Monads in Swift -

gistfile1.swift

public final class Box<T>
{
    let value : T
    
    public init(_ value : T) {
        self.value = value
    }
}

func flatten<T>(box : Box<Box <T>>) -> Box <T> {
    return box.value
}

func map<T,U>(box : Box<T>, f : T -> U) -> Box<U> {
    return Box(f(box.value))
}

infix operator  >>=  {associativity left}
func >>= <T,U>(box : Box<T>, f : T -> Box<U>) -> Box<U> {
    return flatten(map(box,f))
}


func constructor<T>(value: T)->Box<T> {
    return Box(value)
}


let f = {Box($0 + 1)}
let a = 1

let x = Box(a) >>= f
let y = f(a)

let x1 = Box(1)
let y1 = Box(1) >>= constructor

let double = {Box(2 * $0)}
let triple = {Box(3 * $0)}

let x2 = Box(1) >>= double >>= triple
let y2 = Box(1) >>= {double($0) >>= triple}

let box = Box(4)
let sum3 = {Box($0 + 3)}
let toString = {Box(String($0 + 0))}
let iReallyLike = {Box("I really like the number " + $0)}

let luckyNumber7 = box >>= sum3 >>= toString >>= iReallyLike