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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