Free Monads in Swift

Free.swift

//
//  Free.swift
//  Swift_Extras
//
//  Created by Robert Widmann on 9/19/14.
//  Copyright (c) 2014 Robert Widmann. All rights reserved.
//

import Foundation

public enum Free<F : Functor, A> {
	case Pure(A)
	case Roll(K1<Free<F, A>>)
}

extension Free : Functor {
	typealias B = Any

	typealias FA = Free<F, A>

	typealias FB = Free<F, B>

	public static func fmap<B>(f : A -> B) -> Free<F, A> -> Free<F, B> {
		return { (let ftor) in
			switch ftor {
				case .Pure(let a):
					return .Pure(f(a))
				case .Roll(let y):
					return .Roll(Free.fmap({ F.fmap(f)($0) })(y))
			}
		}
	}
}

public func <%><F : Functor, A, B>(f: A -> B, io : Free<F, A>) -> Free<F, B> {
	return Free.fmap(f)(io)
}

public func <^ <F : Functor, A, B>(x : A, io : Free<F, B>) -> Free<F, A> {
	return Free.fmap(const(x))(io)
}