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View issue in M7 - NoSuchElementException thrown from iterator.toIterable.flatten (via flatMaps on views) #6271

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scabug opened this issue Aug 22, 2012 · 4 comments
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@scabug scabug commented Aug 22, 2012

affects 2.10.0 M7.

Reproducable via the repl (as long as -optimise isn't on of course).

val viewed : Iterable[Iterable[Iterable[Int]]] =
List(List(List(0).view).view).view

val filtered = viewed.flatMap{ x => x.flatMap( y => List( y.filter{ _
> 0 } ) ) }

filtered.iterator.toIterable.flatten

will throw the following exception and stack trace:

NoSuchElementException("next on empty iterator") from
at scala.collection.Iterator$$anon$2.next(Iterator.scala:38)
at scala.collection.Iterator$$anon$2.next(Iterator.scala:36)
at scala.collection.Iterator$$anon$14.next(Iterator.scala:394)
at scala.collection.IterableLike$class.head(IterableLike.scala:91)
at scala.collection.SeqViewLike$AbstractTransformed.head(SeqViewLike.scala:43)
at scala.collection.immutable.Stream.scala$collection$immutable$Stream$$flatten1$1(Stream.scala:909)
at scala.collection.immutable.Stream.flatten(Stream.scala:914)
at scala.collection.immutable.Stream.flatten(Stream.scala:185)

Drop the innermost view and it won't and you'll get the
correct Stream() as the result. Using Nil.view will work, using
List(1 ...) will work, anything as long as the first predicate to
Iterator.filter works.

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@scabug scabug commented Aug 22, 2012

Imported From: https://issues.scala-lang.org/browse/SI-6271?orig=1
Reporter: Chris (chris)
Other Milestones: 2.10.0

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@scabug scabug commented Aug 22, 2012

Chris (chris) said:
Jason investigated suggesting scala/scala@caf7eb6 is the culprit.

The code works as expected in M6 (and below to 2.8.1).

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@scabug scabug commented Sep 5, 2012

@jsuereth said:
Reduced test case:

List(0).view.flatMap(_ => List(List(1).view filter (_ < 1))).iterator.toIterable.flatten
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@scabug scabug commented Sep 5, 2012

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