Proposal: Make Semigroup as a superclass of Monoid

[Merijn Verstraaten]

-2 from me.

Not because I oppose Semigroup as a superclass of Monoid, but because of brittle migration path.

I would much rather put the resurrection of DefaultSuperclassInstances [1], IntrinsicSuperclasses [2], or some new proposal on the roadmap that would allow us to more easily refactor and update the typeclass hierarchy without breaking everything all the time.

Cheers,
Merijn

[1] - https://ghc.haskell.org/trac/ghc/wiki/DefaultSuperclassInstances
[2] - https://ghc.haskell.org/trac/ghc/wiki/IntrinsicSuperclasses
> On 30 Mar 2015, at 5:39, John Alfred Nathanael Chee <cheecheeo at gmail.com> wrote:
> 
> Jeremy,
> 
> Do you propose:
> 
> Monoid a => Monoid (Maybe a)
> 
> be changed to:
> 
> Semigroup a => Monoid (Maybe a)
> 
> as the documentation indicates?
> 
> +1, but I think there are some technicalities that I think would benefit from a more concrete proposal including as a patch or hackage analysis.
> 
> On Sun, Mar 29, 2015 at 8:16 PM, M Farkas-Dyck <strake888 at gmail.com> wrote:
> On 29/03/2015 at 05:20:33 -0700, Jeremy wrote:
> > 1. GHC 7.12 will include Semigroup and NonEmpty in base.
> 
> +1 for Semigroup at least.
> 
> NonEmpty is simply the cofree comonad of Maybe, so I think we ought to define it either as such or to be compatibly redefinitile as such, e.g.
> 
> now:
> data NonEmpty a = a .: Maybe (NonEmpty a)
> 
> so potentially later:
> data Cofree f a = a .: f (Cofree f a)
> type NonEmpty = Cofree Maybe
> 
> as otherwise we shall have various code using nonsame isomorphic types which one must convert or coerce between for no good reason.
> 
> +1 for NonEmpty if so defined, -1 otherwise.
> 
> > 2. GHC >7.12 will define Monoid as a subclass of Semigroup.
> 
> +1
> 
> On 29/03/2015 at 11:23:19 -0400, David Feuer wrote:
> > I guess I should get a dog in the fight—I think <> should be the Semigroup method.
> 
> +1
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> Love in Jesus Christ, John Alfred Nathanael Chee
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