These "futures" or "yet another promises" are based on Task/TaskCompletionSource. They extend the capacities of the Tasks by bringing:
- Covariance of the completion result.
- Optional extension points to map errors (exceptions or cancellation) to regular results.
The read only and covariant model are the 2 interfaces ICompletion (no result) and
ICompletion<out TResul> : ICompletion.
The real stuff are the sources of completion: CompletionSource
and CompletionSource<TResult>.
A Completable is not, in itself, the important object here (note that ICompletable<TResult>
is not covariant). A completable simply holds a completion and carries the OnError/OnCanceled/OnCompleted extension points,
it is more to be used as an implementation detail than to expose it: the 3 extension points should typically be explicitly implemented.