Monad in Swift

monad.swift

// A trivial type just for this example
final class Id <A> {
    let raw: A
    init(_ a: A) { self.raw = a }
}

// A typeclass of the Monad
infix operator >>| { associativity left precedence 95 }
public protocol Monad {
    typealias Elem
    func >>| (fa: Self, f: Elem -> Self) -> Self /* (^_^;) ...? */
    class func pure(a: Elem) -> Self
}

// An instance of the Monad typeclass
extension Id: Monad {
    typealias Elem = A // from Id<A>
    class func pure(a: Elem) -> Id<A> { return Id(a) }
}
func >>| <A, B>(fa: Id<A>, f: A -> Id<B>) -> Id<B> { return f(fa.raw) }

// test
func pure<A, MA: Monad where MA.Elem == A>(a: A) -> MA {
    return MA.pure(a)
}
let o = Id("(^_^)") >>| { pure($0.utf16Count) } >>| { pure(Double($0*$0)) }
println("\(o.raw)") // ==> 25.0