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
fromsemigroups-0.18
tobase
.
- (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 ofMonoid
Phase 2b (GHC 8.4) ghc:#14191
- Make
Semigroup
a superclass ofMonoid
Phase 3
- Deprecate manual definitions of
mappend
(c.f. "Monad of noreturn
Proposal") - encourage overriding the current default-implementation of
(<>)
viaMINIMAL
pragma
Phase 4
- Move the now deprecated
mappend
method out of theMonoid
class, and possibly turnmappend
into a legacy top-level binding (c.f. "Monad of noreturn
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