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20638631
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2013-12-17T15:50:25.643
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2013-12-17T15:56:45.190
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2013-12-17T15:56:45.190
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Just to flesh out the answers, you could also define this as a recursive function using pattern matching: <pre><code>def flatMap[A, B](list: List[A])(f: A => List[B]): List[B] = list match { case (x::xs) => f(x) ++ flatMap(xs)(f) case _ => Nil } </code></pre> Or make it explicitly tail-recursive: <pre><code>import scala.annotation.tailrec def flatMapTailRec[A, B](list: List[A])(f: A => List[B]): List[B] = { @tailrec def _flatMap(result: List[B])(input: List[A])(f: A => List[B]): List[B] = input match { case (x::xs) => _flatMap(f(x) ++ result)(xs)(f) case _ => result } _flatMap(List[B]())(list)(f) } </code></pre> I did a bit of quick, non-rigorous benchmarking, using sample input of: <pre><code>val input = (0 to 1000).map(_ => (0 to 1000).toList).toList </code></pre> In order from fastest to slowest: <ul> <li><code>flatMap(input)(x => x)</code> <ul> <li>0.02069937453 seconds</li> </ul></li> <li><code>flatMapTailRec(input)(x => x)</code> <ul> <li>0.02335651054 seconds</li> </ul></li> <li><code>input.flatMap(x => x)</code> <ul> <li>0.0297564358 seconds</li> </ul></li> <li><code>flatMapFoldLeft(input)(x => x)</code> <ul> <li>12.940458234 seconds</li> </ul></li> </ul> I'm a little surprised that <code>foldLeft</code> comes out so much slower than the others. It would be interested to see how <code>flatMap</code> is actually defined in the source. I tried looking myself, but there's too much to go through at the moment >_>. Edit: As Daniel Sobral pointed out in the comments of another answer, these implementations are restricted to <code>List[A]</code>. You could write a more generic implementation that would work for any mappable type. The code would quickly become much more complicated.
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