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ModuleQuickCheck-2.15.0.1Haskell2010

Test.QuickCheck

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:

Example1 expression
quickCheck prop_reverse+++ OK, passed 100 tests.

To run more tests you can use the withMaxSuccess combinator:

Example1 expression
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

Example1 expression
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
  • 27 types
  • 7 classes
  • 143 values

Running tests

9 declarations
valuequickCheck :: Testable prop => prop -> IO ()
#

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.

datadata Args
#

Args specifies arguments to the QuickCheck driver

Constructors

  • Args
    • replay :: Maybe (QCGen, Int)

      Should we replay a previous test? Note: saving a seed from one version of QuickCheck and replaying it in another is not supported. If you want to store a test case permanently you should save the test case itself.

    • maxSuccess :: Int

      Maximum number of successful tests before succeeding. Testing stops at the first failure. If all tests are passing and you want to run more tests, increase this number.

    • maxDiscardRatio :: Int

      Maximum number of discarded tests per successful test before giving up

    • maxSize :: Int

      Size to use for the biggest test cases

    • chatty :: Bool

      Whether to print anything

    • maxShrinks :: Int

      Maximum number of shrinks to do before giving up. Setting this to zero turns shrinking off.

Instances2Read, Show
  • Read ArgsDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Test
  • Show ArgsDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Test
datadata Result
#

Result represents the test result

Constructors

Instances1Show
  • Show ResultDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Test

Running tests verbosely

valueverboseCheck :: Testable prop => prop -> IO ()
#

Tests a property and prints the results and all test cases generated to stdout. This is just a convenience function that means the same as quickCheck . verbose.

Note: for technical reasons, the test case is printed out after the property is tested. To debug a property that goes into an infinite loop, use within to add a timeout instead.

valueverboseCheckWith :: Testable prop => Args -> prop -> IO ()
#

Tests a property, using test arguments, and prints the results and all test cases generated to stdout. This is just a convenience function that combines quickCheckWith and verbose.

Note: for technical reasons, the test case is printed out after the property is tested. To debug a property that goes into an infinite loop, use within to add a timeout instead.

valueverboseCheckWithResult :: Testable prop => Args -> prop -> IO Result
#

Tests a property, using test arguments, produces a test result, and prints the results and all test cases generated to stdout. This is just a convenience function that combines quickCheckWithResult and verbose.

Note: for technical reasons, the test case is printed out after the property is tested. To debug a property that goes into an infinite loop, use within to add a timeout instead.

valueverboseCheckResult :: Testable prop => prop -> IO Result
#

Tests a property, produces a test result, and prints the results and all test cases generated to stdout. This is just a convenience function that combines quickCheckResult and verbose.

Note: for technical reasons, the test case is printed out after the property is tested. To debug a property that goes into an infinite loop, use within to add a timeout instead.

Testing all properties in a module

These functions test all properties in the current module, using Template Haskell. You need to have a {-# LANGUAGE TemplateHaskell #-} pragma in your module for any of these to work.

valuequickCheckAll :: Q Exp
#

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!

Testing polymorphic properties

valuepolyQuickCheck :: Name -> ExpQ
#

Test a polymorphic property, defaulting all type variables to Integer.

Invoke as $(polyQuickCheck 'prop), where prop is a property. Note that just evaluating quickCheck prop in GHCi will seem to work, but will silently default all type variables to ()!

$(polyQuickCheck 'prop) means the same as quickCheck $(monomorphic 'prop). If you want to supply custom arguments to polyQuickCheck, you will have to combine quickCheckWith and monomorphic yourself.

If you want to use polyQuickCheck in the same file where you defined the property, the same scoping problems pop up as in quickCheckAll: see the note there about return [].

The Arbitrary typeclass: generation of random values

1 declaration
classclass Arbitrary a where
#

Random generation and shrinking of values.

QuickCheck provides Arbitrary instances for most types in base, except those which incur extra dependencies. For a wider range of Arbitrary instances see the quickcheck-instances package.

Methods

  • arbitrary :: Gen a

    A generator for values of the given type.

    It is worth spending time thinking about what sort of test data you want - good generators are often the difference between finding bugs and not finding them. You can use sample, label and classify to check the quality of your test data.

    There is no generic arbitrary implementation included because we don't know how to make a high-quality one. If you want one, consider using the testing-feat or generic-random packages.

    The QuickCheck manual goes into detail on how to write good generators. Make sure to look at it, especially if your type is recursive!

  • shrink :: a -> [a]

    Produces a (possibly) empty list of all the possible immediate shrinks of the given value.

    The default implementation returns the empty list, so will not try to shrink the value. If your data type has no special invariants, you can enable shrinking by defining shrink = genericShrink, but by customising the behaviour of shrink you can often get simpler counterexamples.

