[feature-request] Ability to process Promises from Polyglot applications

[mdsina]

We have project based on Project Reactor (https://projectreactor.io) that have own event-loop for message processing.
It's would be great if Polyglot will have an ability to manually process Promises from Polyglot-based applications on HotSpot, as for example.
Currently only Node runtime can use internal API for Promises processing.

[eleinadani]

Hi @mdsina,

do you have an example of how such an API would look like?

[mdsina]

Hi @eleinadani
I apologize this would like iterable chain of callbacks, that I can rebuild with API of Reactor (or something else)
Also there is a case when callback return new callback and we should add them to stack of callbacks chain to process them.
Not sure how does Node do that, but looks like if I can grab callbacks from promise we can build own processing on them

[eleinadani]

@mdsina could you maybe provide us a concrete example of JavaScript+Java code for this scenario?

[pmlopes]

I believe what @mdsina wants (and I'd to know if it's possible too) is access the executor as described in https://promisesaplus.com/ sections 2.2.4

Where it's suggested that the platform code should be used.

In vert.x I've added a full promise polyfill to control the executor, but if there's a way to define one for the native implementation then it would simplify everyone's code I guess.

[mdsina]

@pmlopes thanks for info.
Looks like Promise resolving is not so easy if I need to follow https://promisesaplus.com/
Also polyfill is a good idea, but in our case we use async\await syntax. Transpile async\await to Promises requires a more complex solution with JS parsing.

@eleinadani A more complex example I can provide later. But I need to access promise value. Currently Promise value is not accessible through public methods.

Context context = ...
Value val = context.eval("js", "async callFunction(ctx) {" +
"await res = util.post(ctx.get('I'))" +
"return res;" +
"}" + 
"return callFunction();"
);

// there are code that get promise value and bind Polyglot Context to reactive stream and execute promise values on context as different tasks on workers

[pmlopes]

async/await can be handled by graaljs the polyfill works for me with async await too.

[mdsina]

@pmlopes Can you please share your polyfill or some part if possible?

[pmlopes]

https://github.com/reactiverse/es4x/blob/develop/es4x/src/main/resources/io/reactiverse/es4x/polyfill/promise.js

[mdsina]

@pmlopes Oh, I see. You just replace defualt with your own implementation. Thanks.

[eleinadani]

Graal.js already supports some interoperability between Java code and JS promises. For example, JS code can await on a Java "thenable" object, and Java code can register a Java lambda function to be called once a JS Promise resolves.

To clarify what is currently supported, we added a few unit tests here

@mdsina from your code above it seems to me that you want to await on a Java object, and return a JS Promise back to Java. As you can see from our unit tests, we might already support this scenario. If not, could you please give us a more concrete exaple of what is missing?

@pmlopes, maybe what you need is also already covered in our tests above? If not, happy to discuss how we could improve our interop support for promises

[pmlopes]

I think what isn't totally clear is where is the executor happening in graaljs (not node). I think some documentation should be added to explain how this works:

var promise = new Promise(function(resolve, reject) {
  resolve("Stuff worked!");
  // so the question is... who runs the line above?
  // can we intercept it to be sure it runs where we would like it to run?
  // say our own executor thread?
});

promise.then(res => print(res));
print("after then, before resolve");

Which outputs the right stuff:

after then, before resolve
Stuff worked!