How to produce an Enumerator[T] given Enumerator[Future[T]]

Question

I'm trying to get my head around the Iteratee library in Play 2.2.x. I'm writing a function that generates a stream of data which is expensive to compute. Here's the basic idea, replacing the expensive to compute part with a square: ``` def expensiveFunction(x: Int): Future[Int] = Future.successful(x * x) def expensiveRange(start: Int, end: Int): Enumerator[Future[Int]] = { Enumerator.enumerate(start to end).map(i => expensiveFunction(i)) } ``` If I call `expensiveRange` I get an `Enumerator[Future[Int]]`, which is not what I want since it seems not to take advantage of the explicit asynchrony of the Iteratee pattern. It seems I should be able to transform this into an `Enumerator[Int]` which under the covers uses the futures I return instead of creating another layer of `Future`. Am I right in my thinking that transforming the result to `Enumerator[Int]` is desirable? If so, what is the idiomatic way to do so?

Accepted Answer

You could convert `Enumerator[Future[Int]]` to `Enumerator[Int]` this way:

```
val enumF = Enumerator((1 to 10).map{Future(_)}:_*) // Enumerator[Future[Int]]

val enum = enumF &> Enumeratee.mapM(identity) // Enumerator[Int]
```

But you don't need this conversion since, as Travis Brown noted, you could rewrite your method like this:

```
def expensiveRange(start: Int, end: Int): Enumerator[Int] = {
  Enumerator.enumerate(start to end) &> Enumeratee.mapM(expensiveFunction)
}
```

How to produce an Enumerator[T] given Enumerator[Future[T]]

Solution

Problem