Lazy Monad

Contents.swift

enum Lazy<T> {
    case Function(()->T)
    case Value(T)
    
    var value: T {
        switch self {
        case .Function(let f): return f()
        case .Value(let v): return v
        }
    }
}

extension Lazy {
    func flatMap<U>(function: T->Lazy<U>) -> Lazy<U> {
        return Lazy<U>.Function({function(self.value).value})
    }
    func map<U>(function: T->U) -> Lazy<U> {
        return self.flatMap({.Value(function($0))})
    }
}

print(Lazy.Value(3).map({$0 + 1}).flatMap({ number in
    .Function({"number:\(number)"})
}).value)