Question

In the following situation

trait T {

 @tailrec
 def consume[A](as: Stream[A]): Unit = {
    if (as.isEmpty) ()
    else consume(as.tail)
  }
 }

object O extends T

calling O.consume(Range(1, N).toStream) with N big enough, the program will run out of memory, or at least will consume O(N) instead of the needed O(1).

Answer 1

The tail-recursive method is generated for the trait. The method entry in the extender of the trait (here O) forwards the call to the method of the trait, but while doing so, it keeps a reference to the head of the Stream.

Thus the method is tail-recursive, but memory still can’t be released. Remedy: Don’t define Stream functions in traits, just directly in objects.

An alternative is scalaz’s EphemeralStream, which holds weak references to the stream head and tail, and recomputes them on demand.