IteratorOnce[T]#copyToArray could return number of elems written rather than the array written to

joshlemer · July 7, 2018, 10:51pm

I’m not sure if this is where the Scala team wants people to discuss even minor ideas like this, but the website (https://www.scala-lang.org/contribute/bug-reporting-guide.html) indicates one should be very sure that what they are submitting is a bug before opening an issue. Wouldn’t it be better to have some kind of issue tracker for ideas/design/things that aren’t necessarily bugs?

Anyways I have just come to suggest that these three copyToArray methods on IterableOnce could return the number of elements written to the array passed in, rather than returning the array itself. My reasoning is that

the number of elements written to the array is non-deterministic – the IteratorOnce may run out of elements at any time. It doesn’t seem very easy to determine which elements written to the array are actually newly written elements after the method returns since of course they all have the same type.
The caller of the method of course already has access to the Array[T], so not much is lost by not returning the array, except if anyone really wants to avoid assigning the array to an identifier.

The methods in question are:

github.com

scala/scala/blob/2.13.x/src/library/scala/collection/IterableOnce.scala#L645


  *  or the end of the array is reached.
  *
  *  @param  xs     the array to fill.
  *  @param  start  the starting index of xs.
  *  @tparam B      the type of the elements of the array.
  *
  *  @usecase def copyToArray(xs: Array[A], start: Int): Unit
  *
  *  $willNotTerminateInf
  */
def copyToArray[B >: A](xs: Array[B], start: Int = 0): xs.type = {
  val it = iterator
  var i = start
  while (i < xs.length && it.hasNext) {
    xs(i) = it.next()
    i += 1
  }
  xs
}


/** Copy elements to an array.

github.com

scala/scala/blob/2.13.x/src/library/scala/collection/IterableOnce.scala#L673


  *  @param  start  the starting index of xs.
  *  @param  len    the maximal number of elements to copy.
  *  @tparam B      the type of the elements of the array.
  *
  *  @note    Reuse: $consumesIterator
  *
  *  @usecase def copyToArray(xs: Array[A], start: Int, len: Int): Unit
  *
  *    $willNotTerminateInf
  */
def copyToArray[B >: A](xs: Array[B], start: Int, len: Int): xs.type = {
  val it = iterator
  var i = start
  val end = start + math.min(len, xs.length - start)
  while (i < end && it.hasNext) {
    xs(i) = it.next()
    i += 1
  }
  xs
}

Thanks!

Edit: Sorry, I found the issue tracker, at github.com/scala/bug. I would delete this thread but it doesn’t look like that is possible on discord.