Implement Semigroup as a superclass of Monoid Proposal (Phase 1)

More details in prime:Libraries/Proposals/SemigroupMonoid

Phase 1 (GHC 7.12)

Phase 1 consists in (at the very least) moving Data.Semigroup and Data.List.NonEmpty into base for GHC 7.12 (aka 8.0).

If there's enough time we will also implement a warning as part of the first phase:

Add a warning about definitions of an operator named (<>) that indicate it will be coming into Prelude in 7.14. We should warn about missing Semigroup instances at any use site of (<>) as they'll break in 7.14.

Edited Mar 10, 2019 by Herbert Valerio Riedel

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