quickCheck

Tests a property and prints the results to stdout. By default up to 100 tests are performed, which may not be enough to find all bugs. To run more tests, use withMaxSuccess. If you want to get the counterexample as a Haskell value, rather than just printing it, try the quickcheck-with-counterexamples package.

Perform an efficient check on a string, to quickly determine if the string is in a particular normalization form. A Nothing result indicates that a definite answer could not be determined quickly, and a more thorough check is required, e.g. with isNormalized. The user may have to convert the string to its normalized form and compare the results. A result of Just True or Just False indicates that the string definitely is, or is not, in the given normalization form.

Create a Hedgehog Gen from a QuickCheck Gen. By default the Gen created will not have any shrinking, you can use Gen.shrink if you have a shrink function which you would like to apply.

Automatic testing of Haskell programs QuickCheck is a library for random testing of program properties. The programmer provides a specification of the program, in the form of properties which functions should satisfy, and QuickCheck then tests that the properties hold in a large number of randomly generated cases. Specifications are expressed in Haskell, using combinators provided by QuickCheck. QuickCheck provides combinators to define properties, observe the distribution of test data, and define test data generators. Most of QuickCheck's functionality is exported by the main Test.QuickCheck module. The main exception is the monadic property testing library in Test.QuickCheck.Monadic. If you are new to QuickCheck, you can try looking at the following resources:

• The official QuickCheck manual. It's a bit out-of-date in some details and doesn't cover newer QuickCheck features, but is still full of good advice.
• https://begriffs.com/posts/2017-01-14-design-use-quickcheck.html, a detailed tutorial written by a user of QuickCheck.

The quickcheck-instances companion package provides instances for types in Haskell Platform packages at the cost of additional dependencies.

The QuickCheck manual gives detailed information about using QuickCheck effectively. You can also try https://begriffs.com/posts/2017-01-14-design-use-quickcheck.html, a tutorial written by a user of QuickCheck. To start using QuickCheck, write down your property as a function returning Bool. For example, to check that reversing a list twice gives back the same list you can write:

import Test.QuickCheck

prop_reverse :: [Int] -> Bool
prop_reverse xs = reverse (reverse xs) == xs

You can then use QuickCheck to test prop_reverse on 100 random lists:

>>> quickCheck prop_reverse
+++ OK, passed 100 tests.

To run more tests you can use the withMaxSuccess combinator:

>>> quickCheck (withMaxSuccess 10000 prop_reverse)
+++ OK, passed 10000 tests.

To use QuickCheck on your own data types you will need to write Arbitrary instances for those types. See the QuickCheck manual for details about how to do that. When testing fails quickCheck will try to give you a minimal counterexample to your property: @ import Test.QuickCheck prop_reverse_bad :: [Int] -> Bool prop_reverse_bad xs = reverse xs == xs

>>> quickCheck prop_reverse_bad
*** Failed! Falsified (after 3 tests and 3 shrinks):
[0,1]
@

However, beware because not all properties that ought to fail will fail when you expect them to:

>>> quickCheck $  x y -> x == y
+++ Ok, passed 100 tests.

That's because GHCi will default any type variables in your property to (), so in the example above quickCheck was really testing that () is equal to itself. To avoid this behaviour it is best practise to monomorphise your polymorphic properties when testing:

>>> quickCheck $  x y -> (x :: Int) == y
*** Failed! Falsified (after 4 tests and 3 shrinks):
0
1

This module allows to use QuickCheck properties in tasty.

Provides Arbitrary instances for protocol types.

QuickCheck related utilities.

QuickCheck integration for Predicate

Use QuickCheck generators and Arbitrary instances with Hedgehog.

Servant.QuickCheck provides utilities related to using QuickCheck over an API. Rather than specifying properties that individual handlers must satisfy, you can state properties that ought to hold true of the entire API. While the API must be described with servant types, the server being tested itself need not be implemented with servant-server (or indeed, written in Haskell). The documentation of the Useful predicates sections is meant to serve as a set of helpful pointers for learning more about best practices concerning REST APIs.

Functions for introducing QuickCheck tests into a Sandwich test tree. Modelled after Hspec's version. Documentation can be found here.

Test all properties in the current module. The name of the property must begin with prop_. Polymorphic properties will be defaulted to Integer. Returns True if all tests succeeded, False otherwise. To use quickCheckAll, add a definition to your module along the lines of

return []
runTests = $quickCheckAll

and then execute runTests. Note: the bizarre return [] in the example above is needed on GHC 7.8 and later; without it, quickCheckAll will not be able to find any of the properties. For the curious, the return [] is a Template Haskell splice that makes GHC insert the empty list of declarations at that point in the program; GHC typechecks everything before the return [] before it starts on the rest of the module, which means that the later call to quickCheckAll can see everything that was defined before the return []. Yikes!

Tests a property, produces a test result, and prints the results to stdout.

Tests a property, using test arguments, and prints the results to stdout.

Tests a property, using test arguments, produces a test result, and prints the results to stdout.