Mapping the left side of an Either

[raine]

Let's explore if these functions in data.either would be useful in Sanctuary:

• orElse
• bimap
• leftMap

[benperez]

Nice suggestions! In Haskell Data.Either is an instance of Data.Bifunctor. Which would give us bimap, first (or leftMap), and second (or rightMap/fmap). orElse seems interesting too, what's an example use case for that?

[benperez]

I'm going to go ahead and take a crack at leftMap and biMap. @albrow and I had a need for this earlier today.

[davidchambers]

Are you still interested in working on this, @benperez? If not, I'm happy to investigate these options.

[blperez]

I can check it out this weekend after I submit my ramda PR.

[davidchambers]

That would be wonderful!

[benperez]

@blperez is an imposter, I'll take a look when if I get a chance this weekend.

[iliocatallo]

any news on this side?

[davidchambers]

I think we should implement Either#bimap in accordance with fantasyland/fantasy-land#137:

Either#bimap :: Either a b ~> (a -> c) -> (b -> d) -> Either c d

I'm preparing to move to a different continent in a couple of days, but I should be able to devote significant time to Sanctuary once I'm in Europe. If you'd like to contribute a pull request for this addition, though, please do so. :)

[jlmorgan]

orElse, orElseGet and orElseThrow are tied to extract/get.

Examples:

S.Right(1).extract();
// => 1
S.Left("NaN").extract();
// => ?

S.Right(1).orElse(2);
// => 1
S.Left("NaN").orElse(2);
// => 2

S.Right(1).orElseGet(() => 2);
// => 1
S.Left("NaN").orElseGet(() => 2);
// => 2

S.Right(1).orElseThrow(() => new Error("fatal: because reasons");
// => 1
S.Left("NaN").orElseThrow(() => new Error("fatal: because reasons");
// => throws Error

With extract being a possible mixed type, you would likely end up with getOrElse, getOrElseGet, and getOrElseThrow to avoid marrying to Comonad.

[davidchambers]

With extract being a possible mixed type

In other words, Either a b cannot support extract.

[jlmorgan]

Basically. Hence the get prefixed versions of the method that require you to provide a fallback value.

[jlmorgan]

Static versions come in handy when going through a map as well rather than having to purge all the lefts/nothings/failures.

[davidchambers]

Thanks to #229 we have fromEither :: b -> Either a b -> b. :)