Semigroup (as superclass of) Monoid Proposal

THIS DESCRIPTION IS STILL WORK IN PROGRESS

Please comment on ​ghc:#10365 if you notice some show-stopper issue

Introducing Semigroup as a superclass of Monoid has been proposed several times (in reverse chronological order):

• ​http://thread.gmane.org/gmane.comp.lang.haskell.libraries/24494
• ​http://thread.gmane.org/gmane.comp.lang.haskell.libraries/19649
• TODO …

Final API

The final API (suitable for Haskell Report inclusion) we want to end up with is

module Data.Semigroup where

class Semigroup a where
    (<>) :: a -> a -> a

    sconcat :: NonEmpty a -> a
    sconcat (a :| as) = go a as
      where
        go b (c:cs) = b <> go c cs
        go b []     = b

    -- GHC extension, not needed for Haskell Report
    stimes :: Integral b => b -> a -> a
    stimes y0 x0 = {- default impl -}

module Data.Monoid where

class Semigroup a => Monoid a where
    mempty  :: a

    mconcat :: [a] -> a
    mconcat = foldr (<>) mempty

    -- GHC extension, not needed for Haskell Report
    mtimes :: Integral b => b -> a -> a
    mtimes y0 x0 = {- default impl -}

-- GHC Extension: Legacy alias not needed for Haskell Report
mappend :: Semigroup a => a -> a -> a
mappend = (<>)

Migration plan

Phase 1 (GHC 8.0) #10365

• Move Data.Semigroup & Data.List.NonEmpty from semigroups-0.18 to base.

• (maybe) Implement a warning about definitions of an operator named (<>) that indicate it will be coming into Prelude in 8.2. We should warn about missing Semigroup instances at any use site of (<>) as they'll break in 8.2.

Phase 2a

• move Semigroup class into prelude in anticipation of it becoming a superclass of Monoid

Phase 2b (either merge with 2a or with 3)

• Make Semigroup a superclass of Monoid

Phase 3

• Deprecate manual definitions of mappend (c.f. "Monad of no return Proposal")
• encourage overriding the current default-implementation of (<>) via MINIMAL pragma

Phase 4

• Move the now deprecated mappend method out of the Monoid class, and possibly turn mappend into a legacy top-level binding (c.f. "Monad of no return Proposal")

Writing compatible code

import Data.Semigroup -- re-exports Data.Monoid (w/ Semigroup((<>)))

instance Semigroup Foo where
  (<>) = …

instance Monoid Foo where
  mempty = …

#if !(MIN_VERSION_base(5,0,0))
  -- assumption: Semigroup becomes superclass w/ base-5.0.0
  mappend = (<>)
#endif

---

TODO …integrate migration roadmap outlined in ​http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/24526