Maybe in Swift

gistfile1.swift

enum Maybe<A> {
    case Nothing, Just(A)
    
    func map<B> (f: A -> B) -> Maybe<B> {
        switch self {
        case .Nothing: return .Nothing
        case .Just(let a): return .Just(f(a))
        }
    }
    
    func andThen<B> (f: A -> Maybe<B>) -> Maybe<B> {
        switch self {
        case .Nothing: return .Nothing
        case .Just(let a): return f(a)
        }
    }
    
    func withDefault(def: A) -> A {
        switch self {
        case .Nothing: return def
        case .Just(let a): return a
        }
    }
}

func liftMay2<A,B,C>(f: (A, B) -> C, m1: Maybe<A>, m2: Maybe<B>) -> Maybe<C> {
    return m1.andThen({a in return m2.andThen({b in return .Just(f(a, b))}) })
}

liftMay2(+, .Just(3), .Just(4)).withDefault(0)