Futures

Tab Atkins Jr. jackalmage at gmail.com
Tue Apr 23 11:02:58 PDT 2013


On Tue, Apr 23, 2013 at 6:12 AM, David Bruant <bruant.d at gmail.com> wrote:
> Anyway, my point was that there exist libraries in which "then" is a
> function and the object with this function isn't a promise. These libraries
> will end up in a terrible confusion when used with Futures.
> You think you're resolving a future with an object and... oops! the built-in
> Future algorithm confused your object for a promise. Suddenly, not only are
> you not resolving your promise with the value, but your .then method is
> called unexpectedly.

Agreed.  I'll note, though, that if the semantics of .resolve() are
non-recursive (it strips off one layer of promise/future, but no
further), then this becomes somewhat less of an issue, as it can be
worked around - if you want to return something that's not a promise,
just wrap it in a fulfilled promise (via some sugar function - I've
proposed such in www-dom).  That wrapper will be unwrapped straight
away, and the future accepted with the value within.

The big problem only arrives if you both treat all thenables as
promises/futures, *and* have recursive resolve semantics, because then
you can't *ever* return a non-promise with a .then() method.

I'd prefer that resolve only strip away a branded future, not an
arbitrary thenable, and for there to be a way to brand a thenable as a
future.  That way, using other-library promises just requires one trip
through the branding function and they can interoperate.  Existing
libraries tend to expose the same mechanism already, so that the
other-library promises expose all the proper methods of the "main"
library.

But I want non-recursive resolve *more*, and if I was forced to choose
one, would take monadic resolve over branded futures.

~TJ


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