[commit: ghc] ghc-7.10: Add Eq, Ord, Show, and Read instances for Const (aaca7bd)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : ghc-7.10
Link       : http://ghc.haskell.org/trac/ghc/changeset/aaca7bdfa6202895946ac253a3196c4efa5747b7/ghc

>---------------------------------------------------------------

commit aaca7bdfa6202895946ac253a3196c4efa5747b7
Author: Fumiaki Kinoshita <fumiexcel at gmail.com>
Date:   Wed Jan 14 20:41:30 2015 +0900

    Add Eq, Ord, Show, and Read instances for Const
    
    As suggested in
    
    https://www.haskell.org/pipermail/libraries/2013-October/021531.html
    
    this adds the following instances
    
     - `Show a => Show (Const a b)`
     - `Read a => Read (Const a b)`
     - `Eq a   => Eq   (Const a b)`
     - `Ord a  => Ord  (Const a b)`
    
    The Read/Show instances are defined in such a way as if `Const` was defined
    without record-syntax (i.e. as `newtype Const a b = Const a`)
    
    Addresses #9984
    
    (cherry picked from commit c71fb84b8c9ec9c1e279df8c75ceb8a537801aa1)


>---------------------------------------------------------------

aaca7bdfa6202895946ac253a3196c4efa5747b7
 libraries/base/Control/Applicative.hs | 14 +++++++++++---
 libraries/base/changelog.md           |  3 ++-
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/libraries/base/Control/Applicative.hs b/libraries/base/Control/Applicative.hs
index a0627e4..02062e2 100644
--- a/libraries/base/Control/Applicative.hs
+++ b/libraries/base/Control/Applicative.hs
@@ -61,11 +61,19 @@ import Data.Functor ((<$>))
 import GHC.Base
 import GHC.Generics
 import GHC.List (repeat, zipWith)
-import GHC.Read (Read)
-import GHC.Show (Show)
+import GHC.Read (Read(readsPrec), readParen, lex)
+import GHC.Show (Show(showsPrec), showParen, showString)
 
 newtype Const a b = Const { getConst :: a }
-                  deriving (Generic, Generic1, Monoid)
+                  deriving (Generic, Generic1, Monoid, Eq, Ord)
+
+instance Read a => Read (Const a b) where
+    readsPrec d = readParen (d > 10)
+        $ \r -> [(Const x,t) | ("Const", s) <- lex r, (x, t) <- readsPrec 11 s]
+
+instance Show a => Show (Const a b) where
+    showsPrec d (Const x) = showParen (d > 10) $
+                            showString "Const " . showsPrec 11 x
 
 instance Foldable (Const m) where
     foldMap _ _ = mempty
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index 76a6a19..83ae5e4 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -176,7 +176,8 @@
   * There are now `Foldable` and `Traversable` instances for `Either a`,
    `Const r`, and `(,) a`.
 
-  * There is now a `Monoid`, `Generic`, and `Generic1` instance for `Const`.
+  * There are now `Show`, `Read`, `Eq`, `Ord`, `Monoid`, `Generic`, and
+    `Generic1` instances for `Const`.
 
   * There is now a `Data` instance for `Data.Version`.