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# 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