Friday, July 4, 2014

Scala Tail Recursion confusion

I was looking at a video of Martin Odersky's keynote during Scala Days 2014 and there was a sample tail recursion code that confused me:

private def sameLength[T, U](xs: List[T], ys: List[U]): Boolean = {
  if (xs.isEmpty) ys.isEmpty
  else ys.nonEmpty && sameLength(xs.tail, ys.tail)

On a quick glance, this did not appear to be tail recursive to me, as there is the && operation that needs to be called after the recursive call.

However, thinking a little more about it, && is a short-circuit operator and the recursive operation would get called only if the ys.nonEmpty statement evaluates to true, thus maintaining the definition of a tail recursion.

The decompiled class clarifies this a little more, surprisingly the && operator does not appear anywhere in the decompiled code!:

public <T, U> boolean org$bk$sample$SameLengthTest$$sameLength(List<T> xs, List<U> ys)
    for (; ys.nonEmpty(); xs = (List)xs.tail()) ys = (List)ys.tail();
      xs.isEmpty() ? ys.isEmpty() : 

If the operator were changed to something that does not have short-circuit behavior, the method of course will not be a tail-recursion at that point, say a hypothetical method with the XOR operator:

private def notWorking[T, U](xs: List[T], ys: List[U]): Boolean = {
  if (xs.isEmpty) ys.isEmpty
  else ys.nonEmpty ^ notWorking(xs.tail, ys.tail)

Something fairly basic that tripped me up today!

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