Add a Concat Iterator

[Keno]

If you have an iterator that returns iterators, it is to concat them into a single iterators. Alternatively, this might be named Flatten.

[StefanKarpinski]

Is this similar to chain? cf JuliaLang/julia#11842

[Keno]

Yes, except you can pass an iterator itself, instead of a list of iterators, which is helpful if the toplevel iteration is expensive and/or stateful.

[ghost]

I like it, but should we not export contact as opposed to Concat to be consistent with zip, enumerate, et al.?

[stevengj]

@Keno, shouldn't the chain implementation just take any iterator over iterators, then?

[stevengj]

Or maybe this should just be renamed to chain here if that is a more common name for this? Though I thought flatten was pretty recognizable.

[Keno]

The problem I see is that if you add a generic iterator of iterators, you can't do it statelessly. I think you can with chain.

[stevengj]

If that's true, chain(x) should have two implementations, one for x::Any, and one for x::AbstractVector. But we should still have one function (whatever it is called) that handles both cases, since the behavior is the same.

[Keno]

This now exists in Base.