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import Foundation
/// A Functor ==> Monadic type that encapsulates completion block
/// and their associated async task thereby providing a nice
/// synchronous and flow oriented programming model.
public struct Promise<T> {
public typealias Completion = (T) -> Void
/// This is the async task that we are abstracting over
var aTask: ((Completion?) -> Void)? = nil
/// When creating a promise, you should care to call the first argument closure
/// that is provided with the eventual result value.
public init(_ task: @escaping ((Completion?) -> Void)) {
self.aTask = task
/// Constructor for creating promise out of normal values.
/// This is the lifting function; equivalent to `return` in haskell
public init(_ value: T) {
self.aTask = { aCompletion in
/// `then` is equivalent to `fmap`/`map`. It makes Promises Functorial.
/// This deals with synchronous side of the world.
/// If you want to create a Prmose inside of then, you are better off
/// using `bind`
@discardableResult public func then<U>(_ transform: @escaping (T) -> U) -> Promise<U> {
return Promise<U>{ upcomingCompletion in
self.aTask?() { tk in
let transformed = transform(tk)
/// `bind` is equivalent to `>>=`. It makes Promises Monadic
/// If all you want is to do synchronous tasks with the result
/// its better to use the functorial `then`. However, bind is
/// implemented in terms of `then` and `join`.
public func bind<U>(_ transform: @escaping (T) -> Promise<U>) -> Promise<U> {
let transformed = then(transform)
return Promise.join(transformed)
static public func join<A>(_ input: Promise<Promise<A>>) -> Promise<A> {
return Promise<A>{ aCompletion in
input.then { innerPromise in
innerPromise.then { innerValue in
/// Call this method on any Promise type to execute and fulfill the promise.
/// This is important because the design of Promises is to not evaluate immediately
/// but create a expression that can be executed/ passed or stored. The expression
/// can be internally be optimized or lazily evaluated.
public func execute() {
extension Promise: Functor {
public typealias A = T
public typealias B = Any
public typealias AB = Promise<B>
/// fmap is the same as `then`
public func fmap<B>(_ t: @escaping (T) -> B) -> Promise<B> {
return then(t)
/// Already implements `bind`
extension Promise: Monad { }