@hackage quickcheck-with-counterexamples1.2

When QuickCheck finds a counterexample, it prints it out but doesn't save it so that the programmer can access it. This can be annoying when debugging.

This library provides a small extension to QuickCheck that returns counterexamples as normal Haskell values. To use it, just import Test.QuickCheck.Counterexamples instead of Test.QuickCheck. The library is largely compatible with normal QuickCheck, but the return types of quickCheck and related functions are changed to include a counterexample.

Here is an example of getting counterexamples. Suppose we have the following property:

prop_reverse_append :: [Int] -> [Int] -> Property prop_reverse_append xs ys = reverse (xs++ys) === reverse xs ++ reverse ys

If we look at the type of quickCheck, we see that it will return a counterexample:

:t quickCheck prop_reverse_append quickCheck prop_reverse_append :: IO (Maybe ([Int] :&: [Int] :&: ()))

The Maybe is there because we get Nothing if the property succeeds; ":&:" is a datatype of pairs.

If we run QuickCheck, we can get the counterexample as a normal Haskell value:

Just (xs :&: ys :&: ()) <- quickCheck prop_reverse_append *** Failed! Falsifiable (after 5 tests and 4 shrinks):
[0] [1] [1,0] /= [0,1]

:t xs xs :: [Int]

xs [0]

ys [1]