Hello,
I'd like to share an idea / experiment, trying to make asynchronous operations
cancellable by using the (still in stage 0 proposal) bind operator.
My first goal was to cancel an asynchronous operation, so not just a
cancellable Promise, but a whole chain of promises. Also, I wanted that the
code outside of my operation, if not aware that the code may cancel, just sees
a classic Promise that may reject with a "cancelled" exception. And I wanted an
object to represent the state of the asynchronous operation (cancelled or not).
So I came up with a `CancelToken` (inspired by a lot of previous proposals and
discussions I've seen). And I use an instance of that token, and the
bind/pipeline operator, to write my chain of promises:
```js
const token = new CancelToken();
asyncFoo()
::token.then(asyncBar)
::token.then(value => console.info(value))
::token.catch(exception => console.error(exception))
::token.iscancelled(() => console.debug('Operation cancelled'))
::token.finally(clean)
```
To cancel such chain, I thought to either provide a function when constructing
the token, or propagate the state of another token (using listeners):
```js
const token = new CancelToken(abort => button.onclick = abort);
const otherToken = new CancelToken(token); // token may be falsy
```
And there are probably a lot of ideas to derive from that:
```js
if (token.cancelled) { /*...*/ }
token.onCancelListener(callback);
setCancellableTimeout(func, 1000, token);
// ...
```
I think the pipeline operator, no matter the final syntax or behaviour, can be
very helpful to handle cancellation. Sorry if that’s been discussed before, I
haven’t find it.
Best Regards.
Jeremy
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