LeanCheck
=========
[![LeanCheck's Build Status][build-status]][build-log]
[![LeanCheck on Hackage][hackage-version]][leancheck-on-hackage]
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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's Haddock documentation]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html
[tutorial on property-based testing with LeanCheck]: https://github.com/rudymatela/leancheck/blob/master/doc/tutorial.md
[`Listable`]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html#t:Listable
[`holds`]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html#v:holds
[`counterExample`]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html#v:counterExample
[`check`]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html#v:check
[`checkFor`]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html#v:checkFor
[`checkResult`]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html#v:checkResult
[`checkResultFor`]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html#v:checkResultFor
[`tiers`]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html#v:tiers
[`list`]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html#v:list
[`deriveListable`]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html#v:deriveListable
[property-based testing]: https://github.com/rudymatela/leancheck/blob/master/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
[symbol `>`]: https://www.haskell.org/haddock/doc/html/ch03s08.html#idm140354810780208
[build-status]: https://travis-ci.org/rudymatela/leancheck.svg?branch=master
[build-log]: https://travis-ci.org/rudymatela/leancheck
[hackage-version]: https://img.shields.io/hackage/v/leancheck.svg
[leancheck-on-hackage]: https://hackage.haskell.org/package/leancheck
[stackage-lts-badge]: http://stackage.org/package/leancheck/badge/lts
[stackage-nightly-badge]: http://stackage.org/package/leancheck/badge/nightly
[leancheck-on-stackage]: http://stackage.org/package/leancheck
[leancheck-on-stackage-lts]: http://stackage.org/lts/package/leancheck
[leancheck-on-stackage-nightly]: http://stackage.org/nightly/package/leancheck