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  • Promise capability support

Promise capability support

# Isiah Meadows (6 years ago)

Sometimes, it's very convenient to have those resolve/reject functions as separate functions. However, when logic gets complex enough and you need to send them elsewhere, save a continuation, etc., it'd be much more convenient to just have a capability object exposed more directly rather than go through the overhead and boilerplate of going through the constructor with all its callback stuff and everything.

It's surprisingly not as uncommon as you'd expect for me to do this:

let resolve, reject
let promise = new Promise((res, rej) => {
    resolve = res
    reject = rej
})

But doing this repeatedly gets old, especially when you've had to write it several dozen times already. And it comes up frequently when you're writing lower-level async utilities that require saving promise state and resolving it in a way that's decoupled from the promise itself.

So here's what I propose:

  • Promise.newCapability() - This basically returns the result of this, just wrapped in a suitable object whose prototype is %PromiseCapabilityPrototype% (internal, no direct constructor). It's subclass-safe, so you can do it with subclasses as appropriate, too.
  • capability.resolve(value) - This invokes the implicit resolver created for it, spec'd as [[Resolve]].
  • capability.reject(value) - This invokes the implicit rejector created for it, spec'd as [[Reject]].
  • capability.promise - This returns the newly created promise.

Yes, this is effectively a deferred API, but revealing constructors are a bit too rigid and wasteful for some use cases.

Isiah Meadows me at isiahmeadows.com, www.isiahmeadows.com

# Jordan Harband (6 years ago)

I don't think the Deferred pattern is a good primitive to have in the language, and it's a pretty trivial primitive to write yourself if you need it.

# Bob Myers (6 years ago)

I've used this pattern exactly twice in the large-scale app I'm working on now. One of those I was able to eliminate after I thought harder about the problem. The other I eventually replaced with the following kind of pattern:

function createPromise(resolver, rejector) {
  return new Promise((resolve, reject) {
    resolver.then(resolve);
    rejector.then(reject);
    });
}

Obviously the way this works it that to create a promise "controllable" from "the outside", you create your own resolver and rejector promises to pass to createPromise, such that they trigger when you need them to. To put it a different way, instead of getting back and passing around deferred-like objects, which seems to be a massive anti-pattern to me, the client creates their own promise-controlling promises designed to trigger at the right time.

Bob

# kai zhu (6 years ago)

my use-case is the exact opposite. in integration-level javascript, its impractical to have independent rejectors/error-handlers for dozens of integrated-components. its much easier to debug/refactor integration-code with a single, universal rejector/error-handler (all it does is log the error-stack and return a 404/500 to the server-request, or crash the app if client-side).

on server-side, you can't do much except 404/500 if the db-request times-out, JSON.parse fails, etc.

on client-side, crashing the app (and notifying user an error occured, and retry the request) is pretty much the only thing you can do when the server 404/500's.

# Isiah Meadows (6 years ago)

First, I do get that not all uses of deferred-like objects really merit the need for a deferred. For example, here, I saved the resolver and pulled the state out from the main closure to make the state easier to follow. You could argue a deferred isn't really necessary since I only care about the resolve function, and nothing else. It's also pretty trivial to factor it back into a closure where it was originally.

But it's when external forces control them indirectly through a state machine or similar, that's when it becomes necessary. I have some closed-source uses, but here's a couple concrete examples I have in OSS code:

  1. Here, I have to treat it like a continuation because I have to wait for an IPC protocol sequence to complete before it resolves/rejects: isiahmeadows/invoke-parallel/blob/master/lib/api.js#L144-L147
  2. Here, I have to treat it like a continuation because it's placed into a job queue driven by mainly the completion of child processes: isiahmeadows/website/blob/570db369cfca2b8a4a525be4e4621c854788b4d0/scripts/exec-limit.js#L71-L73

There is literally no other way to handle these beyond using a fake deferred, thanks to the fact they aren't resolved directly in response to any external forces, but indirectly as the result of a state machine transition or similar. I can't even pass them around where I need them, because there's a giant process wall I have to cross each time. And it's this kind of use case that drove me to request this. The resolver functions get in my way, they take up more memory than necessary, and I've found myself occasionally adding separate arrays of resolver/rejector functions so I can also avoid the indirection of calling them.

