[Git][ghc/ghc][master] Fix `instance Bounded a => Bounded (Down a)` (#18716)

[Marge Bot]

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


Commits:
9ad51bc9 by David Beacham at 2020-10-27T13:59:35-04:00
Fix `instance Bounded a => Bounded (Down a)` (#18716)

* Flip `minBound` and `maxBound` to respect the change in ordering
* Remove awkward `Enum` (and hence `Integral`) instances for
  `Data.Ord.Down`
* Update changelog

- - - - -


2 changed files:

- libraries/base/Data/Ord.hs
- libraries/base/changelog.md


Changes:

=====================================
libraries/base/Data/Ord.hs
=====================================
@@ -28,11 +28,11 @@ import Data.Bits (Bits, FiniteBits)
 import Foreign.Storable (Storable)
 import GHC.Ix (Ix)
 import GHC.Base
-import GHC.Enum (Bounded, Enum)
+import GHC.Enum (Bounded(..))
 import GHC.Float (Floating, RealFloat)
 import GHC.Num
 import GHC.Read
-import GHC.Real (Fractional, Integral, Real, RealFrac)
+import GHC.Real (Fractional, Real, RealFrac)
 import GHC.Show
 
 -- |
@@ -66,10 +66,29 @@ clamp (low, high) a = min high (max a low)
 
 -- | The 'Down' type allows you to reverse sort order conveniently.  A value of type
 -- @'Down' a@ contains a value of type @a@ (represented as @'Down' a@).
+--
 -- If @a@ has an @'Ord'@ instance associated with it then comparing two
 -- values thus wrapped will give you the opposite of their normal sort order.
 -- This is particularly useful when sorting in generalised list comprehensions,
--- as in: @then sortWith by 'Down' x@
+-- as in: @then sortWith by 'Down' x at .
+--
+-- >>> compare True False
+-- GT
+--
+-- >>> compare (Down True) (Down False)
+-- LT
+--
+-- If @a@ has a @'Bounded'@ instance then the wrapped instance also respects
+-- the reversed ordering by exchanging the values of @'minBound'@ and
+-- @'maxBound'@.
+--
+-- >>> minBound :: Int
+-- -9223372036854775808
+--
+-- >>> minBound :: Down Int
+-- Down 9223372036854775807
+--
+-- All other instances of @'Down' a@ behave as they do for @a at .
 --
 -- @since 4.6.0.0
 newtype Down a = Down
@@ -81,12 +100,9 @@ newtype Down a = Down
       , Semigroup -- ^ @since 4.11.0.0
       , Monoid    -- ^ @since 4.11.0.0
       , Bits       -- ^ @since 4.14.0.0
-      , Bounded    -- ^ @since 4.14.0.0
-      , Enum       -- ^ @since 4.14.0.0
       , FiniteBits -- ^ @since 4.14.0.0
       , Floating   -- ^ @since 4.14.0.0
       , Fractional -- ^ @since 4.14.0.0
-      , Integral   -- ^ @since 4.14.0.0
       , Ix         -- ^ @since 4.14.0.0
       , Real       -- ^ @since 4.14.0.0
       , RealFrac   -- ^ @since 4.14.0.0
@@ -114,6 +130,13 @@ instance (Show a) => Show (Down a) where
 instance Ord a => Ord (Down a) where
     compare (Down x) (Down y) = y `compare` x
 
+-- | Swaps @'minBound'@ and @'maxBound'@ of the underlying type.
+--
+-- @since 4.14.0.0
+instance Bounded a => Bounded (Down a) where
+    minBound = Down maxBound
+    maxBound = Down minBound
+
 -- | @since 4.11.0.0
 instance Functor Down where
     fmap = coerce


=====================================
libraries/base/changelog.md
=====================================
@@ -44,6 +44,9 @@
   * `catMaybes` is now implemented using `mapMaybe`, so that it is both a "good
     consumer" and "good producer" for list-fusion (#18574)
 
+  * Correct `Bounded` instance and remove `Enum` and `Integral` instances for
+    `Data.Ord.Down`.
+
 ## 4.14.0.0 *TBA*
   * Bundled with GHC 8.10.1
 



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/9ad51bc9d2ad9168abad271f715ce73d3562218a

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/9ad51bc9d2ad9168abad271f715ce73d3562218a
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