Hi, I am trying to use the quickcheck to generate some test-data to test an api, along the lines of http://www.haskell.org/haskellwiki/QuickCheck_as_Test_Set_Generator. For the sake of example, I choose the following function to test data Result = Valid | Invalid api_under_test :: (Int,Int) -> Result api_under_test (x,y) | (x == 1) = Valid | otherwise = Invalid I had the following valid-generator which worked quite easily(trivial) validCombinations= [ (1,1), (1,2) ] validGen = elements validCombinations prop_valid_api_under_test = forAll validGen $ \xs -> (api_under_test xs) == (Valid) Now, I want to have a complement to state: forall tuples not in validCombinations, the api_under_test must return "Invalid". (i.e.) prop_invalid_api_under_test = forAll invalidGen $ \xs -> (api_under_test xs) == (Invalid) However, inspite of all googling, and reading the various docs including quickcheck, I am at loss on how I can elegantly define the "invalidGen" generator. One possible way I can think is to have a customized generator, that would generate two random numbers and then look up the tuple generated against the validCombinations list. However, I feel there just might be a better way of solving this. Any suggestion on how I should be trying to solve this. - Srikanth