Semigroup (as superclass of) Monoid Proposal

THIS DESCRIPTION IS STILL WORK IN PROGRESS

Please comment on ​ghc:#10365 or ​ghc:#14191 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 Prelude 
    ( Semigroup((<>))
    , Monoid(mempty, mconcat)
    -- …
    ) where

-- …

module Data.Semigroup where

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

    -- not (re)exported from Prelude
    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
    -- & not (re)exported from Prelude
    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
    -- & not (re)exported from Prelude
    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) ​ghc:#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 phase 2.

Phase 2a (GHC 8.4) ​ghc:#14191

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

Phase 2b (GHC 8.4) ​ghc:#14191

• 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

Recommended Variant

The code below is expected to be -Wcompat -Wall clean (see also ​https://groups.google.com/forum/#!msg/haskell-core-libraries/PyxpE2ebS9Q/Ni0ywo_GCgAJ)

import Data.Semigroup as Sem
-- base >= 4.8: `Monoid` class is exported via `Prelude`
-- base < 4.11: re-exports `Monoid` class & common newtype wrappers
-- base >= 4.11: doesn't reexport `Monoid` class anymore

instance Sem.Semigroup Foo where
  (<>) = …

instance Monoid Foo where
  mempty = …

#if !(MIN_VERSION_base(4,11,0))
  -- this is redundant starting with base-4.11 / GHC 8.4
  -- if you want to avoid CPP, you can define `mappend = (<>)` unconditionally
  mappend = (<>)
#endif

If you need compatiblity with GHC prior to version 8.0 you can avoid -XCPP by depending conditionally on semigroups via

if !impl(ghc >= 8.0)
  build-depends: semigroups == 0.18.*

to provide a legacy Semigroup class.

Alternative w/ conditionally defined Semigroup instance & -XCPP

This variant can be used if you require support for GHC < 8.0 and you do not want to depend on the semigroups package.

#if MIN_VERSION_base(4,9,0)
-- Data.Semigroup was added in base-4.9
import Data.Semigroup as Sem
#endif
#if !(MIN_VERSION_base(4,8,0))
-- starting with base-4.8, Monoid is rexported from Prelude
import Data.Monoid
#endif

appendFoo :: Foo -> Foo -> Foo
appendFoo = …

#if MIN_VERSION_base(4,9,0)
instance Sem.Semigroup Foo where
  (<>) = appendFoo
#endif

instance Monoid Foo where
  mempty = …

#if MIN_VERSION_base(4,11,0)
-- starting with base-4.11, mappend definitions are redundant;
-- at some point `mappend` will be removed from `Monoid`
#elif MIN_VERSION_base(4,9,0)
  mappend = (Sem.<>)
#else // base < 4.9
-- prior to GHC 8.0 / base-4.9 where no `Semigroup` class existed
  mappend = appendFoo
#endif

---

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