Assimilation of legacy cancellable promises

[bergus]

Most of the existing implementations do offer a .cancel() method on promises. It would be super slick if we could invoke this when assimilating such a cancellable thenable together with a token (apart from already passing the token into then). I'd imagine doing something like

…
token.subscribe(reason => {
   const cancel = thenable.cancel;
   if (typeof cancel == "function")
       cancel.call(thenable, reason);
});
then.call(thenable, resolve, reject, token);

in the assimilation sequence (PromiseResolveThenableJob).

This would allow us to do something like

const Bluebird = require("bluebird");
Bluebird.configure({ cancellation: true });
const bp = … // some cancellable Bluebird promise

const {token, cancel} = CancelToken.source();
const promise = Promise.resolve(bp, token);

// or

const p = … // some ES8 promise
p.then(val => {
     const bp = … // some cancellable Bluebird promise
     return bp;
}, token)

where cancel() later would invoke bp.cancel().

Is this feasible? What do you think? Pinging @petkaantonov @benjamingr

[bergus]

I guess the more reasonable solution would be update legacy libraries to deal with the token they are passed, just like tasks should do. No need to hassle with this in the spec.