In general, I don't like using deferreds if I can help it - it's nothing but boilerplate for the common case. Here's what I usually prefer in order, provided I can help it:

  • The return value itself.
  • async/await
  • Promise.prototype.finally or some similar abstraction.
  • Promise.prototype.then/Promise.prototype.catch
  • Promise.resolve/Promise.reject
  • Promise.try or some similar abstraction.
  • Promise.all([...])/`Promise.race([...])
  • new Promise(...) using the callbacks directly.
  • new Promise(...), converting the result to a pseudo-deferred.

I'm not asking about this because I enjoy deferreds - they're nothing but useless boilerplate for the vast majority of use cases. In fact, I actively try to avoid it most of the time. I'm just asking for an escape hatch in case the simple stuff becomes boilerplate, one mirroring how the spec already deals with those complex scenarios. Very few things hit that breaking point when the callbacks become boilerplate, but low-level async code requiring a dedicated state machine driven by both calls and external effects has a habit of hitting that very quickly.

Isiah Meadows me at isiahmeadows.com, www.isiahmeadows.com

# Isiah Meadows (6 years ago)

For code that high level, I don't even usually deal with promises in the first place except when fetching data, saving data, and awaiting renders and transforms (sometimes). Raw event handlers are good enough for pretty much everything else, and when they aren't, you're almost certainly dealing with a stream or similar specialized data type that isn't a promise.

Either way, deferreds are the worst abstraction to use here, and you won't even likely see it much in Java code at this level (and deferreds is really only what Java has).

Isiah Meadows me at isiahmeadows.com, www.isiahmeadows.com

# Michael Theriot (6 years ago)

So I run into this issue when waiting on multiple events I don't initiate. What I do is create a subclass of promise that exposes these calls.

Not saying that's the ideal way to do it, but solvable in userland and without modifying the base class.

# Augusto Moura (6 years ago)

I think what Jordan means, it's that the deferred has it use case, but probably we don't want it in Javascript native library. There's a lot of mature libraries implementing deferred wrappers and most of them are Promise like compatible, and even if you cannot use libraries or don't want to, you can easily implement a Promise extension and use it yourself.

Interesting enough, I got a really weird case (reads contraintuitive, I'm pretty sure the semantics of the error are right) extending the Promise class to exemplify a simple Deferred implementation, the code:

class Deferred extends Promise {
  constructor(factory) {
    super((resolve, reject) => {
      Object.assign(this, { reject, resolve });
      factory(resolve, reject);
    });
  }
}

const d = new Deferred(() => {});

The problem is the usage of this before calling the super constructor (even when the using in the super call itself). I wonder with it there are any ways of capturing the super constructor arguments in a Base class using class syntax. You probably can get the arguments in a old "function class" syntax (can be done weakmaps too). We can probably start ~yet~ another thread on Promises, about this problem (supposing there's no way of passing this to the promise factory).

# Richard Gibson (6 years ago)

On Fri, Jul 20, 2018 at 12:15 PM Augusto Moura <augusto.borgesm at gmail.com>

wrote:

Interesting enough, I got a really weird case (reads contraintuitive, I'm pretty sure the semantics of the error are right) extending the Promise class to exemplify a simple Deferred implementation, the code:

class Deferred extends Promise {
  constructor(factory) {
    super((resolve, reject) => {
      Object.assign(this, { reject, resolve });
      factory(resolve, reject);
    });
  }
}

const d = new Deferred(() => {});

The problem is the usage of this before calling the super constructor (even when the using in the super call itself).

Isn't the solution the pattern we've already seen?

class Deferred extends Promise {
  constructor(factory) {
    const methods = {};
    super((resolve, reject) => Object.assign(methods, { resolve, reject }));
    Object.assign(this, methods);
    factory(this.resolve, this.reject);
  }
}
# Augusto Moura (6 years ago)

You are right, I didn't know you can use variables before calling the super constructor (a Java thing). So yeah, it's pretty easy to extend a Promise to externalize resolve and reject

PS: Sorry about my last email last paragraph grammar, I'm yet getting used to write long texts in English

# Isiah Meadows (6 years ago)

Just a heads up: that will throw, since the Promise constructor executes its callback synchronously, and thus this isn't set up in the callback until after it's called. You should probably rewrite it the callback to store them locally and then set the properties and call the factory after:

let resolve, reject
super((res, rej) => { resolve = res; reject = rej })

this.resolve = resolve
this.reject = reject
try {
    if (factory != null) factory(resolve, reject)
} catch (e) {
    reject(e)
}

It's subtle, but it does make a difference.