    Most implementations of shrink should try at least three things:

    1. Shrink a term to any of its immediate subterms. You can use subterms to do this.

    2. Recursively apply shrink to all immediate subterms. You can use recursivelyShrink to do this.

    3. Type-specific shrinkings such as replacing a constructor by a simpler constructor.

    For example, suppose we have the following implementation of binary trees:

    data Tree a = Nil | Branch a (Tree a) (Tree a)

    We can then define shrink as follows:

    shrink Nil = []
shrink (Branch x l r) =
  -- shrink Branch to Nil
  [Nil] ++
  -- shrink to subterms
  [l, r] ++
  -- recursively shrink subterms
  [Branch x' l' r' | (x', l', r') <- shrink (x, l, r)]

    There are a couple of subtleties here:

    • QuickCheck tries the shrinking candidates in the order they appear in the list, so we put more aggressive shrinking steps (such as replacing the whole tree by Nil) before smaller ones (such as recursively shrinking the subtrees).

    • It is tempting to write the last line as [Branch x' l' r' | x' <- shrink x, l' <- shrink l, r' <- shrink r] but this is the wrong thing! It will force QuickCheck to shrink x, l and r in tandem, and shrinking will stop once one of the three is fully shrunk.

    There is a fair bit of boilerplate in the code above. We can avoid it with the help of some generic functions. The function genericShrink tries shrinking a term to all of its subterms and, failing that, recursively shrinks the subterms. Using it, we can define shrink as:

    shrink x = shrinkToNil x ++ genericShrink x
  where
    shrinkToNil Nil = []
    shrinkToNil (Branch _ l r) = [Nil]

    genericShrink is a combination of subterms, which shrinks a term to any of its subterms, and recursivelyShrink, which shrinks all subterms of a term. These may be useful if you need a bit more control over shrinking than genericShrink gives you.

    A final gotcha: we cannot define shrink as simply shrink x = Nil:genericShrink x as this shrinks Nil to Nil, and shrinking will go into an infinite loop.

    If all this leaves you bewildered, you might try shrink = genericShrink to begin with, after deriving Generic for your type. However, if your data type has any special invariants, you will need to check that genericShrink can't break those invariants.

Instances114Arbitrary, …

Helper functions for implementing shrink

valueshrinkNothing :: a -> [a]
#

Returns no shrinking alternatives.

valueshrinkList :: (a -> [a]) -> [a] -> [[a]]
#

Shrink a list of values given a shrinking function for individual values.

valueshrinkMap :: Arbitrary a => (a -> b) -> (b -> a) -> b -> [b]
#

Map a shrink function to another domain. This is handy if your data type has special invariants, but is almost isomorphic to some other type.

shrinkOrderedList :: (Ord a, Arbitrary a) => [a] -> [[a]]
shrinkOrderedList = shrinkMap sort id

shrinkSet :: (Ord a, Arbitrary a) => Set a -> [Set a]
shrinkSet = shrinkMap fromList toList
valueshrinkMapBy :: (a -> b) -> (b -> a) -> (a -> [a]) -> b -> [b]
#

Non-overloaded version of shrinkMap.

valueshrinkBoundedEnum :: (Bounded a, Enum a, Eq a) => a -> [a]
#

Shrink an element of a bounded enumeration.

Example
data MyEnum = E0 | E1 | E2 | E3 | E4 | E5 | E6 | E7 | E8 | E9
   deriving (Bounded, Enum, Eq, Ord, Show)
Example1 expression
shrinkBoundedEnum E9[E0,E5,E7,E8]
Example1 expression
shrinkBoundedEnum E5[E0,E3,E4]
Example1 expression
shrinkBoundedEnum E0[]

Lifting of Arbitrary to unary and binary type constructors

classclass Arbitrary1 (f :: Type -> Type) where
#

Lifting of the Arbitrary class to unary type constructors.

Methods

Instances15Arbitrary1, …
classclass Arbitrary2 (f :: Type -> Type -> Type) where
#

Lifting of the Arbitrary class to binary type constructors.

Methods

Instances4Arbitrary2

The Gen monad: combinators for building random generators

1 declaration
newtypenewtype Gen a
#

A generator for values of type a.

The third-party packages QuickCheck-GenT and quickcheck-transformer provide monad transformer versions of Gen.

Instances5Monad, Functor, MonadFix, Applicative, Testable
  • Monad GenDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Gen
  • Functor GenDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Gen
  • MonadFix GenDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Gen
  • Applicative GenDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Gen
  • Testable prop => Testable (Gen prop)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Property

Generator combinators

valuechoose :: Random a => (a, a) -> Gen a
#

Generates a random element in the given inclusive range. For integral and enumerated types, the specialised variants of choose below run much quicker.

valuechooseAny :: Random a => Gen a
#

Generates a random element over the natural range of a.

valueoneof :: HasCallStack => [Gen a] -> Gen a
#

Randomly uses one of the given generators. The input list must be non-empty.

valuefrequency :: HasCallStack => [(Int, Gen a)] -> Gen a
#

Chooses one of the given generators, with a weighted random distribution. The input list must be non-empty.

valuegrowingElements :: HasCallStack => [a] -> Gen a
#

Takes a list of elements of increasing size, and chooses among an initial segment of the list. The size of this initial segment increases with the size parameter. The input list must be non-empty.

valuesized :: (Int -> Gen a) -> Gen a
#

Used to construct generators that depend on the size parameter.

