Proposal: Make Semigroup as a superclass of Monoid

Reid Barton rwbarton at
Mon May 4 14:19:30 UTC 2015

On Sun, Mar 29, 2015 at 8:20 AM, Jeremy <voldermort at> wrote:

> 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

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
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
as a superclass of Monoid.

Reid Barton
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