@swift-4.1.3
// // main.swift // Math // // Created by tarunon on 2018/04/16. // Copyright © 2018年 tarunon. All rights reserved. // import Foundation public protocol MapType { associatedtype Domain associatedtype Codomain init(_ fnc: @escaping (Domain) -> Codomain) func applied(to x: Domain) -> Codomain } public struct Map<Domain, Codomain>: MapType { internal let fnc: (Domain) -> Codomain public init(_ fnc: @escaping (Domain) -> Codomain) { self.fnc = fnc } public func applied(to x: Domain) -> Codomain { return fnc(x) } } public protocol EndType: MapType where Domain == Codomain { static var identity: Self { get } func composed(with f: Self) -> Self } public extension EndType { public static var identity: Self { return _identity(of: Self.self) } public func composed(with g: Self) -> Self { return _composed(with: self, g) } } func _identity<E: EndType>(of type: E.Type = E.self) -> E { return E { x in x } } func _composed<E: EndType>(with f: E, _ g: E) -> E { return E { x in f.applied(to: g.applied(to: x)) } } extension Map: EndType where Domain == Codomain { } public extension Array where Element: EndType { func composed() -> Element { return self.reduce(_identity(of: Element.self)) { (res, f) in _composed(with: res, g: f) } } } let f = Map { x in 2 * x } let g = [f, f].composed() print(g.applied(to: 1)) (edited)
