Semigroup (as superclass of) Monoid Proposal


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):

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
        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 = (<>)

TODO …integrate migration roadmap outlined in

Last modified 21 months ago Last modified on Nov 24, 2015 8:03:43 PM