Partial application in Swift

partial_application.swift

//: Playground - noun: a place where people can play

import Foundation

func apply<A, B>(fn: (A) -> B, _ a: A) -> B {
    return fn(a)
}


func apply<A, B, C>(fn: (A, B) -> C, _ a: A) -> B -> C {
    return { b in return fn(a, b) }
}

func apply<A, B, C>(fn: (A, B) -> C, _ a: A, _ b: B) -> C {
    return fn(a, b)
}


func apply<A, B, C, D>(fn: (A, B, C) -> D, _ a: A) -> (B, C) -> D {
    return { b, c in return fn(a, b, c) }
}

func apply<A, B, C, D>(fn: (A, B, C) -> D, _ a: A, _ b: B) -> C -> D {
    return { c in return fn(a, b, c) }
}

func apply<A, B, C, D>(fn: (A, B, C) -> D, _ a: A, _ b: B, _ c: C) -> D {
    return fn(a, b, c)
}


func apply<A, B, C, D, E>(fn: (A, B, C, D) -> E, _ a: A) -> (B, C, D) -> E {
    return { b, c, d in return fn(a, b, c, d) }
}

func apply<A, B, C, D, E>(fn: (A, B, C, D) -> E, _ a: A, _ b: B) -> (C, D) -> E {
    return { c, d in return fn(a, b, c, d) }
}

func apply<A, B, C, D, E>(fn: (A, B, C, D) -> E, _ a: A, _ b: B, _ c: C) -> D -> E {
    return { d in return fn(a, b, c, d) }
}

func apply<A, B, C, D, E>(fn: (A, B, C, D) -> E, _ a: A, _ b: B, _ c: C, _ d: D) -> E {
    return fn(a, b, c, d)
}


let addTwo = apply(+, 2)
addTwo(3) // => 5

[1, 2, 3, 4, 5].map(apply(*, 2)) // => [2, 4, 6, 8, 10]

class Greeter {
    let greeting: String
    
    init(greeting: String = "Hello") {
        self.greeting = greeting
    }
    
    func greet(name: String = "World") -> String {
        return "\(greeting), \(name)"
    }
}


func apply<A, B, C>(fn: A -> B -> C, a: A) -> B -> C {
    return { b in fn(a)(b) }
}

let greeter = Greeter()
let greetFn = apply(Greeter.greet, greeter)
greetFn("World") // Hello, world