Alternative instance for Const

[Andreas Abel]

 > 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 . <chessai1996 at gmail.com 
> <mailto: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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