Extrapolate
Extrapolate is a property-based testing library for Haskell
capable of reporting generalized counter-examples.
Installing Extrapolate
To install the latest version of Extrapolate from Hackage using cabal, just:
$ cabal update
$ cabal install extrapolate
To test if it installed correctly, follow through the next section.
Starting from Cabal v3, you need to pass --lib
to cabal install
:
$ cabal install extrapolate --lib
Using Extrapolate
To use Extrapolate, you first import the Test.Extrapolate
module,
then pass any properties you with to test to the function check
:
$ ghci
> import Test.Extrapolate
> check $ \x y -> x + y == y + (x :: Int)
+++ OK, passed 360 tests.
> import Data.List (nub)
> check $ \xs -> nub xs == (xs :: [Int])
*** Failed! Falsifiable (after 3 tests):
[0,0]
Generalization:
x:x:_
The operator +
is commutative. The function nub
is not an identity.
Configuring the number of tests
To increase the number of tests, use the for
combinator:
$ ghci
> import Test.Extrapolate
> check `for` 1000 $ \x y -> x + y == y + (x :: Int)
+++ OK, passed 1000 tests.
Customizing the background functions (allowed in side-conditions)
To customize the background functions, use the withBackground
combinator:
$ ghci
> import Test.Extrapolate
> import Data.List (nub)
> let hasDups xs = nub xs /= (xs :: [Int])
> check `withBackground` [constant "hasDups" hasDups] $ \xs -> nub xs == (xs :: [Int])
*** Failed! Falsifiable (after 3 tests):
[0,0]
Generalization:
x:x:_
Conditional Generalization:
xs when hasDups xs
Perhaps the example above is silly (hasDups
is the negation of the property
itself!), but it illustrates the use of withBackground
.
The combinators for
and withBackground
can be used in conjunction:
> check `for` 100 `withBackground` [...] $ property
Don’t forget the dollar sign $
.
Another 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:_)
This hopefully makes it easier to find the source of the bug. In this case,
the faulty sort function discards repeated elements.
Further reading
For more examples, see the eg folder.
For type signatures, other options and uses,
see Extrapolate’s API documentation.
There are two other tools for Haskell capable of producing generalized
counter-examples: SmartCheck and Lazy SmallCheck 2012.
Extrapolate was accepted for presentation at IFL 2017, see the
IFL paper about Extrapolate.
Extrapolate is also subject to a chapter in a PhD Thesis (2017).