There is no such general extension as far as I know, but the same code that produced warnings about impending AMP in 7.8 should work for this.
On 03/29/2015 08:20 AM, Jeremy 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.
I wasn't aware GHC provided a mechanism to do something like this. If it does, that is great news indeed. Can you point to the GHC extension (or, if not implemented yet, the ticket for creating the extension)?
Just to clarify, I'm +1 for the proposal. Even without any GHC magic, I'd be +1 for making Semigroup a superclass of Monoid even if it went through the same code-breaking steps as AMP.
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