I guess I'd need to add some tests. But please let me know if this feature is acceptable.

If this is to be done, I think it should be done consistently for filter, Iterators.filter, map, Iterators.map, reduce, mapreduce, foldl and foldr.

If this is to be done, I think it should be done consistently for filter, Iterators.filter, map, Iterators.map, reduce, mapreduce, foldl and foldr.

See #35293

From triage: we can't do this for map because the 0-collection case is useful (this has been discussed before). However it can be done for all the other listed functions.

• For mapreduce I assume you'd need to pass two functions up front? The result should always be a 1-argument function that accepts a collection/iterator.
• reduce, foldl, and foldr also need to accept an init keyword arg (on the first call, IMO).
• Idea from @StefanKarpinski : add mapper(f) to handle the x->map(f, x) case.

Is this good to go now?

Bump. Merge?