I think the iffy feeling comes from the proposal breaking the distributive
law. For example:
(Right f <|> Right g) <*> Left x
= Left x
but
(Right f <*> Left x) <|> (Right g <*> Left x)
= Left (x <> x)
On Thu, Jun 14, 2018, 22:42 Tony Morris
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.
_______________________________________________ Libraries mailing listLibraries@haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries