Extrapolate

Extrapolate is a property-based testing library for Haskell capable of reporting generalized counter-examples.

Example

Consider the following (faulty) sort function and a property about it:

sort :: Ord a => [a] -> [a]
sort []      =  []
sort (x:xs)  =  sort (filter (< x) xs)
             ++ [x]
             ++ sort (filter (> x) xs)

prop_sortCount :: Ord a => a -> [a] -> Bool
prop_sortCount x xs  =  count x (sort xs) == count x xs
  where
  count x = length . filter (== x)

After testing the property, Extrapolate returns a fully defined counter-example along with a generalization:

> import Test.Extrapolate
> check (prop_sortCount :: Int -> [Int] -> Bool)
*** Failed! Falsifiable (after 4 tests):
0 [0,0]

Generalization:
x (x:x:xs)

This hopefully makes it easier to find the source of the bug. In this case, the faulty sort function discards repeated elements.

More documentation

For more examples, see the eg folder.