[Haskell-cafe] ANN: LeanCheck v0.8.0 -- enumerative property testing
Rudy Matela
rudy at matela.com.br
Sun Nov 18 20:27:33 UTC 2018
Hello Haskell CafĂ©,
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
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