Alternative instance for Const

[Zemyla]

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 . <chessai1996 at 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.
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