Monads.swift

// Monads! >>= >> return

import Foundation

infix operator >>= { associativity left }

func >>=<A,B> (left: Optional<A>, right: A -> Optional<B>) -> Optional<B> {
    if left == nil {
        return nil
    } else {
        return right(left!)
    }
}

// should be >> but Swift uses that for bitwise shift
infix operator >>> { associativity left }

func >>><A,B> (left: Optional<A>, right: Optional<B>) -> Optional<B> {
    return left >>= { (_: A) -> Optional<B> in return right }
}

func excited(x: String) -> Optional<String>{
    return Optional(x + "!")
}

func makeInt(x: String) -> Optional<Int> {
    return x.toInt()
}

func doubleIt(x: Int) -> Optional<Int> {
    return Optional(x + x)
}

// we need this because Playground doesn't support the Printable protocol :(
func toString(x: Int) -> Optional<String> {
    return Optional(x.description)
}

let yay = Optional("yay")
let hey = Optional("hey")
let two = Optional(2)
let n:Optional<String> = nil

yay >>= excited >>= excited                         // Optional("yay!!")
yay >>= excited >>= excited >>> hey >>= excited     // Optional("hey!")
n >>= excited >>= excited >>> hey >>= excited       // nil
two >>= doubleIt >>= toString >>= excited           // Optional("4!")