Github user srowen commented on the issue:
https://github.com/apache/spark/pull/16276
Yes you're hitting a key point, that `flatten` only makes sense on an RDD
of `TraversableOnce`. I think we'd implement this in the way that
`DoubleRDDFunctions` adds operations for only RDDs of doubles instead of adding
this implicit transformation.
In that case you can get an `iterator` directly from each element, and then
concatenating them. I think it goes something like:
```
iter => iter.foldLeft(Iterator.empty)((a,b) => a ++ b.toIterator)
```
... but I'm sure that's not 100% correct.
Anyway the complexity makes me wonder if it's worth doing unless the lack
of a direct method is a bottleneck. (We could also look at whether Spark
internally could replace flatMap(x => x) with the new method)
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