SendersAndReceivers.swift

/*
 * This is all Eric Nieblers work, see: https://youtu.be/h-ExnuD6jms
 */

import Foundation

protocol Receiver<T> {
    associatedtype T

    func setValue(_ value: T)
}

func calcInt() -> (any Receiver<Int>) -> Void {
    return { r in
        Thread {
            r.setValue(42)
        }.start()
    }
}

struct ThenSender<T>: Receiver {
    let p: any Receiver<T>
    let next: (T) -> T

    init(_ p: any Receiver<T>, _ next: @escaping (T) -> T) {
        self.p = p
        self.next = next
    }

    func setValue(_ value: T) {
        p.setValue(next(value))
    }
}

func then<T>(_ task: @escaping (any Receiver<T>) -> Void, _ next: @escaping (T) -> T) -> (any Receiver<T>) -> Void {
    return { p in
        task(ThenSender(p, next))
    }
}

class Shared<T> {
    var mtx = pthread_mutex_t()
    var cv = pthread_cond_t()
    var data: T!

    init() {
        pthread_mutex_init(&mtx, nil)
        pthread_cond_init(&cv, nil)
    }
}

struct Promise<T>: Receiver {
    var shared: Shared<T>

    func _set(_ value: T) {
        pthread_mutex_lock(&shared.mtx)
        shared.data = value
        pthread_mutex_unlock(&shared.mtx)
        pthread_cond_signal(&shared.cv)
    }

    func setValue(_ value: T) {
        _set(value)
    }
}

func sync_wait<T>(_ task: @escaping (any Receiver<T>) -> Void) -> T {
    let shared = Shared<T>()

    // start
    task(Promise<T>(shared: shared))

    // get
    func _get() {
        pthread_mutex_lock(&shared.mtx)
        while shared.data == nil {
            pthread_cond_wait(&shared.cv, &shared.mtx)
        }
        pthread_mutex_unlock(&shared.mtx)
    }

    _get()

    return shared.data
}

let start = calcInt()
let next = then(start, { i in return i / 2 })


print("Value: \(sync_wait(next))")