Combinator to merge two futures yielding the same types

[ghost]

Supposed to solve #270.

I chose the name Branch to be consistent with future similar types with more variants (if ever useful), like Branch3, Branch4, etc, instead of Haskell's Either.

[alexcrichton]

Thanks for the PR! Could this be a new module, src/future/branch.rs, with reexports in src/future/mod.rs? (Like the other combinators).

I also wonder if we should call this Branch2 to have the naming be extensible into more variants later?

[alexcrichton]

cc @tailhook you tend to have a lot of insights on naming these combinators and such, does this look ok?

[Stebalien]

FYI, @bluss has already created an either crate for use cases like this so you might consider simply implementing Future on Either. Multiple branches can be achieved through recursion, albeit not quite as efficiently at the moment (better layout optimizations may fix that).

[alexcrichton]

I'd prefer to keep the dependencies slim for now if possible, but that's certainly an option to keep open (and can be done in parallel if need be)

[carllerche]

Personally I prefer the either name. I don't really think it has a hard implication of two variants.

[ghost]

I initially called it IfElse.

I don't really mind, but I've tried not to use Either because it's not very readable in Haskell, where it means both Result and "Branch combinator".

I'm just realising it's not a good reason, since Rust has an actual Result.

[spinda]

@pijul Do you plan on updating this so it can be merged in?

[ghost]

Update in which way?

[alexcrichton]

Ah yeah I'd still like to discuss naming a bit here before landing (there's quite a few possibilities). @aturon do you have thoughts on the particular names used?

[tailhook]

Well, the naming part is unclear here (this is actually why I haven't submitted a similar PR a while ago :) ).

Personally, I like current version:

if something {
  Branch2::A(something)
} else {
  Branch2::B(something_else)
}

I.e. it reads like branch 1 from 2 (but backwards).

The IfElse and Either could be okay but I think more than two branches are at least as useful as join5 that we already have.

[aturon]

My preference is for Either, and then later Either3, Either4 etc if needed. Mostly because of the strong precedent and straightforward reading.

Another question is what to call the variants. I've also seen these called Left and Right or First and Second, but just using single letters as this PR does seems like an improvement to me!

[alexcrichton]

Ok, @pijul want to rename this to Either but leave the names A and B?

[ghost]

Sure!

[alexcrichton]

Thanks!