Re: Alternative instance for Const

No, the Alternative instance should be based on semirings instead. The one you propose isn't distributive.

+1. -1 on non-distributive default instances. On 2020-03-21 21:22, Zemyla wrote:

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 . mailto:chessai1996@gmail.com> wrote:

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 mailto:Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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