Maybe monad written in Swift

mabe-monad.swift

// from Swiftz
public class K1<A> { public init() {} }

protocol Monad {
    typealias A
    typealias B
    typealias FB = K1<B>
    func bind(A -> FB) -> FB;
    static func ret(A) -> Self
}

enum Maybe<T> : Monad {
    case Just(T)
    case Nothing
    
    typealias A = T
    typealias B = Any
    typealias FB = Maybe<B>
    // crash!! with `typealias K1 = Maybe`
    func bind<B>(f:T -> Maybe<B>) -> Maybe<B> {
        switch self {
        case .Just(let x): return f(x)
        default: return .Nothing
        }
    }
    // compile error happens without `return`!!
    static func ret(a:A) -> Maybe<A> {return .Just(a)}
}

let one = Maybe<Int>.ret(1)

let res = one.bind({(x:Int) in .Just(x+2)})
            .bind({(_:Int) in .Nothing}) // needs (_:Int) for type inference
            .bind({(x:Int) in .Just(x*2)})

switch res {
    case  Maybe<Int>.Just(let x): print("\(x)")
    default: print("nothing")
}