For example, listOf, which uses the size parameter as an upper bound on length of lists it generates, can be defined like this:

listOf :: Gen a -> Gen [a]
listOf gen = sized $ \n ->
  do k <- choose (0,n)
     vectorOf k gen

You can also do this using getSize.

valuegetSize :: Gen Int
#

Returns the size parameter. Used to construct generators that depend on the size parameter.

For example, listOf, which uses the size parameter as an upper bound on length of lists it generates, can be defined like this:

listOf :: Gen a -> Gen [a]
listOf gen = do
  n <- getSize
  k <- choose (0,n)
  vectorOf k gen

You can also do this using sized.

valueresize :: HasCallStack => Int -> Gen a -> Gen a
#

Overrides the size parameter. Returns a generator which uses the given size instead of the runtime-size parameter.

valuescale :: (Int -> Int) -> Gen a -> Gen a
#

Adjust the size parameter, by transforming it with the given function.

valuesuchThat :: Gen a -> (a -> Bool) -> Gen a
#

Generates a value that satisfies a predicate.

valuesuchThatMap :: Gen a -> (a -> Maybe b) -> Gen b
#

Generates a value for which the given function returns a Just, and then applies the function.

valuesuchThatMaybe :: Gen a -> (a -> Bool) -> Gen (Maybe a)
#

Tries to generate a value that satisfies a predicate. If it fails to do so after enough attempts, returns Nothing.

Generators for lists

valuelistOf :: Gen a -> Gen [a]
#

Generates a list of random length. The maximum length depends on the size parameter.

valuelistOf1 :: Gen a -> Gen [a]
#

Generates a non-empty list of random length. The maximum length depends on the size parameter.

valueshuffle :: [a] -> Gen [a]
#

Generates a random permutation of the given list.

valuesublistOf :: [a] -> Gen [a]
#

Generates a random subsequence of the given list.

Generators for particular types

valuearbitrarySizedIntegral :: Integral a => Gen a
#

Generates an integral number. The number can be positive or negative and its maximum absolute value depends on the size parameter.

valuearbitrarySizedFractional :: Fractional a => Gen a
#

Uniformly generates a fractional number. The number can be positive or negative and its maximum absolute value depends on the size parameter.

valuearbitrarySizedBoundedIntegral :: (Bounded a, Integral a) => Gen a
#

Generates an integral number from a bounded domain. The number is chosen from the entire range of the type, but small numbers are generated more often than big numbers. Inspired by demands from Phil Wadler.

Running generators

valuegenerate :: Gen a -> IO a
#

Run a generator. The size passed to the generator is always 30; if you want another size then you should explicitly use resize.

Debugging generators

valuesample :: Show a => Gen a -> IO ()
#

Generates some example values and prints them to stdout.

valuesample' :: Gen a -> IO [a]
#

Generates some example values.

The Function typeclass: generation of random shrinkable, showable functions

14 declarations

Example of use:

Example5 expressions
:{let prop :: Fun String Integer -> Bool    prop (Fun _ f) = f "monkey" == f "banana" || f "banana" == f "elephant":}quickCheck prop*** Failed! Falsified (after 3 tests and 134 shrinks):{"elephant"->1, "monkey"->1, _->0}

To generate random values of type Fun a b, you must have an instance Function a. If your type has a Show instance, you can use functionShow to write the instance; otherwise, use functionMap to give a bijection between your type and a type that is already an instance of Function. See the Function [a] instance for an example of the latter.

For more information, see the paper "Shrinking and showing functions" by Koen Claessen.

datadata Fun a b
#

Generation of random shrinkable, showable functions.

To generate random values of type Fun a b, you must have an instance Function a.

See also applyFun, and Fn with GHC >= 7.8.

Constructors

  • Fun (a :-> b, b, Shrunk) (a -> b)
Instances3Functor, Show, Arbitrary
valueapplyFun :: Fun a b -> a -> b
#

Extracts the value of a function.

Fn is the pattern equivalent of this function.

prop :: Fun String Integer -> Bool
prop f = applyFun f "banana" == applyFun f "monkey"
      || applyFun f "banana" == applyFun f "elephant"
valueapplyFun2 :: Fun (a, b) c -> a -> b -> c
#

Extracts the value of a binary function.

Fn2 is the pattern equivalent of this function.

prop_zipWith :: Fun (Int, Bool) Char -> [Int] -> [Bool] -> Bool
prop_zipWith f xs ys = zipWith (applyFun2 f) xs ys == [ applyFun2 f x y | (x, y) <- zip xs ys]
valueapplyFun3 :: Fun (a, b, c) d -> a -> b -> c -> d
#

Extracts the value of a ternary function. Fn3 is the pattern equivalent of this function.

patternpattern Fn :: (a -> b) -> Fun a b
#

A modifier for testing functions.

prop :: Fun String Integer -> Bool
prop (Fn f) = f "banana" == f "monkey"
           || f "banana" == f "elephant"
patternpattern Fn2 :: (a -> b -> c) -> Fun (a, b) c
#

A modifier for testing binary functions.

prop_zipWith :: Fun (Int, Bool) Char -> [Int] -> [Bool] -> Bool
prop_zipWith (Fn2 f) xs ys = zipWith f xs ys == [ f x y | (x, y) <- zip xs ys]
patternpattern Fn3 :: (a -> b -> c -> d) -> Fun (a, b, c) d
#

A modifier for testing ternary functions.

classclass Function a where
#

The class Function a is used for random generation of showable functions of type a -> b.

