# LeanCheck

LeanCheck is a simple enumerative property-based testing library. Properties
are defined as Haskell functions returning a boolean value which should be
`True`

for all possible choices of argument values. LeanCheck applies
enumerated argument values to these properties in search for a counterexample.
Properties can be viewed as parameterized unit tests.

LeanCheck works by producing *tiers* of test values: a possibly infinite list
of finite sublists of same-and-increasingly-sized values. This enumeration is
similar to Feat’s. However, the ranking and ordering of values are defined
differently. The interface is also different.

Throughout this README lines that begin with the symbol `>`

indicate a line
entered into an interactive interpreter (`ghci`

). The result of evaluating the
expression is then printed on the following line.

## Installing

To install the latest LeanCheck version from Hackage, just run:

```
$ cabal update
$ cabal install leancheck
```

## Checking if properties are True

To check if properties are True,
just use the function `holds`

`:: Testable a => Int -> a -> Bool`

.
It takes *two arguments*:
the *number of values* to test
and a *property* (function returning Bool),
then, it returns a boolean indicating whether the property holds.
See (ghci):

```
> import Test.LeanCheck
> import Data.List
> holds 100 $ \xs -> sort (sort xs) == sort (xs::[Int])
True
> holds 100 $ \xs -> [] `union` xs == (xs::[Int])
False
```

## Finding counter examples

To find counter examples to properties,
you can use the function `counterExample`

`:: Testable a => Int -> a -> Maybe [String]`

.
It takes *two arguments*:
the *number of values* to test
and a *property* (function returning Bool).
Then, it returns Nothing if no results are found or Just a list of Strings
representing the offending arguments to the property.
See (ghci):

```
> import Test.LeanCheck
> import Data.List
> counterExample 100 $ \xs -> sort (sort xs) == sort (xs::[Int])
Nothing
> counterExample 100 $ \xs -> [] `union` xs == (xs::[Int])
Just ["[0,0]"]
> counterExample 100 $ \xs ys -> xs `union` ys == ys `union` (xs::[Int])
Just ["[]","[0,0]"]
```

## Checking properties like in SmallCheck/QuickCheck

To “check” properties like in SmallCheck and QuickCheck
automatically printing results on standard output,
you can use the function `check`

`:: Testable a => a -> IO ()`

.

```
> import Test.LeanCheck
> import Data.List
> check $ \xs -> sort (sort xs) == sort (xs::[Int])
+++ OK, passed 200 tests.
> check $ \xs ys -> xs `union` ys == ys `union` (xs::[Int])
*** Failed! Falsifiable (after 4 tests):
[] [0,0]
```

The function `check`

tests for a maximum of 200 tests.
To check for a maximum of `n`

tests, use `checkFor`

`n`

.
To get a boolean result wrapped in `IO`

, use `checkResult`

or `checkResultFor`

.
There is no “quiet” option, just use `holds`

or `counterExample`

in that case.

## Testing user-defined types

LeanCheck works on properties with `Listable`

argument types.
`Listable`

instances are declared similarly to SmallCheck:

```
data MyType = MyConsA
| MyConsB Int
| MyConsC Int Char
| MyConsD String
instance Listable MyType where
tiers = cons0 MyConsA
\/ cons1 MyConsB
\/ cons2 MyConsC
\/ cons1 MyConsD
```

For types that do not have a constraning data invariant, instances can be
automatically derived with [Template Haskell] by using `deriveListable`

like
so:

```
deriveListable ''MyType
```

The `tiers`

function return a potentially infinite list of finite sub-lists
(tiers). Each successive tier has values of increasing size.

```
tiers :: Listable a => [[a]]
```

For convenience, the function `list`

returns a potentially infinite list
of values of the bound type:

```
list :: Listable a => [a]
```

So, for example:

```
> take 5 (list :: [(Int,Int)])
[(0,0),(0,1),(1,0),(0,-1),(1,1)]
```

The `list`

function can be used to debug your custom instances.

`Listable`

class instances are more customizable than what is described here:
check source comments or haddock documentation for details.

## Further reading

For a detailed documentation of each function, see
LeanCheck’s Haddock documentation.

For an introduction to property-based testing
and a step-by-step guide to LeanCheck, see the
tutorial on property-based testing with LeanCheck
(`doc/tutorial.md`

in the source repository).

LeanCheck is subject to a chapter in a PhD Thesis (2017).