Implements wait_for()

[shikokuchuo]

Closes #142, closes #48.

As discussed in today's Shiny meeting. This PR takes an approach that works universally for any type of promise.

I need time to ponder (i) the function name, (ii) whether it should throw on rejection, and (iii) whether it is correct/optimal to set all = FALSE for later::run_now().

To add:

• Tests
• News item

In the meantime, feel free to comment @schloerke @gadenbuie @cpsievert.

[shikokuchuo]

After putting together this PR, I came across the version of wait_for() @lionel- uses internally for tests in coro. This gives me confidence in (i) the naming of the function and (ii) its behaviour on error.

[gadenbuie]

This looks like a great start @shikokuchuo! Have you reviewed the chromote implementation?

Currently this PR resembles various implementations we've used in package testing, which is a much more controlled environment. For a public, robust and final solution in promises we should be very convinced about the features we do or do not need from the chromote implementation.

[shikokuchuo]

Thanks @gadenbuie I hadn't actually reviewed chromote as I was under the impression that it wasn't an R-level implementation, let alone one based on promises! Luckily it seems we all converge on the same approach.

Chromote additionally has a lot of code to handle interrupts and my gut reaction is that this isn't necessary in the general case, but this should become apparent when I actually write the tests!

[gadenbuie]

Chromote additionally has a lot of code to handle interrupts and my gut reaction is that this isn't necessary in the general case, but this should become apparent when I actually write the tests!

I think this gets at what I meant by the difference between the implementations we've used in testing and a user-facing API. My inclination is that in user-facing code we'd want, or might even need, to support interrupts and other features that users would need and expect in an interactive session.

[schloerke]

Yes. Both @gadenbuie and I would like to have the chromote version with interrupt support.

Thank you, @shikokuchuo !