There is a default implementation for function, which you can use if your type has structural equality. Otherwise, you can normally use functionMap or functionShow.

Methods

Instances53Function, …

The CoArbitrary typeclass: generation of functions the old-fashioned way

8 declarations
classclass CoArbitrary a where
#

Used for random generation of functions. You should consider using Test.QuickCheck.Fun instead, which can show the generated functions as strings.

If you are using a recent GHC, there is a default definition of coarbitrary using genericCoarbitrary, so if your type has a Generic instance it's enough to say

instance CoArbitrary MyType

You should only use genericCoarbitrary for data types where equality is structural, i.e. if you can't have two different representations of the same value. An example where it's not safe is sets implemented using binary search trees: the same set can be represented as several different trees. Here you would have to explicitly define coarbitrary s = coarbitrary (toList s).

Methods

  • coarbitrary :: a -> Gen b -> Gen b

    Used to generate a function of type a -> b. The first argument is a value, the second a generator. You should use variant to perturb the random generator; the goal is that different values for the first argument will lead to different calls to variant. An example will help:

    instance CoArbitrary a => CoArbitrary [a] where
  coarbitrary []     = variant 0
  coarbitrary (x:xs) = variant 1 . coarbitrary (x,xs)
Instances56CoArbitrary, …
value(><) :: (Gen a -> Gen a) -> (Gen a -> Gen a) -> Gen a -> Gen a
#

Deprecated. Use ordinary function composition instead

Combine two generator perturbing functions, for example the results of calls to variant or coarbitrary.

Type-level modifiers for changing generator behavior

20 declarations

These types do things such as restricting the kind of test data that can be generated. They can be pattern-matched on in properties as a stylistic alternative to using explicit quantification.

Examples:

-- Functions cannot be shown (but see Function)
prop_TakeDropWhile (Blind p) (xs :: [A]) =
  takeWhile p xs ++ dropWhile p xs == xs

prop_TakeDrop (NonNegative n) (xs :: [A]) =
  take n xs ++ drop n xs == xs

-- cycle does not work for empty lists
prop_Cycle (NonNegative n) (NonEmpty (xs :: [A])) =
  take n (cycle xs) == take n (xs ++ cycle xs)

-- Instead of forAll orderedList
prop_Sort (Ordered (xs :: [OrdA])) =
  sort xs == xs
newtypenewtype Blind a
#

Blind x: as x, but x does not have to be in the Show class.

Constructors

Instances9Functor, Enum, Eq, Integral, Num, Ord, …
  • Functor BlindDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Enum a => Enum (Blind a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Eq a => Eq (Blind a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Integral a => Integral (Blind a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Num a => Num (Blind a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Ord a => Ord (Blind a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Real a => Real (Blind a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Show (Blind a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Arbitrary a => Arbitrary (Blind a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
newtypenewtype Fixed a
#

Fixed x: as x, but will not be shrunk.

Constructors

Instances10Functor, Enum, Eq, Integral, Num, Ord, …
  • Functor FixedDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Enum a => Enum (Fixed a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Eq a => Eq (Fixed a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Integral a => Integral (Fixed a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Num a => Num (Fixed a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Ord a => Ord (Fixed a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Read a => Read (Fixed a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Real a => Real (Fixed a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Show a => Show (Fixed a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Arbitrary a => Arbitrary (Fixed a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
newtypenewtype OrderedList a
#

Ordered xs: guarantees that xs is ordered.

Constructors

Instances6Functor, Eq, Ord, Read, Show, Arbitrary
newtypenewtype NonEmptyList a
#

NonEmpty xs: guarantees that xs is non-empty.

Constructors

Instances6Functor, Eq, Ord, Read, Show, Arbitrary
datadata InfiniteList a
#

InfiniteList xs _: guarantees that xs is an infinite list. When a counterexample is found, only prints the prefix of xs that was used by the program.

Here is a contrived example property:

prop_take_10 :: InfiniteList Char -> Bool
prop_take_10 (InfiniteList xs _) =
  or [ x == 'a' | x <- take 10 xs ]

In the following counterexample, the list must start with "bbbbbbbbbb" but the remaining (infinite) part can contain anything:

Example1 expression
quickCheck prop_take_10*** Failed! Falsified (after 1 test and 14 shrinks):"bbbbbbbbbb" ++ ...

Constructors

Instances2Show, Arbitrary
newtypenewtype SortedList a
#

Sorted xs: guarantees that xs is sorted.

Constructors

Instances6Functor, Eq, Ord, Read, Show, Arbitrary
newtypenewtype Positive a
#

Positive x: guarantees that x > 0.

Constructors

Instances7Functor, Enum, Eq, Ord, Read, Show, …
  • Functor PositiveDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Enum a => Enum (Positive a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Eq a => Eq (Positive a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Ord a => Ord (Positive a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Read a => Read (Positive a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Show a => Show (Positive a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • (Num a, Ord a, Arbitrary a) => Arbitrary (Positive a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
newtypenewtype Negative a
#

Negative x: guarantees that x < 0.

Constructors

Instances7Functor, Enum, Eq, Ord, Read, Show, …
  • Functor NegativeDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Enum a => Enum (Negative a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Eq a => Eq (Negative a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Ord a => Ord (Negative a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Read a => Read (Negative a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Show a => Show (Negative a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • (Num a, Ord a, Arbitrary a) => Arbitrary (Negative a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
newtypenewtype NonZero a
#

NonZero x: guarantees that x /= 0.

Constructors

Instances7Functor, Enum, Eq, Ord, Read, Show, …
  • Functor NonZeroDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Enum a => Enum (NonZero a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Eq a => Eq (NonZero a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Ord a => Ord (NonZero a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Read a => Read (NonZero a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Show a => Show (NonZero a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • (Num a, Eq a, Arbitrary a) => Arbitrary (NonZero a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
newtypenewtype NonNegative a
#

NonNegative x: guarantees that x >= 0.

Constructors

Instances7Functor, Enum, Eq, Ord, Read, Show, …
newtypenewtype NonPositive a
#

NonPositive x: guarantees that x <= 0.

Constructors

Instances7Functor, Enum, Eq, Ord, Read, Show, …
newtypenewtype Large a
#

Large x: by default, QuickCheck generates Ints drawn from a small range. Large Int gives you values drawn from the entire range instead.

Constructors

Instances11Functor, Enum, Eq, Integral, Num, Ord, …
  • Functor LargeDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Enum a => Enum (Large a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Eq a => Eq (Large a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Integral a => Integral (Large a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Num a => Num (Large a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Ord a => Ord (Large a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Read a => Read (Large a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Real a => Real (Large a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Show a => Show (Large a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Ix a => Ix (Large a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • (Integral a, Bounded a) => Arbitrary (Large a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
newtypenewtype Small a
#

Small x: generates values of x drawn from a small range. The opposite of Large.

Constructors

Instances11Functor, Enum, Eq, Integral, Num, Ord, …
  • Functor SmallDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Enum a => Enum (Small a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Eq a => Eq (Small a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Integral a => Integral (Small a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Num a => Num (Small a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Ord a => Ord (Small a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Read a => Read (Small a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Real a => Real (Small a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Show a => Show (Small a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Ix a => Ix (Small a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Integral a => Arbitrary (Small a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
datadata Smart a
#

Smart _ x: tries a different order when shrinking.

Constructors

Instances3Functor, Show, Arbitrary
  • Functor SmartDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Show a => Show (Smart a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Arbitrary a => Arbitrary (Smart a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
newtypenewtype Shrink2 a
#

Shrink2 x: allows 2 shrinking steps at the same time when shrinking x

Constructors

Instances10Functor, Enum, Eq, Integral, Num, Ord, …
  • Functor Shrink2Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Enum a => Enum (Shrink2 a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Eq a => Eq (Shrink2 a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Integral a => Integral (Shrink2 a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Num a => Num (Shrink2 a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Ord a => Ord (Shrink2 a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Read a => Read (Shrink2 a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Real a => Real (Shrink2 a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Show a => Show (Shrink2 a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
  • Arbitrary a => Arbitrary (Shrink2 a)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Modifiers
newtypenewtype ASCIIString
#

ASCIIString: generates an ASCII string.

Constructors

Instances5Eq, Ord, Read, Show, Arbitrary
newtypenewtype UnicodeString
#

UnicodeString: generates a unicode String. The string will not contain surrogate pairs.

Constructors

Instances5Eq, Ord, Read, Show, Arbitrary
newtypenewtype PrintableString
#

PrintableString: generates a printable unicode String. The string will not contain surrogate pairs.

Constructors

Instances5Eq, Ord, Read, Show, Arbitrary

Property combinators

17 declarations
newtypenewtype Property
#

The type of properties.

Instances1Testable
classclass Testable prop where
#

The class of properties, i.e., types which QuickCheck knows how to test. Typically a property will be a function returning Bool or Property.

Methods

  • property :: prop -> Property

    Convert the thing to a property.

  • propertyForAllShrinkShow :: Gen a -> (a -> [a]) -> (a -> [String]) -> (a -> prop) -> Property

    Optional; used internally in order to improve shrinking. Tests a property but also quantifies over an extra value (with a custom shrink and show function). The Testable instance for functions defines propertyForAllShrinkShow in a way that improves shrinking.

