No, the Alternative instance should be based on semirings instead. The one
you propose isn't distributive.
Lacking general semirings, the best we can do at the moment is newtypes for
each semiring we have. For instance:
newtype NumConst a b = NumConst { getNumConst :: a }
deriving (Functor)
instance Num a => Applicative (NumConst a) where
pure = const (NumConst 1)
(<*>) = (coerce :: (a -> a -> a) -> NumConst a (u -> v) -> NumConst a u
-> NumConst a v) (*)
instance Num a => Alternative (NumConst a) where
empty = NumConst 0
(<|>) = (coerce :: (a -> a -> a) -> NumConst a b -> NumConst a b ->
NumConst a b) (+)
On Sat, Mar 21, 2020, 13:44 chessai .
We already have
instance Monoid m => Applicative (Const m)
we could easily add
instance Monoid m => Alternative (Const m) where empty = coerce mempty (<|>) = coerce (<>)
which trivially satisfies left/right identity and associativity.
I propose we add this instance to base. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries