Js Deferred/Promise/Future compared to functional languages like Scala

Question

I\'m mostly using programming languages like Scala and JavaScript. I\'m trying to understand the similarities and differences in how async reactive programming is used in bo

Answer 1

It seems that in Javascript we call a Deferred the object we resolve to >complete a Promise. In Scala, it seems the Promise is the object you >resolve to get a Future monad.

Can someone tell me if this is right? Is there any good reason for a >different usage of the term Promise between Js and Scala?

In Scala, Promise and Future have separated functionality, Future is a asynchronous computation container, which return you some value in the future, and Promise is the writing part for async-computation, which you can do something as follow

val promise = Promise[String]
val future1 = promise.future
val future2 = future1.map { case s => println(s); s }

future2.onSuccess { case s => println(s + " 2nd time") }

promise.success("promise completed")

Once you execute the last statement, the output will be

promise completed
promise completed 2nd time

In Scala,you read value from Future using onComplete, or you chain it using map, and you write to a Future using it's Promise counterpart

In JS Promise A+ specs, they are bundled together, Promise.then is used for both chaining and retrieving value for side-effect (eg. console.log), to write you will use resolve like code snippet below

var promise = new Promise(function(resolve, reject){
    Thread.sleep(10000);
    resolve("promise completed");
}

Answer 2

I'm trying to understand the similarities and differences in how async reactive programming is used in both languages.

This document here doesn't compare Javascript promises with Scala, but instead Javascript promises with C++ C# and Python: https://github.com/KjellSchubert/promise-future-task. I know thats not exactly what you had asked for, but this might give you some interesting pointers nonetheless.

Answer 3

Yes, and no.

While extremely similar. With JavaScript Promises that comply to the Promises/A+ spec .then is not really a monadic bind and does .map and .flatMap both. Inside a .then handler when you return a promise it will recursively unwrap it.

Promise.delay(1000).then(function() {
    return Promise.delay(1000).then(function () {
        return Promise.delay(2000);
    }).then(function () {
        return Promise.delay(5000)
    });
}).then(function () {
    alert("This is only shown after 8 seconds and not one");
});

(fiddle)

You are correct that the standard JS promise libraries and the A+ spec does not feature monadic promises. They have been discussed, and implementations like fantasy-promises exist. They follow a differnet spec and have little adoption. Also see this. There has been ongoing discussion about it in the language design discussion forum - esdiscuss and a monadic .chain method that does not flatmap and allows for monadic promises is considered but unlikely to make it.

This is for pragmatic reasons. The current way promises are implemented is immensely useful. Rare are the cases you actually want a Future[Future and normally you want continuations to just work in the language. Promises 'borrow' from monads and are 'monadic' in a sense themselves. .then is very close to bind and in my head I use them interchangeably :)

It is impossible to have a Promise[Promise[Value]] like a Future[Future[Value]] in Scala with most promise libraries. You'd have to wrap it in an object and have Promise[Container[Promise[Value]]].

Promise.delay(1000).then(function () {
    return Promise.delay(1000).then(function () {
        return {
            wrap: Promise.delay(2000).then(function () {
                return Promise.delay(5000);
            })
        };
    });
}).then(function () {
    alert("This logs after 1 second");
    // I've also not seen a really solid use case 
    // except TypeScript type inference which is meh
});

(fiddle)

There are also a number of other smaller differences between the two, but generally you are correct in your assertions.

Answer 4

In contrast to Scala, the JS Promise is not a monad, due to the implicit "thenable" unwrapping breaking monadic law. You can, however, implement a callback-based monadic semantics and functionality, serving the same purpose.

See e.g. the cpsfy library.

In addition, there is a structural difference due to .then accepting 2 functions, while .chain accepts only one. However, a chain accepting 2 or even any number of argument functions can be implemented, like e.g. with CPS wrapper from cpsfy:

//function returning CPS function with 2 callbacks
const readFileCps = file => (onRes, onErr) =>  
  require('fs').readFile(file, (err, content) => {
    err ? onErr(err) : onRes(content)
  })

// CPS wraps a CPS function to provide the API methods
const getLines = CPS(readFileCps('name.txt'))
  // map applies function to the file content
  .map(file => file.trim()) 
  .filter(file => file.length > 0)
  // chain applies function that returns CPS function
  .chain(file => readFileCps(file))
  .map(text => text.split('\n'))
// => CPS function with 2 callbacks

// To use, simply pass callbacks in the same order
getLines(
  lines => console.log(lines),  // onRes callback
  err => console.error(err)  // onErr callback
)