Instances9Testable, …
  • Testable DiscardDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Property
  • Testable PropDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Property
  • Testable PropertyDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Property
  • Testable ResultDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Property
  • Testable BoolDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.Property
  • Testable ()Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Property
  • Testable prop => Testable (Gen prop)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Property
  • Testable prop => Testable (Maybe prop)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Property
  • (Arbitrary a, Show a, Testable prop) => Testable (a -> prop)Defined in QuickCheck-2.15.0.1 · Test.QuickCheck.Property
valueforAll :: (Show a, Testable prop) => Gen a -> (a -> prop) -> Property
#

Explicit universal quantification: uses an explicitly given test case generator.

valueshrinking
  1. :: Testable prop
  2. => (a -> [a])

    shrink-like function.

  3. -> a

    The original argument

  4. -> (a -> prop)
  5. -> Property
#

Shrinks the argument to a property if it fails. Shrinking is done automatically for most types. This function is only needed when you want to override the default behavior.

value(==>) :: Testable prop => Bool -> prop -> Property
#

Implication for properties: The resulting property holds if the first argument is False (in which case the test case is discarded), or if the given property holds. Note that using implication carelessly can severely skew test case distribution: consider using cover to make sure that your test data is still good quality.

datadata Discard
#

If a property returns Discard, the current test case is discarded, the same as if a precondition was false.

An example is the definition of ==>:

(==>) :: Testable prop => Bool -> prop -> Property
False ==> _ = property Discard
True  ==> p = property p
Discard
Instances1Testable
valuediscard :: a
#

A special error value. If a property evaluates discard, it causes QuickCheck to discard the current test case. This can be useful if you want to discard the current test case, but are somewhere you can't use ==>, such as inside a generator.

value(===) :: (Eq a, Show a) => a -> a -> Property
#

Like ==, but prints a counterexample when it fails.

value(=/=) :: (Eq a, Show a) => a -> a -> Property
#

Like /=, but prints a counterexample when it fails.

valuetotal :: NFData a => a -> Property
#

Checks that a value is total, i.e., doesn't crash when evaluated.

valueioProperty :: Testable prop => IO prop -> Property
#

Do I/O inside a property.

Warning: any random values generated inside of the argument to ioProperty will not currently be shrunk. For best results, generate all random values before calling ioProperty, or use idempotentIOProperty if that is safe.

valueidempotentIOProperty :: Testable prop => IO prop -> Property
#

Do I/O inside a property.

Warning: during shrinking, the I/O may not always be re-executed. Instead, the I/O may be executed once and then its result retained. If this is not acceptable, use ioProperty instead.

Controlling property execution

valueverbose :: Testable prop => prop -> Property
#

Prints out the generated test case every time the property is tested. Only variables quantified over inside the verbose are printed.

Note: for technical reasons, the test case is printed out after the property is tested. To debug a property that goes into an infinite loop, use within to add a timeout instead.

valueverboseShrinking :: Testable prop => prop -> Property
#

Prints out the generated test case every time the property fails, including during shrinking. Only variables quantified over inside the verboseShrinking are printed.

Note: for technical reasons, the test case is printed out after the property is tested. To debug a property that goes into an infinite loop, use within to add a timeout instead.

valuewithMaxSuccess :: Testable prop => Int -> prop -> Property
#

Configures how many times a property will be tested.

For example,

quickCheck (withMaxSuccess 1000 p)

will test p up to 1000 times.

valuewithin :: Testable prop => Int -> prop -> Property
#

Considers a property failed if it does not complete within the given number of microseconds.

Note: if the property times out, variables quantified inside the within will not be printed. Therefore, you should use within only in the body of your property.

Good: prop_foo a b c = within 1000000 ...

Bad: prop_foo = within 1000000 $ \a b c -> ...

Bad: prop_foo a b c = ...; main = quickCheck (within 1000000 prop_foo)

valuediscardAfter :: Testable prop => Int -> prop -> Property
#

Discards the test case if it does not complete within the given number of microseconds. This can be useful when testing algorithms that have pathological cases where they run extremely slowly.

valuewithDiscardRatio :: Testable prop => Int -> prop -> Property
#

Configures how many times a property is allowed to be discarded before failing.

For example,

quickCheck (withDiscardRatio 10 p)

will allow p to fail up to 10 times per successful test.

valuewithMaxShrinks :: Testable prop => Int -> prop -> Property
#

Configure the maximum number of times a property will be shrunk.

For example,

quickCheck (withMaxShrinks 100 p)

will cause p to only attempt 100 shrinks on failure.

valuemapSize :: Testable prop => (Int -> Int) -> prop -> Property
#

Adjust the test case size for a property, by transforming it with the given function.

Conjunction and disjunction

value(.&.) :: (Testable prop1, Testable prop2) => prop1 -> prop2 -> Property
#

Nondeterministic choice: p1 .&. p2 picks randomly one of p1 and p2 to test. If you test the property 100 times it makes 100 random choices.

What to do on failure

valuewitness :: (Typeable a, Show a, Testable prop) => a -> prop -> Property
#

Return a value in the witnesses field of the Result returned by quickCheckResult. Witnesses are returned outer-most first.

