This proposal attempts to clarify the behavioral clauses of the Promises/A proposal, and to extend it to cover the cases where handlers may return a promise.
This proposal intentionally omits the progress handling portion of Promises/A. In practice it has proven to be underspecified and currently does not have an agreed-upon or defacto behavior within the promise implementor community.
Also intentionally omitted is a requirement for calling fulfill and broken handlers either synchronously or asynchronously [1]. Promises/A itself does not specify, and both synchronous and asynchronous approaches exist in the current landscape of popular implementations.
This specification borrows heavily from the Promises/A proposal by Kris Zyp, as well as the UncommonJS Thenable Promises specification by Kris Kowal. All credit goes to those authors.
As with Promises/A, this proposal does not deal with creation of promises.
A promise represents a value that may not be available yet. A promise must be one of three states: pending, fulfilled, or broken:
-
When in the pending state, a promise may transition to either the fulfilled or broken state.
-
When in the fulfilled state, a promise has a value and provides a way to arrange for a function to be called with that value. Once a promise has transitioned to the fulfilled state, it must never transition to any other state.
-
When in the broken state, a promise has a reason (an indication of why it was broken) and provides a way to arrange for a function to be called with that reason. Once a promise has transitioned to the broken state, it must never transition to any other state.
A promise is an object or function that defines a then
method that accepts at least 2 arguments:
promise.then(fulfilled, broken)
-
Both
fulfilled
andbroken
are optional arguments -
If truthy,
fulfilled
must be a function that accepts a value as its first argument.- When
promise
is fulfilled,fulfilled
will be called withpromise
's fulfillment value. fulfilled
will never be called more than once.fulfilled
will never be called ifbroken
has already been called.
- When
-
If truthy,
broken
must be a function that accepts a reason (which must be a value, not a promise) as its first argument.- When
promise
is broken,broken
will be called withpromise
's reason for being broken. broken
will never be called more than once.broken
will never be called iffulfilled
has already been called.
- When
-
then
may be called any number of times. -
fulfilled
andbroken
supplied in one call tothen
must never be called after those supplied to a later call tothen
on the same promise. -
then
must return a promise [2]var promise2 = promise1.then(fulfilled, broken)
- When
promise1
is either fulfilled andfulfilled
is called with the fulfillment value, or broken andbroken
is called with the reason:- If either returns a value,
promise2
must be fulfilled with that value. - If either throws an exception,
promise2
must be broken with the thrown exception as the reason. - If either returns a promise (call it
returnedPromise
),promise2
must be placed into the same state asreturnedPromise
:- If
returnedPromise
is fulfilled,promise2
must be fulfilled with the same fulfillment value. - If
returnedPromise
is broken,promise2
must be broken with the same reason. - If
returnedPromise
is pending,promise2
must also be pending. WhenreturnedPromise
is fulfilled,promise2
must be fulfilled with the same fulfillment value. WhenreturnedPromise
is broken,promise2
must be broken with the same reason.
- If
- If either returns a value,
- When
- Each implementation should document whether it calls handlers synchronously or asynchronously.
- Each implementation should document whether it may produce
promise2
===promise1
, and if so, under what conditions. It is intentionally not specified as to whether the returned promise may be the same promise, or must be a new promise, i.e.promise2
!==promise1
is not a requirement. An implemention is free to allowpromise2
===promise1
, provided it can meet the requirements in this section.
Haha I was totally thinking of doing this, with exactly this name too! Glad you are on top of it; it's great to have others sharing the effort :)
I actually like some of the terminology things going on here:
That is, promises are either pending, fulfilled, or broken. Deferreds can be resolved or rejected. Now we can explain how the latter impacts the former, without being confused by the way that, in the old terminology (pending vs. resolved and fulfilled vs. rejected) "reject" maps rather directly where "resolve" is rather complex.
I feel somewhat strongly that async resolution should be required for compliance. It can be turned off for specific libraries as an optional feature, but the default configuration of most libraries should be async. I realize this would be a breaking change for When, requiring users to specify an option to retrieve the old behavior, but I hope that would be OK. Pinging @wycats.
You say "must be a function," but that's rather imprecise. Should
then
throw an early error? Ignore it? Store it in the internal listeners array, then blow up when calling a non-function.Given that the
then
duck-type test is easy to fool, and has existing landmines in the wild, I'd consider specifying a better method of detection. For example, AMD has something likedefine.amd
. I'd lovethen.aplus
or similar.