Add a number of methods to `EitherOrBoth`

[glandium]

These are methods that I found useful for my use.

I also have a

impl<A, B> EitherOrBoth<EitherOrBoth<A, B>, EitherOrBoth<A, B>> {
    pub fn transpose(self) -> EitherOrBoth<EitherOrBoth<A, A>, EitherOrBoth<B, B>> { ... }
}

that is not part of this PR because it seemed really too niche. (My own real usecase is starting from EitherOrBoth<Either<A, B>, Either<A, B>>)

[phimuemue]

Hi there. Option<EitherOrBoth<L, R>> is isomorphic to (Option<L>, Option<R>), and the latter is arguably more straightforward.

We now have left_and_right which converts an EitherOrBoth to (Option<L>, Option<R>). As such, I'd like to close this, because all the proposed can be implemented in terms of Option-combinators:

• filter_{left|right|any}: Call Option::filter on the respective tuple elements.
• filter_map_{left|right|any}: Call Option::and_then on the respective tuple elements.

I know that calling the Option-combinators on the respective tuple elements may involve creating a temporary, but that could be mitigated by implementing methods on tuples (T0, T1) so that one could e.g write tuple_containing_two_options.map_first_element(Option::filter).

Would your use-case prohibit (Option<L>, Option<R>) instead of Option<EitherOrBoth<L, R>>? I.e. could you use left_and_right in your case?

[glandium]

In my case, Option<EitherOrBoth<L, R>> is more straightforward to use because ? can be used, which leaves me with an EitherOrBoth<L, R>, which is ultimately what I needed.

I'll add that since I submitted this PR, I've... actually stopped needed these methods, so they might very well be very niche-y.

[phimuemue]

I see, thanks for the explanation.

I close this for now and will come back to it if there seems to be more need for them.

Note to myself: Maybe we can implement ‘EitherOrBoth<Option, Option>‘ to have a method ‘ filter_out _none‘, returning the suggested type. Might compose well enough to cover the use cases.

[glandium]

map, filter and filter_map on EitherOrBoth<T, T> might be more easily useful, though, to apply the same closure to both sides.