Pedro Matiello

Futures in Scala

28 Oct 2013 scala concurrency futures

Scala 2.10 introduced futures as a convenient abstraction for concurrent programming. Using futures, one can perform a number of computations in parallel for which the result is expected to be available, at some point, in these Future objects.

A result in a Future can be easily retrieved without blocking the execution flow by setting a callback to be invoked once it’s ready:

val fut = future { slowComputation }

fut.onSuccess {
	case result => useSuccess(result)

This callback is guaranteed to be executed after the future completes successfully. It’s also possible request a callback to be executed if the future fails with an exception:

fut.onFailure {
	case exception => useError(exception)

Alternativelly, a single callback can be defined for handling both success and failure cases:

fut.onComplete {
	case Success(result) => useSuccess(result)
	case Failure(exception) => useError(exception)

Finally, if blocking is acceptable, one can simply wait until the value is available:

Await.result(fut, Duration.Inf)

This isn’t what you probably want, though.

Some convenient operations

Even though futures are easier to handle than most other approaches to concurrency, they are not really a transparent solution. In fact, sometimes they can be a little tricky to use while still retaining its advantages. For instance, how does one, holding a future for some value, obtains a modified version of this value without waiting for its evaluation?

Fortunatelly, the API for futures in Scala is quite helpful in cases like this.

From future of value to future of modified value

Given a future fut for a value v and a function f, it’s quite straightforward to obtain a new future for a value f(v) without waiting for v to be evaluated:

From list of futures to future of list

It’s possible turn a list of futures into a future of a list for the same values. That is, given a List of Future objects List(Future(v1), Future(v2), ...), it is possible to produce a Future object such as Future(List(v1, v2, ...)):


Reduce list of futures to future value

It’s also possible to take a list of futures List(Future(v1), Future(v2), ...) and a reduction function f to produce a new future containing List(v1, v2, ...).reduce(f) as value:


Reduce future of list to future value

Finally, given the future of a list Future(List(v1, v2, ...)) and a reduction function f, a new future containing List(v1, v2, ...).reduce(f) as value can be produced: { list => list.reduce(f) }