Proposal: Add Data.Semigroup to base, as a superclass of Monoid

[Mario Blažević]

On 06/11/13 14:46, John Wiegley wrote:
> 1. I propose that we add the following package to base:
>
>       http://hackage.haskell.org/packages/archive/semigroups/0.9.2/doc/html/Data-Semigroup.html
>
>     This is somewhat in the spirit of the AMP proposal: further improving the
>     correctness of our algebraic abstractions.


     I was wondering how much longer until this proposal came up. +1 
from me in general, but I have some quibbles with details.


> 2. That we make Semigroup a superclass of Monoid, so that (minimally):
>
>       class Semigroup a where
>           (<>) :: a -> a -> a
>
>       class Semigroup a => Monoid a where
>           mempty :: a
>           mconcat :: [a] -> a
>           mconcat = foldr (<>) mempty
>
>       mappend :: Semigroup a => a -> a -> a
>       mappend = (<>)


     +1, though I'd prefer to leave mappend restricted to the Monoid 
class. In the long term, I'd rather have it killed off than kept as a 
synonym for (<>). Besides, it's m(onoid)append.



> 3. (Optional, recommended) There are other useful functions that can be added
>     to Semigroup, such as sconcat and times1p, but I will let Edward speak to
>     whether those should be proposed at this time.


     They should be added in now or never, there's no reason to break 
compatibility twice. I don't think times1p can be accepted in its 
current form as it depends on a different library. Edward, can you make 
a concrete proposal for these?


>
> 4. (Optional, recommended) That we fix the Monoid instance for Maybe to be:
>
>      instance Semigroup a => Monoid (Maybe a) where
>           mempty = Nothing

>      instance Semigroup a => Semigroup (Maybe a) where
>           Just x <> Just y = Just (x <> y)
>           Nothing <> x = x
>           x <> Nothing = x


+1