Functor.swift

import Foundation

struct Box<T> {}

protocol Functor {
    typealias A
    typealias B
    typealias Boxed = Box<B>
    
    func fmap<B>(f: A -> B) -> Boxed
}

extension Array: Functor {
    typealias A = T
    typealias B = Any
    typealias Boxed = [B]
    
    func fmap<B>(f: A -> B) -> Boxed {
        return self.map({ f($0) })
    }
}

extension Optional: Functor {
    typealias A = T
    typealias B = Any
    typealias Boxed = B?
    
    func fmap<B>(f: A -> B) -> Boxed {
        switch self {
        case .Some(let value):
            return Boxed(f(value))
        case .None:
            return .None
        }
    }
}

let n: Int? = 1
let m = n.fmap { x in "\(x)" }