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| author | sheaf <sam.derbyshire@gmail.com> | 2022-02-11 15:33:27 +0100 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2022-02-20 13:57:34 -0500 |
| commit | 845284a5ddceb043b1b1af6f130143f1ef160835 (patch) | |
| tree | aff71eacb5a36c9d71a907974030a59353c443bf | |
| parent | 92ab3ff2fbed8847747b43592546f6b98a6d03ec (diff) | |
| download | haskell-845284a5ddceb043b1b1af6f130143f1ef160835.tar.gz | |
Generically: remove redundant Semigroup constraint
This patch removes a redundant Semigroup constraint on the Monoid
instance for Generically. This constraint can cause trouble when
one wants to derive a Monoid instance via Generically through a type
that doesn't itself have a Semigroup instance, for example:
data Point2D a = Point2D !a !a
newtype Vector2D a = Vector2D { tip :: Point2D a }
deriving ( Semigroup, Monoid )
via Generically ( Point2D ( Sum a ) )
In this case, we should not require there to be an instance
Semigroup ( Point2D ( Sum a ) )
as all we need is an instance for the generic representation of
Point2D ( Sum a ), i.e. Semigroup ( Rep ( Point2D ( Sum a) ) () ).
| -rw-r--r-- | libraries/base/GHC/Generics.hs | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/libraries/base/GHC/Generics.hs b/libraries/base/GHC/Generics.hs index d4f56fd1e6..6ce5b70083 100644 --- a/libraries/base/GHC/Generics.hs +++ b/libraries/base/GHC/Generics.hs @@ -1428,7 +1428,7 @@ instance (Generic a, Semigroup (Rep a ())) => Semigroup (Generically a) where Generically a <> Generically b = Generically (to (from a <> from b :: Rep a ())) -- | @since 4.17.0.0 -instance (Semigroup a, Generic a, Monoid (Rep a ())) => Monoid (Generically a) where +instance (Generic a, Monoid (Rep a ())) => Monoid (Generically a) where mempty :: Generically a mempty = Generically (to (mempty :: Rep a ())) |