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

Reviewed By: ekmett

Differential Revision: https://phabricator.haskell.org/D619

2 files changed, 13 insertions, 4 deletions

diff --git a/libraries/base/Control/Applicative.hs b/libraries/base/Control/Applicative.hs
index a0627e440d..02062e2e19 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 76a6a191dd..83ae5e44c1 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`.