In ghci, for example:

Example3 expressions
[Wit x] <- fmap witnesses . quickCheckResult $ \ x -> witness x $ x == (0 :: Int)*** Failed! Falsified (after 2 tests):1x1:t xx :: Int
valuewhenFail' :: Testable prop => IO () -> prop -> Property
#

Performs an IO action every time a property fails. Thus, if shrinking is done, this can be used to keep track of the failures along the way.

valueexpectFailure :: Testable prop => prop -> Property
#

Indicates that a property is supposed to fail. QuickCheck will report an error if it does not fail.

Analysing test case distribution

4 declarations
valuelabel :: Testable prop => String -> prop -> Property
#

Attaches a label to a test case. This is used for reporting test case distribution.

For example:

prop_reverse_reverse :: [Int] -> Property
prop_reverse_reverse xs =
  label ("length of input is " ++ show (length xs)) $
    reverse (reverse xs) === xs
Example1 expression
quickCheck prop_reverse_reverse+++ OK, passed 100 tests:7% length of input is 76% length of input is 35% length of input is 44% length of input is 6...

Each use of label in your property results in a separate table of test case distribution in the output. If this is not what you want, use tabulate.

valuecollect :: (Show a, Testable prop) => a -> prop -> Property
#

Attaches a label to a test case. This is used for reporting test case distribution.

collect x = label (show x)

For example:

prop_reverse_reverse :: [Int] -> Property
prop_reverse_reverse xs =
  collect (length xs) $
    reverse (reverse xs) === xs
Example1 expression
quickCheck prop_reverse_reverse+++ OK, passed 100 tests:7% 76% 35% 44% 6...

Each use of collect in your property results in a separate table of test case distribution in the output. If this is not what you want, use tabulate.

valueclassify
  1. :: Testable prop
  2. => Bool

    True if the test case should be labelled.

  3. -> String

    Label.

  4. -> prop
  5. -> Property
#

Reports how many test cases satisfy a given condition.

For example:

prop_sorted_sort :: [Int] -> Property
prop_sorted_sort xs =
  sorted xs ==>
  classify (length xs > 1) "non-trivial" $
  sort xs === xs
Example1 expression
quickCheck prop_sorted_sort+++ OK, passed 100 tests (22% non-trivial).
valuetabulate :: Testable prop => String -> [String] -> prop -> Property
#

Collects information about test case distribution into a table. The arguments to tabulate are the table's name and a list of values associated with the current test case. After testing, QuickCheck prints the frequency of all collected values. The frequencies are expressed as a percentage of the total number of values collected.

You should prefer tabulate to label when each test case is associated with a varying number of values. Here is a (not terribly useful) example, where the test data is a list of integers and we record all values that occur in the list:

prop_sorted_sort :: [Int] -> Property
prop_sorted_sort xs =
  sorted xs ==>
  tabulate "List elements" (map show xs) $
  sort xs === xs
Example1 expression
quickCheck prop_sorted_sort+++ OK, passed 100 tests; 1684 discarded.List elements (109 in total): 3.7% 0 3.7% 17 3.7% 2 3.7% 6 2.8% -6 2.8% -7

Here is a more useful example. We are testing a chatroom, where the user can log in, log out, or send a message:

data Command = LogIn | LogOut | SendMessage String deriving (Data, Show)
instance Arbitrary Command where ...

There are some restrictions on command sequences; for example, the user must log in before doing anything else. The function valid :: [Command] -> Bool checks that a command sequence is allowed. Our property then has the form:

prop_chatroom :: [Command] -> Property
prop_chatroom cmds =
  valid cmds ==>
    ...

The use of ==> may skew test case distribution. We use collect to see the length of the command sequences, and tabulate to get the frequencies of the individual commands:

prop_chatroom :: [Command] -> Property
prop_chatroom cmds =
  wellFormed cmds LoggedOut ==>
  'collect' (length cmds) $
  'tabulate' "Commands" (map (show . 'Data.Data.toConstr') cmds) $
    ...
Example1 expression
quickCheckWith stdArgs{maxDiscardRatio = 1000} prop_chatroom+++ OK, passed 100 tests; 2775 discarded:60% 020% 115% 2 3% 3 1% 4 1% 5Commands (68 in total):62% LogIn22% SendMessage16% LogOut

Checking test case distribution

valuecover
  1. :: Testable prop
  2. => Double

    The required percentage (0-100) of test cases.

  3. -> Bool

    True if the test case belongs to the class.

  4. -> String

    Label for the test case class.

  5. -> prop
  6. -> Property
#

Checks that at least the given proportion of successful test cases belong to the given class. Discarded tests (i.e. ones with a false precondition) do not affect coverage.

Note: If the coverage check fails, QuickCheck prints out a warning, but the property does not fail. To make the property fail, use checkCoverage.

