Add arbitrary instances for Down

src/Test/QuickCheck/Arbitrary.hs

@@ -157,6 +157,7 @@ import Control.Monad
 
 import Data.Int(Int8, Int16, Int32, Int64)
 import Data.Word(Word, Word8, Word16, Word32, Word64)
+import Data.Ord
 import System.Exit (ExitCode(..))
 import Foreign.C.Types
 
@@ -1061,6 +1062,17 @@ instance Arbitrary NewlineMode where
 #endif
 #endif
 
+#if defined(MIN_VERSION_base)
+#if MIN_VERSION_base(4,6,0)
+instance Arbitrary1 Down where
+  liftArbitrary = fmap Down
+  liftShrink shr = fmap Down . shr . getDown
+instance Arbitrary a => Arbitrary (Down a) where
+  arbitrary = arbitrary1
+  shrink = shrink1
+#endif
+#endif
+
 -- ** Helper functions for implementing arbitrary
 
 -- | Apply a binary function to random arguments.