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| author | Bodigrim <andrew.lelechenko@gmail.com> | 2022-12-11 01:04:09 +0000 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2022-12-11 12:26:55 -0500 |
| commit | bfd7c1e653c207dd5dea599f16ee4afad03f2ea5 (patch) | |
| tree | 7015f7cc86702e0147b089c618dfdf19659c74c8 | |
| parent | b3e98a926ac05b8e59e3b31b74b019d4ecb462f6 (diff) | |
| download | haskell-bfd7c1e653c207dd5dea599f16ee4afad03f2ea5.tar.gz | |
Document that Bifunctor instances for tuples are lawful only up to laziness
| -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) |