Add Iterator::zip_longest.

[SimonSapin]

Like Iterator::zip, but the returned iterator is as long as the longest of the input iterators rather than shortest, and yields (Option<A>, Option<B>) (which can not both be None) rather than (A, B).

An alternative could be to yield a new EitherOrBoth (?) enum that is either Both(A, B), Left(A), or Right(B).

Precedent: https://docs.python.org/library/itertools.html#itertools.izip_longest

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @aturon (or someone else) soon.

[tikue]

I like the idea of EitherOrBoth better than (Option<A>, Option<B>). As it is now, I worry that usage would require (None, None) => unreachable!() in a lot of places.

[SimonSapin]

I’m slightly reluctant since EitherOrBoth sounds a lot like Either, which was removed: #9157

[tikue]

I don't have a preference on the name, but I think the functionality of EitherOrBoth leverages the type system better. Perhaps a name like ZippedOneOrBoth is easier to reason about? In any case, I don't think the name is as bad as Either was, because that was a general utility that should in most cases be replaced with a more specific enum and was otherwise redundant with Result, whereas this would be an enum specific to the Iterator's zip_longest method.

[SimonSapin]

It’s not so much about the name, it’s an a very similar type used to exist in std and was removed. @aturon, what do you think?

[thestinger]

I don't see what Either being removed has to do with this. The rationale for removing was that custom sum types are almost always more appropriate because they can communicate more information - although they lose the ability to have many useful convenience methods out of the box, at least without making deriving for it.

[aturon]

I'm personally fine with accepting this on an #[experimental] basis, and seeing whether we get enough usage to warrant a permanent place in std. I'm also in favor of a custom enum; EitherOrBoth is fine by me. (It could always have a convenience method to turn it into a pair of options).

[aturon]

cc @alexcrichton

[SimonSapin]

Rebased and switched to EitherOrBoth.

I’ve also published this on crates.io:

https://crates.io/crates/zip-longest
https://github.com/SimonSapin/rust-std-candidates

[aturon]

@SimonSapin The itertools crate might actually be a good place for this to go for the moment. I can easily imaging us importing adapters from that crate over time, as they prove their worth.

[SimonSapin]

@aturon: Ok. Feel free to postpone this PR to after 1.0.

Whoever maintains the itertools crate: feel free to copy or re-export the zip_longest crate.

[aturon]

Whoever maintains the itertools crate: feel free to copy or re-export the zip_longest crate.

Paging @bluss!

I'm going to close this for now. Thanks for your understanding, @SimonSapin.

[bluss]

Done. :-)

http://bluss.github.io/rust-itertools/doc/itertools/trait.Itertools.html#method.zip_longest

itertools is using the same license as Rust itself, for the very reason that it should be easy to lift things back and forth.