Turn a side-effecting function returning Option into an Iterator

Question

Assume we have a side-effecting "producer" function f: () => Option[T] which returns a Some when repeatedly called until a future point at which it will forever return None. (e.g. a wrapped Java API producing null at EOF might well have this sort of behaviour).

Is it possible to wrap this function into something like a TraversableOnce or an Iterator, with the following constraints:

• Standard Scala library constructs preferred
• The sequence could be arbitrarily long and holding all the values, e.g. in a Stream is not wanted
• Similarly there must be no possibility of stack overflow
• The visible user source code should not use a var
• Thread safety is not required

There are some useful methods on the Iterator object, but nothing that exactly matches my use case. Any ideas welcome!

Answer 1

This does the trick:

def wrap[T](f: () => Option[T]): Iterator[T] = {
  Iterator.continually(f()).takeWhile(_.isDefined).flatten      
}

REPL test:

scala> :paste
// Entering paste mode (ctrl-D to finish)
var i = 0
def sideEffectingFunc(): Option[Int] = {
  i += 1
  if (i < 10) Some(i)
  else None
}
// Exiting paste mode, now interpreting.

i: Int = 0
sideEffectingFunc: ()Option[Int]

scala> val it = wrap(sideEffectingFunc)
it: Iterator[Int] = non-empty iterator

scala> it.toList
res1: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9)