From https://figure.ink/blog/2021/6/13/cleaning-up-async-without-swift-55

From https://figure.ink/blog/2021/6/13/cleaning-up-async-without-swift-55 · GitHub

import Foundation typealias Continuation<Ret> = (Ret) -> Void typealias ContinuationMonad<Value> = (@escaping Continuation<Value>) -> Void typealias Transform<T,U> = (T) -> ContinuationMonad<U>

func async<Value>(_ wrappedValue: Value) -> ContinuationMonad<Value> { { $0(wrappedValue) } }

func flatMap<T,U>(_ monad: @escaping ContinuationMonad<T>, _ transform: @escaping Transform<T,U>) -> ContinuationMonad<U> { { continuation in monad { wrappedValue in transform(wrappedValue)(continuation) } } }

public func curry<T, U, Z>(_ ƒ: @escaping (T, U)->Z) -> (T) -> (U) -> Z { { t in { u in ƒ(t, u) } } }

precedencegroup BackApplication { associativity: left } infix operator <|: BackApplication

public func <| <T, U>(ƒ: (T)->U, t: T) -> U { ƒ(t) }

precedencegroup FlatMapOperator { higherThan: BackApplication associativity: left } infix operator >>=: FlatMapOperator

func >>=<T,U> (lhs: @escaping ContinuationMonad<T>, rhs: @escaping Transform<T,U>) -> ContinuationMonad<U> { flatMap(lhs, rhs) }

func fetchAvatar(user: UUID, completion: @escaping Continuation<URL>) { // Delay added to simulate an actual async task. DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(1)) { completion(URL(string: "/\(user.uuidString)")!) } }

// For the sake of demonstration, we’re pulling a // string out of the url rather than fetching an // image actual image. func fetchImage(url: URL, completion: @escaping Continuation<String>) { DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(1)) { completion(url.path) } }

func fetchPortrait(user: UUID, completion: @escaping Continuation<String>) { async(user) >>= curry(fetchAvatar) >>= curry(fetchImage) <| completion }

fetchPortrait(user: UUID()) { print($0) } //> After two seconds, prints a UUID.