Add function shrinkBoundedEnum to Test.QuickCheck.Extra.

Though the `QuickCheck` library provides a handy `arbitraryBoundedEnum`
function, it currently doesn't provide a `shrinkBoundedEnum`
counterpart. Users have asked for a function like this here:

nick8325/quickcheck#343

This commit adds our own version, for now. Perhaps, later on, we can
submit a PR to the `QuickCheck` repository to add this function, and
delete our own version.

lib/test-utils/src/Test/QuickCheck/Extra.hs

@@ -27,6 +27,7 @@ module Test.QuickCheck.Extra
 
       -- * Shrinking
     , liftShrinker
+    , shrinkBoundedEnum
     , shrinkInterleaved
     , shrinkMapWith
     , groundRobinShrink
@@ -207,6 +208,36 @@ genSized2With f genA genB = uncurry f <$> genSized2 genA genB
 interleaveRoundRobin :: [[a]] -> [a]
 interleaveRoundRobin = concat . L.transpose
 
+-- | Shrinks a 'Bounded' 'Enum' value.
+--
+-- Example:
+--
+-- @
+-- data MyEnum = A0 | A1 | A2 | A3 | A4 | A5 | A6 | A7
+--    deriving (Bounded, Enum, Eq, Ord, Show)
+-- @
+--
+-- >>> shrinkBoundedEnum A7
+-- [A0,A4,A6]
+--
+-- >>> shrinkBoundedEnum A4
+-- [A0,A2,A3]
+--
+-- >>> shrinkBoundedEnum A0
+-- []
+--
+-- See 'arbitraryBoundedEnum'.
+--
+shrinkBoundedEnum :: forall a. (Eq a, Enum a, Bounded a) => a -> [a]
+shrinkBoundedEnum a
+    | a == minBound =
+        []
+    | otherwise =
+        toEnum <$> filter (>= minBoundInt) (shrinkIntegral $ fromEnum a)
+  where
+    minBoundInt :: Int
+    minBoundInt = fromEnum (minBound @a)
+
 -- | Shrink the given pair in interleaved fashion.
 --
 -- Successive shrinks of the left and right hand sides are interleaved in the