LeanCheck’s Build Status LeanCheck on Hackage LeanCheck on Stackage LTS LeanCheck on Stackage Nightly

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.


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])
> holds 100 $ \xs -> [] `union` xs == (xs::[Int])

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])

> 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)])

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.

Providers for Tasty, test-framework and Hspec

The following providers allow including LeanCheck properties into Tasty, test-framework or Hspec test suites.

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).


Changelog for LeanCheck


  • export tiersFractional from Core and main module;
  • improve Listable instance for Floats and Doubles;
  • improve Show instance for functions;
  • improve Haddock documentation;
  • remove experimental function enumeration modules, in favour of the working ListsOfPairs enumeration;
  • add special String and Char types to Utils.Types;
  • fix bug in the Natural type of the Utils.Types modules;
  • force non-negativity in Natural and Nat types from Utils.Types;
  • rename some exported symbols in the ShowFunction module;
  • improve tests of LeanCheck itself.


  • Add a changelog.md file with the contents of git tag annotations: git tag -ln99.


  • Add experimental Test.LeanCheck.Generic module with automatic derivation of Listable instances through GHC.Generics;
  • Improve Haddock documentation.


  • Fix tests on systems with case-insensitive filesystems, like:
    • Windows;
    • Mac OS;
  • Fix tests on GHC 8.6.

This release fixes just the tests of LeanCheck itself. The LeanCheck library is otherwise unaffected.


  • Add list of providers on README;
  • Minor fix in haddock.


  • Fix bug: add missing Hugs backport file to source distribution (GHC users were not affected by this);
  • Improve tests so I don’t forget to include files in the source distribution (cabal sdist) again.


  • Significantly improve documentation;
  • Slightly improve tests.


  • LeanCheck now works on Hugs-200607 (only minor changes were needed);
  • Implement functions that calculate statistics: Test.LeanCheck.Stats;
  • More stuff on Utils: rational, okNum;
  • Improve tests;
  • Improve build scripts;
  • Minor assorted fixes.


  • Improved cabal file;
  • Cabal package now has all files checked in on git repo;
  • Add functions to compute Listable statistics (and some stubs);
  • Improve tests;
  • Code improvements (refactoring).


The only change in relation to v0.6.6 is a fixed build on Travis (the reference output files were outdated). The code of the tool is otherwise unchanged.


  • Improve showing of functional counter-examples.


  • Export ordering from ‘Test.LeanCheck.TypeBinding’;
  • Improve documentation;
  • Improve tests.

Earlier versions

Please refer to the git commit history.

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