Alternatively :) or more like, something to think about, an Alt instance for Either.

class Alt f where
  (<!>) :: f a -> f a -> f a

instance Alt Either where
  Right b <!> _ = Right b
  Left a <!> x = x

https://hackage.haskell.org/package/semigroupoids/docs/Data-Functor-Alt.html

There is something iffy about Monoid e => Alternative (Either e) but I can't put my finger on it.

On 06/14/2018 08:37 PM, Nathan van Doorn wrote:

Proposal: add:

instance Monoid e => Alternative (Either e) where

  empty = Left mempty

  Left a <|> Left b = Left (a `mappend` b)

  Right a <|> _ = Right a

  _ <|> Right b = Right b

instance Monoid e => MonadPlus (Either e) where

  ...

to base.

This is a reasonably obvious instance which I am pretty sure is law abiding.

It'd be useful for defining a series of computations which may fail, where we only care about one success.

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