Proposal: Make Semigroup as a superclass of Monoid

[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.

2. GHC >7.12 will define Monoid as a subclass of Semigroup.

Stage 2 could be delayed - or in the extreme case, cancelled - if the
warnings following stage 1 indicate that the proposal would cause
significant breakage of existing code, although this is not anticipated.

The rationale for this change is:

1. Semigroup is a popular package (this is relevant in combination with the
following point).
2. Using an existing Monoid as a Semigroup requires redefining it (or
WrappedMonoid), leading to much boilerplate and duplication.
3. NonEmpty is often redefined by beginners (or those who don't think it's
worth having an extra dependency for).

[1] https://mail.haskell.org/pipermail/libraries/2013-June/020188.html



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