Isiah Meadows me at isiahmeadows.com, www.isiahmeadows.com

# Isiah Meadows (6 years ago)

I think what Jordan means, it's that the deferred has it use case, but probably we don't want it in Javascript native library. There's a lot of mature libraries implementing deferred wrappers and most of them are Promise like compatible, and even if you cannot use libraries or don't want to, you can easily implement a Promise extension and use it yourself.

Jordan, is this accurate? It wasn't what I read of it, unless you sent something that mistakenly missed the list.

If that is the case, I don't see much precedent: Promise.resolve/Promise.reject also hit that bar just as quickly, if not quicker:

// Exact polyfills, assuming `IsPromise` as per 25.6.1.6 is exposed globally.
// https://tc39.github.io/ecma262/#sec-ispromise
Promise.resolve = Promise.resolve || function (value) {
    if (IsPromise(value) && value.constructor === Promise) return value
    let resolve
    let promise = new this((r, _) => { resolve = r })
    resolve(value)
    return promise
}

Promise.reject = Promise.reject || function (value) {
    let reject
    let promise = new this((_, r) => { reject = r })
    reject(value)
    return promise
}

Isiah Meadows me at isiahmeadows.com, www.isiahmeadows.com

# Jordan Harband (6 years ago)

The use cases for "i want a resolved or rejected promise for a value/reason" are much more numerous than for "i want a deferred", so yes, I don't think a Deferred belongs in the language, and I think Promise.resolve/reject absolutely clear that bar.

# Augusto Moura (6 years ago)

Reject and resolve static methods are not introducing a new ~maybe dangerous~ pattern into the language, they are just isolating a factory for a common use case (creating a promise wrapping a well know value at the time of execution), deferreds add a whole lot of indirection in the table, that might lay some traps for non-experienced developers and promote some bad designs or confusing code.

# Isiah Meadows (6 years ago)

Fair enough. I was just hoping that the internal mechanism itself could be exposed, since the spec already has to use it (as do implementations 99% of the time). In case you're concerned about the potential for abuse, I do have this:

  1. It's different than what you'd get with new Promise(...) itself. This alone should push people away from it - you don't see a lot of repeated map.set("foo", foo).set("bar", bar) when new Map([["foo", foo], ["bar", bar]]) is sufficient.
  2. The name is relatively long and it sticks out very well - the factory's name is about twice as long to type than new Promise and it clearly doesn't return a Promise directly.
  3. It's not a constructor, but a method. This makes it harder to contort it into the traditional OOP madness, since methods aren't subclassable.
  4. The name clearly states it's creating a new capability, a new "controller" of sorts for a promise. This is way more explicit than defer, which sounds like you're "deferring" something (when you're not), it'd would be much less prone to confusion and misuse.
  5. Instead of the deferred type choosing how the promise is created, the relationship is inverted. And since promise subclassing is a bit annoying already, you're less likely to see the subclassing crap happen around this.

I specifically don't want to make it simple, idiomatic, and easy to use, because 1. it doesn't even properly capture errors that might occur, and 2. it's a relatively unsafe API most won't really need. However, it's easier to integrate into state-driven control flow, and that's why I would like to see it added.

In case you're curious about other legitimate use cases:

  • Simplifying returning a promise fulfilled when an event emits next, or rejects if it's beat by an error event - if this is built-in, you could drop most of the memory and computational overhead by just dealing with the capability directly rather than invoking a closure.
  • Returning a promise fulfilled when a socket completes, without the overhead of an event emitter.
  • Returning a promise fulfilled on next stream flush, rejected if an error occurs - these are usually pooled, so the factory is a bit wasteful with memory.
  • Sending a worker thread a message and returning a promise fulfilled with the response, rejected on error. When the message channel contains unrelated messages as well, you can't use normal promises with this.

Also, I created a quick prolyfill for my Promise.newCapability here: gist.github.com/isiahmeadows/14ddaae1ae2e73df3115259a0aff1804

Isiah Meadows me at isiahmeadows.com, www.isiahmeadows.com

# Herbert Vojčík (6 years ago)

Isiah Meadows wrote on 20. 7. 2018 3:13:

Sometimes, it's very convenient to have those resolve/reject functions as separate functions. However, when logic gets complex enough and you need to send them elsewhere, save a continuation, etc., it'd be much more convenient to just have a capability object exposed more directly rather than go through the overhead and boilerplate of going through the constructor with all its callback stuff and everything.

