# 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).