Hello Haskell Café, A new version of LeanCheck is out (v0.8.0). LeanCheck is a property testing library (like QuickCheck) that tests values enumeratively. _Whats new?_ Among several changes, the most notable and significant are: * improvements in [LeanCheck's Haddock documentation]; * removal of a few experimental function enumeration modules; * and improved reporting of functional counter-examples (see below). [LeanCheck's changelog] provides more details. Take for example the following higher-order property that takes a functional argument and states an equivalence between `foldl` and `foldr`: prop_foldlr' :: (Int->Int->Int) -> Int -> [Int] -> Bool prop_foldlr' f z xs = foldl (flip f) z (reverse xs) == foldr f z xs You can check that it is correct by: > import Test.LeanCheck > import Test.LeanCheck.Function > check prop_foldlr' +++ OK, passed 200 tests. Now here is an incorrect version of the above property: prop_foldlr :: (A -> A -> A) -> A -> [A] -> Bool prop_foldlr f z xs = foldr f z xs == foldl f z xs You can check that it is incorrect by: > check prop_foldlr *** Failed! Falsifiable (after 75 tests): \x _ -> case x of 0 -> 1 _ -> 0 0 [0,0] LeanCheck reports the smallest counterexample it finds. The functional argument is now reported very concisely: a function that returns 1 whenever the first argument is 0 and returns 0 otherwise. Here's one last incorrect example property with two functional arguments: prop_mapFilter :: (Int->Int) -> (Int->Bool) -> [Int] -> Bool prop_mapFilter f p xs = filter p (map f xs) == map f (filter p xs) > check prop_mapFilter *** Failed! Falsifiable (after 36 tests): \_ -> 0 \x -> case x of 0 -> True _ -> False [1] The functions `map` and `filter` do not commute, the three values above are a counterexample. You can find LeanCheck on [Hackage] or [GitHub]. As usual, you can install it with: $ cabal install leancheck [Hackage]: https://hackage.haskell.org/package/leancheck [GitHub]: https://github.com/rudymatela/leancheck [LeanCheck's changelog]: https://hackage.haskell.org/package/leancheck/changelog [LeanCheck's Haddock documentation]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html