Github user StephanEwen commented on the pull request:
https://github.com/apache/incubator-flink/pull/84#issuecomment-50518588
Throwing an error for a second `iterator()` call would be possible, but
quite some change.
For example in CoGroup, I use the simply `Collections.emptySet()` as the
Iterable for empty sides. The `KeyGroupedIterable` for the reduce function also
simply returns itself as the iterator, which means the original lightweight
grouping logic can stay the same.
Also, the some of old internal tests (Record-based) make use of the
possibility to mix the *Iterable* and the *Iterator*. I could change that, if
needed.
We could make this change, but I would like to hear some opinion on whether
this is actually more a confusing trap ("Why the heck do I get an empty set the
second time I iterate over the input"), or whether this is actually understood
that these inputs are what Scala calls a *TraversableOnce*
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