isLeft and isRight should be documented

[sanctuary-js]

isLeft and isRight should be documented

[davidchambers]

Another option is to provide and document the case-folding function:

either :: (a -> c) -> (b -> c) -> Either a b -> c

What do you think of this idea, @dotnetCarpenter?

[dotnetCarpenter]

@davidchambers Is that because you dislike Either.isLeft and Either.isRight?

Since js does not have pattern matching, both properties are extremely useful. I keep wishing fluture had something akin.

I used both a lot while developing PR#308 in sanctuary-def (not sure they are gonna stay in the finished PR though).
See line 1542 and line 1571

[davidchambers]

I prefer the case-folding function because it does not expose implementation details.

Here is a function-based implementation that can provide either but not isLeft and isRight properties:

//    Left :: a -> Either a b
const Left = a => left => right => left (a);

//    Right :: b -> Either a b
const Right = b => left => right => right (b);

//    either :: (a -> c) -> (b -> c) -> Either a b -> c
const either = f => g => e => e (f) (g);

//    showEither :: (Showable a, Showable b) => Either a b -> String
const showEither = either (a => 'Left (' + show (a) + ')')
                          (b => 'Right (' + show (b) + ')');

isLeft :: Either a b -> Boolean and isRight :: Either a b -> Boolean can be defined in terms of either. :)