@swiftbot
import Foundation public protocol MapType { associatedtype DomainType associatedtype CodomainType init(_ fnc: @escaping (DomainType) -> CodomainType) func applied(to x: DomainType) -> CodomainType } 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 DomainType == CodomainType { static var identity: Self { get } func composed(with f: Self) -> Self } public extension EndType { public static var identity: Self { return Self { x in x } } public func composed(with g: Self) -> Self { return Self { x in // ココで EXC_BAD_ACCESS self.applied(to: g.applied(to: x)) } } } extension Map: EndType where Domain == Codomain { } public extension Array where Element == Map<Int, Int> { func composed() -> Element { return self.reduce(Element.identity) { (res, f) in res.composed(with: f) } } } let f: Map<Int, Int> = Map { x in 2 * x } let fs: Array<Map<Int, Int>> = [f, f, f] let g: Map<Int, Int> = fs.composed() let t: Int = g.applied(to: 1) print(t)