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| author | Gabriella Gonzalez <Gabriel439@gmail.com> | 2022-07-24 12:54:21 -0700 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2022-07-25 09:42:40 -0400 |
| commit | 2a773259fae8ae178a80538223b3e7a9bda93717 (patch) | |
| tree | 7e584b3a56976de4174ba0ae087ffb6550557a30 | |
| parent | 918620d9e2f9e4a0122c57d7fdddbe34626f34b0 (diff) | |
| download | haskell-2a773259fae8ae178a80538223b3e7a9bda93717.tar.gz | |
Default implementation for mempty/(<>)
Approved by: https://github.com/haskell/core-libraries-committee/issues/61
This adds a default implementation for `mempty` and `(<>)` along
with a matching `MINIMAL` pragma so that `Semigroup` and `Monoid`
instances can be defined in terms of `sconcat` / `mconcat`.
The description for each class has also been updated to include the
equivalent set of laws for the `sconcat`-only / `mconcat`-only
instances.
| -rw-r--r-- | libraries/base/GHC/Base.hs | 22 | ||||
| -rw-r--r-- | libraries/base/changelog.md | 2 |
2 files changed, 23 insertions, 1 deletions
diff --git a/libraries/base/GHC/Base.hs b/libraries/base/GHC/Base.hs index 0ddd256426..c63ec1264f 100644 --- a/libraries/base/GHC/Base.hs +++ b/libraries/base/GHC/Base.hs @@ -211,6 +211,12 @@ infixr 6 <> -- -- [Associativity] @x '<>' (y '<>' z) = (x '<>' y) '<>' z@ -- +-- You can alternatively define `sconcat` instead of (`<>`), in which case the +-- laws are: +-- +-- [Unit]: @'sconcat' ('pure' x) = x@ +-- [Multiplication]: @'sconcat' ('join' xss) = 'sconcat' ('fmap' 'sconcat' xss)@ +-- -- @since 4.9.0.0 class Semigroup a where -- | An associative operation. @@ -218,6 +224,7 @@ class Semigroup a where -- >>> [1,2,3] <> [4,5,6] -- [1,2,3,4,5,6] (<>) :: a -> a -> a + a <> b = sconcat (a :| [ b ]) -- | Reduce a non-empty list with '<>' -- @@ -248,6 +255,8 @@ class Semigroup a where stimes :: Integral b => b -> a -> a stimes = stimesDefault + {-# MINIMAL (<>) | sconcat #-} + -- | The class of monoids (types with an associative binary operation that -- has an identity). Instances should satisfy the following: @@ -257,6 +266,13 @@ class Semigroup a where -- [Associativity] @x '<>' (y '<>' z) = (x '<>' y) '<>' z@ ('Semigroup' law) -- [Concatenation] @'mconcat' = 'foldr' ('<>') 'mempty'@ -- +-- You can alternatively define `mconcat` instead of `mempty`, in which case the +-- laws are: +-- +-- [Unit]: @'mconcat' ('pure' x) = x@ +-- [Multiplication]: @'mconcat' ('join' xss) = 'mconcat' ('fmap' 'mconcat' xss)@ +-- [Subclass]: @'mconcat' ('toList' xs) = 'sconcat' xs@ +-- -- The method names refer to the monoid of lists under concatenation, -- but there are many other instances. -- @@ -271,7 +287,9 @@ class Semigroup a => Monoid a where -- -- >>> "Hello world" <> mempty -- "Hello world" - mempty :: a + mempty :: a + mempty = mconcat [] + {-# INLINE mempty #-} -- | An associative operation -- @@ -297,6 +315,8 @@ class Semigroup a => Monoid a where {-# INLINE mconcat #-} -- INLINE in the hope of fusion with mconcat's argument (see !4890) + {-# MINIMAL mempty | mconcat #-} + -- | @since 4.9.0.0 instance Semigroup [a] where (<>) = (++) diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md index adc18af65c..06570b5371 100644 --- a/libraries/base/changelog.md +++ b/libraries/base/changelog.md @@ -11,6 +11,8 @@ * Add `applyWhen` to `Data.Function`. * Add functions `mapAccumM` and `forAccumM` to `Data.Traversable`, per the [Core Libraries proposal](https://github.com/haskell/core-libraries-committee/issues/65). + * Add default implementation of `(<>)` in terms of `sconcat` and `mempty` in + terms of `mconcat`. ## 4.17.0.0 *TBA* |