Haskell: QuickCheck property fails tests using implications

Question

I've got the following property I want to test using quickcheck:

prop_zip xs ys = length xs == length ys ==> 
    unzip (zip xs ys) == (xs,ys)

Eventhough it seems to be logically right according to the definition of zip and unzip, that this property should be correct fo lists of the same length, the quickcheck ends with:

*** Gave up! Passed only 49 tests.

Thanks in advance for any hint or advice!

Answer 1

Preconditions that are difficult to meet by generating random test cases are often a bad idea in QuickCheck. Instead, you should use generators carefully to construct test cases that meet the precondition automatically.

For example, in this case, you can use forAll to generate the second list to be of the same length as the first list:

prop_zip' (xs :: [a]) =
  forAll (vectorOf (length xs) arbitrary) $ \ (ys :: [a]) ->
  unzip (zip xs ys) == (xs, ys)

(I'm additionally using ScopedTypeVariables here to determine the type of the second list. You might want to change this to suit your specific needs.)