Futures (Scala)

Future is a Monad

The first method for working with Future functionally is map. This method takes a Function which performs some operation on the result of the Future, and returning a new result. The return value of the map method is another Future that will contain the new result:

val f1 = Future {
  "Hello" + "World"
}

val f2 = f1 map { x =>
  x.length
}

val result = f2.get

In this example we are joining two strings together within a Future. Instead of waiting for this to complete, we apply our function that calculates the length of the string using the map method. Now we have a second Future that will eventually contain an Int. When our original Future completes, it will also apply our function and complete the second Future with it’s result. When we finally get the result, it will contain the number 10. Our original Future still contains the string “HelloWorld” and is unaffected by the map.

The map method is fine if we are modifying a single Future, but if 2 or more Futures are involved map will not allow you to combine them together:

val f1 = Future {
  "Hello" + "World"
}

val f2 = Future {
  3
}

val f3 = f1 map { x =>
  f2 map { y =>
    x.length * y
  }
}

val result = f2.get.get

The get method had to be used twice because f3 is a Future[Future[Int]] instead of the desired Future[Int]. Instead, the flatMap method should be used:

val f1 = Future {
  "Hello" + "World"
}

val f2 = Future {
  3
}

val f3 = f1 flatMap { x =>
  f2 map { y =>
    x.length * y
  }
}

val result = f2.get

For Comprehensions

Since Future has a map and flatMap method it can be easily used in a ‘for comprehension’:

val f = for {
  a <- Future(10 / 2) // 10 / 2 = 5
  b <- Future(a + 1)  //  5 + 1 = 6
  c <- Future(a - 1)  //  5 - 1 = 4
} yield b * c         //  6 * 4 = 24

val result = f.get

Something to keep in mind when doing this is even though it looks like parts of the above example can run in parallel, each step of the for comprehension is run sequentially. This will happen on separate threads for each step but there isn’t much benefit over running the calculations all within a single Future. The real benefit comes when the Futures are created first, and then combining them together.

Composing Futures

The example for comprehension above is an example of composing Futures. A common use case for this is combining the replies of several Actors into a single calculation without resorting to calling get or await to block for each result. First an example of using get:

val f1 = actor1 ? msg1
val f2 = actor2 ? msg2

val a: Int = f1.get
val b: Int = f2.get

val f3 = actor3 ? (a + b)

val result: String = f3.get

Here we wait for the results from the first 2 Actors before sending that result to the third Actor. We called get 3 times, which caused our little program to block 3 times before getting our final result. Now compare that to this example:

val f1 = actor1 ? msg1
val f2 = actor2 ? msg2

val f3 = for {
  a: Int    <- f1
  b: Int    <- f2
  c: String <- actor3 ? (a + b)
} yield c

val result = f3.get

Here we have 2 actors processing a single message each. Once the 2 results are available (note that we don’t block to get these results!), they are being added together and sent to a third Actor, which replies with a string, which we assign to ‘result’.

This is fine when dealing with a known amount of Actors, but can grow unwieldy if we have more then a handful. The sequence and traverse helper methods can make it easier to handle more complex use cases. Both of these methods are ways of turning, for a subclass T of Traversable, T[Future[A]] into a Future[T[A]]. For example:

// oddActor returns odd numbers sequentially from 1
val listOfFutures: List[Future[Int]] = List.fill(100)(oddActor ? GetNext)

// now we have a Future[List[Int]]
val futureList = Future.sequence(listOfFutures)

// Find the sum of the odd numbers
val oddSum = futureList.map(_.sum).get

To better explain what happened in the example, Future.sequence is taking the List[Future[Int]] and turning it into a Future[List[Int]]. We can then use map to work with the List[Int] directly, and we find the sum of the List.

The traverse method is similar to sequence, but it takes a T[A] and a function T => Future[B] to return a Future[T[B]], where T is again a subclass of Traversable. For example, to use traverse to sum the first 100 odd numbers:

val oddSum = Future.traverse((1 to 100).toList)(x => Future(x * 2 - 1)).map(_.sum).get

This is the same result as this example:

val oddSum = Future.sequence((1 to 100).toList.map(x => Future(x * 2 - 1))).map(_.sum).get

But it may be faster to use traverse as it doesn’t have to create an intermediate List[Future[Int]].

Then there’s a method that’s called fold that takes a start-value, a sequence of Future:s and a function from the type of the start-value and the type of the futures and returns something with the same type as the start-value, and then applies the function to all elements in the sequence of futures, non-blockingly, the execution will run on the Thread of the last completing Future in the sequence.

val futures = for(i <- 1 to 1000) yield Future(i * 2) // Create a sequence of Futures

val futureSum = Futures.fold(0)(futures)(_ + _)

That’s all it takes!

If the sequence passed to fold is empty, it will return the start-value, in the case above, that will be 0. In some cases you don’t have a start-value and you’re able to use the value of the first completing Future in the sequence as the start-value, you can use reduce, it works like this:

val futures = for(i <- 1 to 1000) yield Future(i * 2) // Create a sequence of Futures

val futureSum = Futures.reduce(futures)(_ + _)

Same as with fold, the execution will be done by the Thread that completes the last of the Futures, you can also parallize it by chunking your futures into sub-sequences and reduce them, and then reduce the reduced results again.

This is just a sample of what can be done, but to use more advanced techniques it is easier to take advantage of Scalaz, which Akka has support for in its akka-scalaz module.

Scalaz

Akka also has a Scalaz module (Add-on Modules) for a more complete support of programming in a functional style.