Functor.swift

//
//  main.swift
//  Functor
//
//  Created by sam on 10/4/16.
//  Copyright © 2016 Sam Moore. All rights reserved.
//

import Foundation

var str: String = "Hello, playground"

// Protocol of a "function"
// OUT OF DATE

protocol Functor {
    associatedtype In
    associatedtype Out

    init<U>(x: In, f: In -> U)
    
    func apply(x: In) -> Out
}

// Operator

infix operator ∆ { }

func ∆<In, Out> (lhs: In, rhs: In -> Out) -> Out {
    return rhs(lhs)
}

// Functors

func map<In, Out>(f: In -> Out) -> In -> Out {
    return { x in f(x) }
}

func filter<In>(f: In -> Bool) -> In -> In? {
    return { x in if f(x) { return x } else { return nil } }
}

// TODO: fix associatvitiy
let string = ("Hello"
    ∆ map({ x in x + ", world!"}))
    ∆ filter({ x in x.containsString("Hello")})

print(string)