Conditional execution of Applicative expressions. For example,

when debug (putStrLn "Debugging")

will output the string Debugging if the Boolean value debug is True, and otherwise do nothing.

void value discards or ignores the result of evaluation, such as the return value of an IO action.

Examples

>>> void Nothing
Nothing
>>> void (Just 3)
Just ()

>>> void (Left 8675309)
Left 8675309
>>> void (Right 8675309)
Right ()

>>> void [1,2,3]
[(),(),()]

>>> void (1,2)
(1,())

>>> mapM print [1,2]
1
2
[(),()]
>>> void $ mapM print [1,2]
1
2

The reverse of when.

A variant of labelledExamples that takes test arguments and returns a result.

A variant of labelledExamples that returns a result.

A variant of labelledExamples that takes test arguments.

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:

>>> labelledExamples prop_delete
*** Found example of count x xs == 0
0
[]
[]

*** Found example of count x xs == 1
0
[0]
[]

*** Found example of count x xs >= 2
5
[5,5]
[5]

+++ OK, passed 100 tests:
78% count x xs == 0
21% count x xs == 1
 1% count x xs >= 2

Test all properties in the current module. This is just a convenience function that combines quickCheckAll and verbose.

verboseCheckAll has the same issue with scoping as quickCheckAll: see the note there about return [].

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!

List all properties in the current module.

$allProperties has type [(String, Property)].

allProperties has the same issue with scoping as quickCheckAll: see the note there about return [].

Test all properties in the current module, using a custom quickCheck function. The same caveats as with quickCheckAll apply.

$forAllProperties has type (Property -> IO Result) -> IO Bool. An example invocation is $forAllProperties quickCheckResult, which does the same thing as $quickCheckAll.

forAllProperties has the same issue with scoping as quickCheckAll: see the note there about return [].

Monomorphise an arbitrary property by defaulting all type variables to Integer.

For example, if f has type Ord a => [a] -> [a] then $(monomorphic 'f) has type [Integer] -> [Integer].

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

Test a polymorphic property, defaulting all type variables to Integer. This is just a convenience function that combines verboseCheck and monomorphic.

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

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 [].

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.

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.

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.

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.

Re-run a property with the seed and size that failed in a run of quickCheckResult.

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

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 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.

The default test arguments

Check if the test run result was a success

Args specifies arguments to the QuickCheck driver

Result represents the test result

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

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

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

Take the disjunction of several properties.

Disjunction: p1 .||. p2 passes unless p1 and p2 simultaneously fail.

Take the conjunction of several properties.

Conjunction: p1 .&&. p2 passes if both p1 and p2 pass.

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.

Like forAllShrink, but without printing the generated value.

Like forAllShrink, but with an explicitly given show function.

Like forAll, but tries to shrink the argument for failing test cases.

Like forAll, but without printing the generated value.

Like forAll, but with an explicitly given show function.

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

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.

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)

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.

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 ...

>>> quickCheck prop_chatroom
+++ OK, passed 100 tests; 2909 discarded:
56% 0
17% 1
10% 2
 6% 3
 5% 4
 3% 5
 3% 7

Commands (111 in total):
51.4% LogIn
30.6% SendMessage
18.0% LogOut

Table 'Commands' had only 18.0% LogOut, but expected 25.0%

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

>>> 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) $
    ...

>>> quickCheckWith stdArgs{maxDiscardRatio = 1000} prop_chatroom
+++ OK, passed 100 tests; 2775 discarded:
60% 0
20% 1
15% 2
 3% 3
 1% 4
 1% 5

Commands (68 in total):
62% LogIn
22% SendMessage
16% LogOut

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

>>> quickCheck prop_sorted_sort
+++ OK, passed 100 tests; 135 discarded (26% non-trivial).

Only 26% non-trivial, but expected 50%

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

>>> quickCheck prop_sorted_sort
+++ OK, passed 100 tests (22% non-trivial).

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

>>> quickCheck prop_reverse_reverse
+++ OK, passed 100 tests:
7% 7
6% 3
5% 4
4% 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.

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

>>> quickCheck prop_reverse_reverse
+++ OK, passed 100 tests:
7% length of input is 7
6% length of input is 3
5% length of input is 4
4% 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.

The standard parameters used by checkCoverage: certainty = 10^9, tolerance = 0.9. See Confidence for the meaning of the parameters.

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)

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)

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

In ghci, for example:

>>> [Wit x] <- fmap witnesses . quickCheckResult $ \ x -> witness x $ x == (0 :: Int)
*** Failed! Falsified (after 2 tests):
1
>>> x
1
>>> :t x
x :: Int

Configure the maximum size a property will be tested at.

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.

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.

Configures how many times a property will be tested.

For example,

quickCheck (withMaxSuccess 1000 p)

will test p up to 1000 times.

Modifies a property so that it will be tested repeatedly. Opposite of once.

Modifies a property so that it only will be tested once. Opposite of again.

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

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.

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.

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.

Performs an IO action after the last failure of a property.

Adds the given string to the counterexample if the property fails.

Adds the given string to the counterexample if the property fails.

Disables shrinking for a property altogether. Only quantification inside the call to noShrinking is affected.

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.

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

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.

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.

The type of properties.

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

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

The statistical parameters used by checkCoverage.

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

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]

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"

The basic building block for Function instances. Provides a Function instance by mapping to and from a type that already has a Function instance.

Provides a Function instance for types isomorphic to Void.

An actual Function Void instance is defined in quickcheck-instances.

Provides a Function instance for types with Show and Read.

Provides a Function instance for types with Integral.

Provides a Function instance for types with RealFrac.

Provides a Function instance for types with Bounded and Enum. Use only for small types (i.e. not integers): creates the list [minBound..maxBound]!

A modifier for testing functions.

prop :: Fun String Integer -> Bool
prop (Fn f) = f "banana" == f "monkey"
           || f "banana" == f "elephant"

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]

A modifier for testing ternary functions.

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.

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.

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

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

Ordered xs: guarantees that xs is ordered.

NonEmpty xs: guarantees that xs is non-empty.

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:

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

Sorted xs: guarantees that xs is sorted.

Positive x: guarantees that x > 0.

Negative x: guarantees that x < 0.