Add quickCheckCounterexample

src/Test/QuickCheck.hs

@@ -45,6 +45,10 @@ module Test.QuickCheck
   , quickCheckWith
   , quickCheckWithResult
   , quickCheckResult
+#ifndef NO_TYPEABLE
+  , quickCheckCounterexample
+  , quickCheckWithCounterexample
+#endif
   , recheck
   , isSuccess
     -- ** Running tests verbosely

src/Test/QuickCheck/Property.hs

@@ -269,6 +269,13 @@ coerceCounterexample (Cex a) = case cast a of
 castCounterexample :: Typeable a => Counterexample -> Maybe a
 castCounterexample (Cex a) = cast a
 
+data Counterexamples = NoCounterexamples
+                     | forall a. (Typeable a, Show a) => a :! Counterexamples
+
+toCounterexamples :: [Counterexample] -> Counterexamples
+toCounterexamples [] = NoCounterexamples
+toCounterexamples (Cex a : ces) = a :! toCounterexamples ces
+
 #define COUNTEREXAMPLES(a) , theCounterexamples a
 #else
 #define COUNTEREXAMPLES(a)

src/Test/QuickCheck/Test.hs

@@ -202,6 +202,23 @@ quickCheckWithResult :: Testable prop => Args -> prop -> IO Result
 quickCheckWithResult a p =
   withState a (\s -> test s (property p))
 
+#ifndef NO_TYPEABLE
+-- | Test a property and get counterexamples as a result. Can be used like:
+--
+-- @
+-- $> x :! _ <- quickCheckCounterexample $ \ x -> withCounterexample (x :: Int) (x > 0)
+-- *** Failed! Falsified (after 1 test):
+-- 0
+-- $> x
+-- 0
+quickCheckCounterexample :: Testable prop => prop -> IO Counterexamples
+quickCheckCounterexample = quickCheckWithCounterexample stdArgs
+
+-- | Test a property, using test arguments, and get counterexamples as a result.
+quickCheckWithCounterexample :: Testable prop => Args -> prop -> IO Counterexamples
+quickCheckWithCounterexample args p = toCounterexamples . counterexamples <$> quickCheckWithResult args p
+#endif
+
 -- | Re-run a property with the seed and size that failed in a run of 'quickCheckResult'.
 recheck :: Testable prop => Result -> prop -> IO ()
 recheck res@Failure{} = quickCheckWith stdArgs{ replay = Just (usedSeed res, usedSize res)} . once