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.
In this README, lines ending with `-- >` indicate expected return values.
Installing
----------
To install the latest LeanCheck version from Hackage, just run:
$ 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
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 this
[tutorial on property-based testing with LeanCheck].
[LeanCheck's Haddock documentation]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html
[tutorial on property-based testing with LeanCheck]: doc/tutorial.md
[`Listable`]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html#t:Listable
[property-based testing]: doc/tutorial.md
[Feat]: https://hackage.haskell.org/package/testing-feat
[SmallCheck]: https://hackage.haskell.org/package/smallcheck
[QuickCheck]: https://hackage.haskell.org/package/QuickCheck