For example:

prop_sorted_sort :: [Int] -> Property
prop_sorted_sort xs =
  sorted xs ==>
  cover 50 (length xs > 1) "non-trivial" $
  sort xs === xs
Example1 expression
quickCheck prop_sorted_sort+++ OK, passed 100 tests; 135 discarded (26% non-trivial).Only 26% non-trivial, but expected 50%
valuecoverTable
  1. :: Testable prop
  2. => String
  3. -> [(String, Double)]
  4. -> prop
  5. -> Property
#

Checks that the values in a given table appear a certain proportion of the time. A call to coverTable table [(x1, p1), ..., (xn, pn)] asserts that of the values in table, x1 should appear at least p1 percent of the time that table appears, x2 at least p2 percent of the time that table appears, and so on.

Note: If the coverage check fails, QuickCheck prints out a warning, but the property does not fail. To make the property fail, use checkCoverage.

Continuing the example from the tabular combinator...

data Command = LogIn | LogOut | SendMessage String deriving (Data, Show)
prop_chatroom :: [Command] -> Property
prop_chatroom cmds =
  wellFormed cmds LoggedOut ==>
  'tabulate' "Commands" (map (show . 'Data.Data.toConstr') cmds) $
    ...

...we can add a coverage requirement as follows, which checks that LogIn, LogOut and SendMessage each occur at least 25% of the time:

prop_chatroom :: [Command] -> Property
prop_chatroom cmds =
  wellFormed cmds LoggedOut ==>
  coverTable "Commands" [("LogIn", 25), ("LogOut", 25), ("SendMessage", 25)] $
  'tabulate' "Commands" (map (show . 'Data.Data.toConstr') cmds) $
    ... property goes here ...
Example1 expression
quickCheck prop_chatroom+++ OK, passed 100 tests; 2909 discarded:56% 017% 110% 2 6% 3 5% 4 3% 5 3% 7Commands (111 in total):51.4% LogIn30.6% SendMessage18.0% LogOutTable 'Commands' had only 18.0% LogOut, but expected 25.0%
valuecheckCoverage :: Testable prop => prop -> Property
#

Check that all coverage requirements defined by cover and coverTable are met, using a statistically sound test, and fail if they are not met.

Ordinarily, a failed coverage check does not cause the property to fail. This is because the coverage requirement is not tested in a statistically sound way. If you use cover to express that a certain value must appear 20% of the time, QuickCheck will warn you if the value only appears in 19 out of 100 test cases - but since the coverage varies randomly, you may have just been unlucky, and there may not be any real problem with your test generation.

When you use checkCoverage, QuickCheck uses a statistical test to account for the role of luck in coverage failures. It will run as many tests as needed until it is sure about whether the coverage requirements are met. If a coverage requirement is not met, the property fails.

Example:

quickCheck (checkCoverage prop_foo)
valuecheckCoverageWith :: Testable prop => Confidence -> prop -> Property
#

Check coverage requirements using a custom confidence level. See stdConfidence.

An example of making the statistical test less stringent in order to improve performance:

quickCheck (checkCoverageWith stdConfidence{certainty = 10^6} prop_foo)
datadata Confidence
#

The statistical parameters used by checkCoverage.

Constructors

  • Confidence
    • certainty :: Integer

      How certain checkCoverage must be before the property fails. If the coverage requirement is met, and the certainty parameter is n, then you should get a false positive at most one in n runs of QuickCheck. The default value is 10^9.

      Lower values will speed up checkCoverage at the cost of false positives.

      If you are using checkCoverage as part of a test suite, you should be careful not to set certainty too low. If you want, say, a 1% chance of a false positive during a project's lifetime, then certainty should be set to at least 100 * m * n, where m is the number of uses of cover in the test suite, and n is the number of times you expect the test suite to be run during the project's lifetime. The default value is chosen to be big enough for most projects.

    • tolerance :: Double

      For statistical reasons, checkCoverage will not reject coverage levels that are only slightly below the required levels. If the required level is p then an actual level of tolerance * p will be accepted. The default value is 0.9.

      Lower values will speed up checkCoverage at the cost of not detecting minor coverage violations.

Instances1Show
  • Show ConfidenceDefined in QuickCheck-2.15.0.1 · Test.QuickCheck.State

Generating example test cases

valuelabelledExamples :: Testable prop => prop -> IO ()
#

Given a property, which must use label, collect, classify or cover to associate labels with test cases, find an example test case for each possible label. The example test cases are minimised using shrinking.

For example, suppose we test delete x xs and record the number of times that x occurs in xs:

prop_delete :: Int -> [Int] -> Property
prop_delete x xs =
  classify (count x xs == 0) "count x xs == 0" $
  classify (count x xs == 1) "count x xs == 1" $
  classify (count x xs >= 2) "count x xs >= 2" $
  counterexample (show (delete x xs)) $
  count x (delete x xs) == max 0 (count x xs-1)
  where count x xs = length (filter (== x) xs)

labelledExamples generates three example test cases, one for each label:

Example1 expression
labelledExamples prop_delete*** Found example of count x xs == 00[][]*** Found example of count x xs == 10[0][]*** Found example of count x xs >= 25[5,5][5]+++ OK, passed 100 tests:78% count x xs == 021% count x xs == 1 1% count x xs >= 2