[Git][ghc/ghc][master] Document that Bifunctor instances for tuples are lawful only up to laziness

[Marge Bot (@marge-bot)]

Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
bfd7c1e6 by Bodigrim at 2022-12-11T12:26:55-05:00
Document that Bifunctor instances for tuples are lawful only up to laziness

- - - - -


1 changed file:

- libraries/base/Data/Bifunctor.hs


Changes:

=====================================
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)
 



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/bfd7c1e653c207dd5dea599f16ee4afad03f2ea5

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/bfd7c1e653c207dd5dea599f16ee4afad03f2ea5
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