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.
