diff options
| -rw-r--r-- | libraries/base/Data/Bifunctor.hs | 13 |
1 files changed, 11 insertions, 2 deletions
diff --git a/libraries/base/Data/Bifunctor.hs b/libraries/base/Data/Bifunctor.hs index f014e5d3d4..217c4a2681 100644 --- a/libraries/base/Data/Bifunctor.hs +++ b/libraries/base/Data/Bifunctor.hs @@ -120,8 +120,17 @@ class (forall a. Functor (p a)) => Bifunctor p where second = bimap id - --- | @since 4.8.0.0 +-- | Class laws for tuples hold only up to laziness. Both +-- 'first' 'id' and 'second' 'id' are lazier than 'id' (and 'fmap' 'id'): +-- +-- >>> first id (undefined :: (Int, Word)) `seq` () +-- () +-- >>> second id (undefined :: (Int, Word)) `seq` () +-- () +-- >>> id (undefined :: (Int, Word)) `seq` () +-- *** Exception: Prelude.undefined +-- +-- @since 4.8.0.0 instance Bifunctor (,) where bimap f g ~(a, b) = (f a, g b) |