# 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

Last modified 19 months ago
Last modified on Nov 24, 2015 8:03:43 PM