0) func _ready(): var numbers := [1, 2, 3, 4] var sum: int = array_sum(numbers) # 10

27 Reduce Example 2 func larger(num1: int, num2: int) -> int: return num1 if num1 > num2 else num2 func get_user_age(user: User) -> int: return user.age func get_highest_user_age() -> int: var user_ages: Array = map(fetch_users(), \ funcref(self, "get_user_age")) # Base value is omitted here return reduce(user_ages, funcref(self, "larger"))

28 Reduce Possible implementation func reduce(input: Array, function: FuncRef, base = null): var accumulator = base var index := 0 if not base and input.size() > 0: accumulator = input[0] index = 1 while index < input.size(): accumulator = function.call_func(accumulator, input[index]) index += 1 return accumulator

29 Lambda Function definition where a value (expression) is expected Helps in cases where the function is simple or doesn t need to be reused in another context Example: var result := map([2, 4, 6], x => x / 2) print(result) # [1, 2, 3]

30 Lambdas in GDScript Well, we re not there yet, but will soon Seriously!

31 Let s talk a bit about theory I promise it s only a bit

32 Functor (No need to flee, it s not that hard) Functor is a type that can be mapped, that is, there s a map function applicable to the type Yes, that s it As we say, Array is a functor because it has* a map function *Technically the function isn t in the Array class, but it doesn t matter conceptually.

33 Monoid Monoid is a type that has a function with three properties: Binary Has two parameters Associative function(a, function(b, c)) == function(function(a, b), c) Has a neutral (identity) element function(a, identity) == function(identity, a) == a It doesn t need to be commutative (function(a, b)!= function(b, a))

34 Monoid Example: integers with sum as the function Binary: (two arguments: 3 and 4 ) Associative: (1 + 2) + 3 = 1 + (2 + 3) = 6 Identity element: = = 2

35 Monoid Array is also a monoid! What s the function that makes it a monoid? Concatenation: Binary: [1, 2] + [3, 4] == [1,2,3,4] Associative: ([1,2] + [3,4]) + [5,6] == [1,2] + ([3,4] + [5,6]) == [1,2,3,4,5,6] Identity element: [] + [1,2] == [1, 2] + [] == [1, 2]

36 Monad Yes, let s talk about monad! As said: A monad is a monoid in an endofunctor category In other words: Monad is a monoid that is also an endofunctor In the context of programming, every functor is an endofunctor Which means...

37 Array is monad! Yes, that s it It s a functor (can be mapped) It s a monoid (regarding the concatenation function) So it s also a monad

38 Promise Encapsulates asynchronous operations Allows to maintain the chain of operations while we wait the promise to be fulfilled That is, the asynchronous operation complete The promise can be treated as the value that it contains Suppress side effects

39 Promise Example: HTTP request (promise implementation) class_name Promise signal success(value) # probably want error and done signals too var done: bool = false func _success(value): done = true emit_signal("success", value)

40 Promise Example: HTTP request (kitten request class) var promise: Promise = null func kitten_request(width: int, height: int) -> Promise: var http := HTTPRequest.new(); add_child(http) http.request(" % [width, height]) http.connect("request_completed", self, "_on_request_completed") promise = Promise.new() return promise func _on_request_completed(result, response_code, headers, body): var image := Image.new() image.load_jpg_from_buffer(body) var texture := ImageTexture.new() texture.create_from_image(image) promise._success(texture)

41 Promise Example: HTTP request (using promises) func _ready(): var kitten_promise: Promise = kitten_request(800, 600) kitten_promise.connect("on_success", self, "on_get_kitten") func on_get_kitten(kitten: Texture): $KittenSprite.texture = kitten

42 Promise

43 Promise in functional programming Promise is also a monad! Monad can also be seen as encapsulation inside a context In an Array, the context is the multiplicity of values In Promise, the context is the promise of a future resolution To put in the context is usually called Lift

44 Promise as a functor The mapping in promise means applying the function to the final resolved value Like the kitten picture It s possible to define Promise.map() which applies the function only when it resolves (that is, when the async operation completes) Remember that map() returns the value inside the context, so Promise.map() also returns a Promise In turn, it s possible to apply other pure operations to a Promise before it is fullfilled

45 Promise map Example var users: Promise = request_users() # Promise of array of users var active_users: Promise = users.map(funcref(self, "filter_active_users")) active_users.connect("success", self, "print_users") func filter_active_users(users: Array) -> Array: return filter(users, funcref(self, "is_active")) func print_users(users): print(users)

46 Monad as side effect isolation Like Promise encapsulates an asynchronous operation, Monads can also encapsulate side effect It s possible to operate with the values inside the lifted context (i.e. inside the Promise) without depending on its effects Example: operating in a text input before the user types anything The function needs only to get the text as argument, not wait for the side effect Keeping side effects and shared state only in a small and specific part of the code

47 Learn a functional language At first it may be hard to assimilate the concepts But the patterns can be applied in other languages and be used to increase code quality Examples of functional languages: Elm, Elixir, Haskell, Clojure, Scala, F#

48 Questions? github.com/vnen twitter.com/vnen

49 Thank you! github.com/vnen twitter.com/vnen