Functors

gistfile1.swift

import Foundation

protocol Mappable {
    typealias T
    func fmap<M: Mappable>(f: (T) -> M) -> M
}

extension Array : Mappable {
    func fmap<M: Mappable>(f: (Array<T>) -> M) -> M {
        return f(self)
    }
}

class MyBox<T> : Mappable {
    var value: T
    init(_ value: T) {
        self.value = value
    }

    func fmap<M : Mappable>(f: (T) -> M) -> M {
        return f(value)
    }
}

func fmap<T, Mt: Mappable, Mu: Mappable where Mt.T == T>(value: Mt, f: (T) -> Mu) -> Mu {
    return value.fmap(f)
}

let cat = "my cat"
let box = MyBox(cat)
fmap(box) { MyBox($0 + " other stuff") }
fmap([1, 2, 3]) { arr in [2, 3, 4] }