It's surprisingly not as uncommon as you'd expect for me to do this:

let resolve, reject
let promise = new Promise((res, rej) => {
     resolve = res
     reject = rej
})

But doing this repeatedly gets old, especially when you've had to write it several dozen times already. And it comes up frequently when you're writing lower-level async utilities that require saving promise state and resolving it in a way that's decoupled from the promise itself.

So here's what I propose:

  • Promise.newCapability() - This basically returns the result of [this][1], just wrapped in a suitable object whose prototype is %PromiseCapabilityPrototype% (internal, no direct constructor). It's subclass-safe, so you can do it with subclasses as appropriate, too.
  • capability.resolve(value) - This invokes the implicit resolver created for it, spec'd as [[Resolve]].
  • capability.reject(value) - This invokes the implicit rejector created for it, spec'd as [[Reject]].
  • capability.promise - This returns the newly created promise.

Yes, this is effectively a deferred API, but revealing constructors are a bit too rigid and wasteful for some use cases.

Don't understand "revealing constructors". Can be done is userland in a few lines. lolg.it/herby/deferred

# Isiah Meadows (6 years ago)

Here's a quick overview of the "revealing constructor" pattern, if it helps: blog.domenic.me/the-revealing-constructor-pattern

Isiah Meadows me at isiahmeadows.com, www.isiahmeadows.com

# Bob Myers (6 years ago)

Yes, I've encountered this "revealing constructor" terminology and find it confusing. I hope it doesn't catch on. A lot of people are likely to try to associate it with the "revealing module" pattern, with which it actually has nothing in common. It's a strange term because this pattern, if one wants to characterize it in terms of "revealing" things, is more about what is not being revealed (to the outside world), not what is being revealed.

It's a useful pattern seen also in the observable constructor, and probably deserves to have a good name, but I can't come up with anything myself, other than maybe the suboptimal "callback-based constructor".

Bob

# Jordan Harband (6 years ago)

It's already caught on - "revealing constructor pattern" is the pattern that describes the Promise executor.

The "revealing module" pattern is obsolete anyways, but it functions on the same principle - using closures to reveal only explicit things instead of everything.

# Bob Myers (6 years ago)

Not to beat a dead horse but

  • No, it hasn't caught on, as evidenced by the recent poster who had never heard of it.
  • Yes, I know it's used to describe the Promise executor pattern.
  • "Revealing module pattern" reveals. The so-called "revealing constructor pattern" does not reveal anything. It hides. So the term is semantically incorrect from the beginning.

Bob

# Jordan Harband (6 years ago)

It allows you to reveal things, selectively, just as the revealing module pattern does.

(separately, the measure of "caught on" is not "there does not exist someone who has not heard of it")

# Bob Myers (6 years ago)

It allows you to reveal things, selectively, just as the revealing module pattern does.

The revealing module pattern is so named precisely because it reveals (to clients) certain aspects of the "module" in question. Yes, it hides other aspects, but only by virtue of not revealing them. If you want to support the terminology of "revealing constructor pattern", could you please state what is being revealed, exactly, to whom? When I call a promise constructor I get back one thing and one thing only, which is a promise. Is the notion that it is the promise which is being revealed? In that sense, all APIs "reveal" their results. If is the intent of using this term that the content of the executor is being revealed to the coder when he views the code? All code is revealed when it is viewed. It would seem to me that to qualify for the name "revealing constructor pattern" it ought to be revealing something.

(separately, the measure of "caught on" is not "there does not exist someone who has not heard of it")

This group is not a court of law with evidentiary rules. That someone with a degree of interest in JS to the extent they are a regular on the ML has never heard of it would seem to mean at least that it has not caught on widely. I myself had worked with and read about ES6 promises extensively before I heard the term. Perhaps in the future I can write things like "It might be argued by some that there is some possibility that the evidence could conceivably be interpreted as indicating that it may be the case that it has not caught on widely"?

In any case, "catching on", however you define it, would seem to require more than a grand total of two Google results for "revealing constructor pattern", one of which is by the person that invented it.

Bob

# Michael Theriot (6 years ago)

FWIW I have never heard of this terminology before either but in practice have used the pattern myself.