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.
LeanCheck implementation is easy to understand. LeanCheck's core is under 200 lines of code.
To install the latest LeanCheck version from Hackage just run:
$ cabal update
$ cabal install leancheck
Starting from Cabal v3.0, you need to pass --lib
as an argument to
cabal install
to install packages globally on the default
user environment:
$ cabal install leancheck --lib
If you already have LeanCheck installed Cabal may refuse to update to the latest version. To update, you need to reset your user's cabal installation with:
rm -rf ~/.cabal/{bin,lib,logs,share,store} ~/.ghc/*/
WARNING: the above command will erase all user-local packages.
LeanCheck has (official) packages available on Stackage, OpenSUSE, Gentoo, Arch Linux and NixOS.
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
As a rule-of-thumb, you should run holds for 500, 1 000, or 10 000 tests. With more than that you may run out-of-memory depending on the types being tested.
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]"]
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.
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.
LeanCheck comes out-of-the-box with Listable
instances for all types in the
Haskell 2010 Language Report with the intentional exception of a few types.
The leancheck-instances package aims to support types in the
Haskell Platform -- $ cabal install leancheck-instances
.
The following providers allow including LeanCheck properties into Tasty, test-framework or Hspec test suites.
$ cabal install tasty-leancheck
;$ cabal install test-framework-leancheck
;$ cabal install hspec-leancheck
.Due to the way it is implemented (using lists of lists), LeanCheck can be quite memory intensive if we set the maximum number of tests of a property to millions of values (YMMV).
For the default maximum number of tests (200) you should be safe on most cases. If you use 1 000 or 10 000 as the maximum number of tests for a property you're also generally safe. More than that, it is in a hit or miss basis.
For more details, see LeanCheck memory usage.
LeanCheck strives to be beginner/student friendly both in the interface and its implementation. For instance, to understand LeanCheck's core, one does not need to understand Monads as they aren't used at all there.
In the name of keeping the implementation easy to understand, a compromise were made in terms of performance (cf. LeanCheck memory usage).
LeanCheck is mostly Haskell 98 compliant and almost Haskell 2010 compliant.
With the exception of Listable
derivation modules (TH and Generics),
the only extension used by LeanCheck is CPP. This is to maintain
compatibility with different compilers. LeanCheck even compiles and runs on
Hugs98 from September 2006.
LeanCheck has 100% Haddock coverage with most functions having examples.
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).
LeanCheck has a list of frequently asked questions and answers.