On Sun, Mar 29, 2015 at 8:20 AM, Jeremy
The proposal to make Semigroup a superclass of Monoid was discussed a while ago [1], and the conclusion was to "put this off until the dust has settled from the AMP and FT changes".
Now that 7.10 is out, I would like to re-propose. The proposed plan is similar to AMP, but less invasive, as (in my subjective experience) user-defined Monoids are much less common than user-defined Monads.
1. GHC 7.12 will include Semigroup and NonEmpty in base. All Monoid instances, and anything else which forms a Semigroup, will have a Semigroup instance. GHC will issue a warning when it encounters an instance of Monoid which is not an instance of Semigroup.
Strongly opposed to adding a NonEmpty type to base. It's a step in the wrong direction: the problem it clumsily tries to address is solved much better by refinement types à la LiquidHaskell, which handles this and other whole classes of problems at once. Now, we don't have LiquidHaskell in GHC yet; but let's not settle for adding a NonEmpty type that we know is an inferior approach to base now, when it will likely be very hard to remove it in the future. I know there are some who use NonEmpty types currently, but I think their needs are just as well (if not better) met by putting the type in a small package outside of base with few dependencies.
2. GHC >7.12 will define Monoid as a subclass of Semigroup.
While it frustrates me to repeatedly see so much time spent by both GHC developers and Haskell library and application programmers on changes like this with fairly small upside, I don't have any fundamental objection to ending up in a state with Semigroup as a superclass of Monoid. Regards, Reid Barton