Promises

Goals

A promise is a small object in memory, dealing with the result of an asynchronous process. This is an attempt to avoid allocations due to asynchronous calls.

Callbacks

when attaching callbacks to a process, there is always a chance for the callback to be missformed.

Coroutine

Coroutines are pretty convenient sometimes, but they cannot return value, and each created routine allocates, and are not reused.

Tasks

await/async is really elegant, but they allocates because it creates Task objects.

Why not accessing the Promise directly?

Promises come from a pool. To reuse it, it takes the developer to release it. The problem with pooled object is that we have no control over the fact that the developer really doesn't keep a reference to the promise.

Deferred and Completer share a key with the active promise and they can interact with it as long as they have the same key.

When a promise is released, its key is reset, so the linked deferred and completer cannot interact with it anymore.

Deferred and Completer are structures. They don't allocate.