Stack-safe unfold - scala

Question

I want to write an unfold for streams.
Fails to compile, because it's not tail recursive.
#annotation.tailrec
def unfold[A, B](a: A)(f: A => Option[(A, B)]): Stream[B] =
f(a).map { case (a, b) => b #:: unfold(a)(f) }.getOrElse(Stream.empty)
... not really working either. (Not lazy)
def unfold[A, B](a: A)(f: A => Option[(A, B)]): Stream[B] = {
#annotation.tailrec
def go(state: A, next: Stream[B]): Stream[B] = {
f(state) match {
case Some((a, b)) => go(a, b #:: next)
case None => Stream.empty
}
}
go(a, Stream.empty)
}
// unfold: [A, B](a: A)(f: A => Option[(A, B)])Stream[B]
unfold(0)(i => Some((i+1, i))).take(10).toList
// java.lang.OutOfMemoryError: GC overhead limit exceeded
// at .$anonfun$res3$1(<console>:18)
// at .$anonfun$res3$1$adapted(<console>:18)
// at $$Lambda$1543/299219071.apply(Unknown Source)
// at .go$1(<console>:19)
// at .unfold(<console>:24)
// ... 27 elided
Kinda ugly, via Iterator.
def unfold[A, B](a: A)(f: A => Option[(A, B)]): Stream[B] = {
new Iterator[B] {
private var state: Option[A] = Some(a)
private var nextResult: Option[B] = None
def next: B =
nextResult.getOrElse(
throw new NoSuchElementException("next on empty iterator"))
def hasNext: Boolean = {
if (nextResult.isDefined) {
true
} else {
state match {
case Some(s) =>
f(s) match {
case Some((nextState, produced)) =>
nextResult = Some(produced)
state = Some(nextState)
true
case None => false
}
case None => false
}
}
}
}.toStream
}

Answer 1

def unfold[A, B](a: A)(f: A => Option[(A, B)]): Stream[B] =
f(a).map { case (a, b) => b #:: unfold(a)(f) }.getOrElse(Stream.empty)
This is correct and lazy (although some REPLs might attempt to print all the stream).
Scala streams have their tails evaluated lazily. I don't think you can affect the tail of a stream in a lazy fashion in a tail-recursive way. However, it's pointless because laziness gives you stack safety:
assert(unfold(0) { _ => Some((0, 0)) } .toString == "Stream(0, ?)")
assert {
try {
unfold(0) { _ => Some((0, 0)) }.take(1000000).toList
true
} catch {
case _: StackOverflowError => false
}
}