I would like the consumer of a promise to be able to cancel an operation. While the consumer of a promise should not be able to directly resolve or reject the deferred, it should be able to say that it no longer cares about the outcome.
There have been several times I've thought "Hmmmm. what if I need to kill this object before the promise completes? How will the code in these resolve/reject callbacks behave? Will they throw exceptions? Do I want that to happen and potentially cause the promise to reject for other listeners too?"
If a promise consumer had the ability to say "I no longer want my callbacks called", that would let me write code that doesn't care about getting called after the objects in the current scope/context are destroyed. If when.js were able to release it's references to the callbacks, that would be even better since the garbage collector could free up the memory associated with those function calls.
It sounds like there are two potentially separate issues here:
Besides the existing ability to cancel a deferred, it'd be nice to be able to safely grant another party the ability to cancel that deferred--similar to the current way in which resolver can safely be given to another party to grant the ability to resolve or reject the promise. Whether that "cancelability" belongs on the promise is a related question ... as is how/whether that cancelability would transfer to chained promises.
The ability to "unobserve" a promise after you've registered to observe via .then() or when(). To do this currently, you'd have to write special code in your promise handlers that bails out if they're called when you no longer care about the promise.
In other words, the two potentially useful, but different behaviors are: canceling the operation itself, and canceling your interest in the outcome of the operation.
If that sounds right, @unscriptable would you file a separate ticket for 2? If not, let me know what I got wrong, and we can try to clarify :)
This is being tracked over in the Promises/A+ cancellation supplemental spec