Functional Programming with Gosu

First Class Citizen

An entity that can be passed around as an argument, returned from a function, modified and assigned to a variable.

First Class Function

A function that is treated as first class citizen.

Higher-Order Functions (HOF)

A function which takes function as an argument and/or that return a function.

Closure

A function that remembers its lexical scope even when the function is executed outside that lexical scope.

function greeterFn() : block() {
  var name="World" //name is a local variable created by init
  var greet = \-> print("Hello ${name}") //greet is an inner function 
                                         //that uses the variable declared 
                                         //in parent function.
  return greet
}

var greeter = greeterFn()
greeter()

Currying

The process of converting a function that takes multiple arguments into a function that takes them one at a time.

var sum = \ a : int, b : int -> a + b
print(sum(1,2))

var curriedSum = \ a : int -> \ b : int -> a + b
print(curriedSum(1)(2))

Function Composition

The act of putting two functions together to form a third function where the output of one function is the input of the other.

uses java.lang.Integer
uses java.lang.Double
uses java.lang.Math

var compose = \ output : block(out : Integer) : String, func : block(param: Double) : Integer -> \ arg : Double -> func(output(arg)) //Definition

var floorToString = compose(\ out -> out.toString(), \ param -> Math.floor(param)) //Usage

print(floorToString(121.212121))

Continuation

The part of the code that's yet to be executed.

uses java.lang.Double

var printAsString = \ num : Double -> print("Given ${num}")

var addOneAndContinue = \ num: Double, cc : block(___num : Double) -> {
  var result = num + 1
  cc(result)
}

addOneAndContinue(2, printAsString)

Purity

A function is pure if the return value is only determined by its input values, and does not produce side effects.

var greet = \ name: String -> print("Hello ${name}")
greet("World")

The following is not pure since it modifies state outside of the function:

var greeting : String
var greet = \ name: String -> {greeting ="Hello ${name}"}
greet("World")
print(greeting)

Side Effects

A function or expression is said to have a side if apart from returning a value, it interacts with (reads from or writes to) external mutable state.

var currentDate = java.util.Date.CurrentDate //Retrieves the date from the system.
print(currentDate)

gw.api.util.Logger.forCategory("side-effect").info('IO is a side effect!')

Idempotent

A function is idempotent if reapplying it to its result does not produce a different result.

print(java.lang.Math.abs(java.lang.Math.abs(10)))

Point-Free Style (Tacit Programming)

Write functions where the definition does not explicitly identify the arguments used. This style usually requires currying or other higher order functions.

uses java.lang.Integer

// Given
var map = \ fn : block(item : int) : int -> \ list : List<Integer> -> list.map<Integer>(\ item -> fn(item))
var add = \ a : int -> \ b : int -> a + b
var nums : List<Integer> = (0..5).toList()

// Not points-free - 'numbers' is an explicit argument
var incrementAll = \ numbers : List<Integer> -> map(add(1))(numbers)

print(incrementAll(nums))

// Points-free - The list is an implicit argument
var incrementAll2 = map(add(1))

print(incrementAll2(nums))

Predicate

A function that returns true or false for a given value.

var predicate = \ a : int -> a > 2

print((1..4).where(\ a -> predicate(a)))

Lambda

An anonymous function that can be treated like a value.

(\ a : int -> a + 1)(1)

Lambda can be assigned to a variable

var add1 = \ a : int -> a + 1
print(add1(1))

Reference

https://github.com/hemanth/functional-programming-jargon
https://en.wiktionary.org/wiki/second-class_object#English
https://en.wiktionary.org/wiki/third-class_object#English