The appendix about Functional Programming introduced the main patterns and principles that help when dealing with computations that imply some effects. Moving a step forward, we can consider also the latency as an effect.
The following table represents a possible classification of the effects in programming.
| One | Many | |
|---|---|---|
| Synchronous | T/Try[T] | Iterable[T] |
| Asynchronous | Future[T] | Observable[T] |
This section will introduce a first abstraction to achieve sequential composition of actions that can take time to complete and that can also fail: Future[T].
Future and Promise are two really powerful abstractions that can be classified in the asynchronous programming category.
Wikipedia defines Futures and Promises as follows:
In computer science, future, promise, and delay refer to constructs used for synchronization in some concurrent programming languages. They describe an object that acts as a proxy for a result that is initially unknown, usually because the computation of its value is yet incomplete. [...]
A future is a read-only placeholder view of a variable, while a promise is a writable, single assignment container which sets the value of the future.
Futures and Promises promotes an asynchronous and non-blocking approach to programming. The scale up to use multiple cores on a machine, but with the currently implementations they don't scale out across nodes.