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

ContinuationMonad.swift

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.