Safe Haskell  Safe 

 quickCheck :: Testable prop => prop > IO ()
 data Args = Args {}
 data Result
 = Success { }
  GaveUp { }
  Failure {
 numTests :: Int
 numShrinks :: Int
 numShrinkTries :: Int
 numShrinkFinal :: Int
 usedSeed :: QCGen
 usedSize :: Int
 reason :: String
 theException :: Maybe AnException
 labels :: [(String, Int)]
 output :: String
  NoExpectedFailure { }
 stdArgs :: Args
 quickCheckWith :: Testable prop => Args > prop > IO ()
 quickCheckWithResult :: Testable prop => Args > prop > IO Result
 quickCheckResult :: Testable prop => prop > IO Result
 verboseCheck :: Testable prop => prop > IO ()
 verboseCheckWith :: Testable prop => Args > prop > IO ()
 verboseCheckWithResult :: Testable prop => Args > prop > IO Result
 verboseCheckResult :: Testable prop => prop > IO Result
 quickCheckAll :: Q Exp
 verboseCheckAll :: Q Exp
 forAllProperties :: Q Exp
 polyQuickCheck :: Name > ExpQ
 polyVerboseCheck :: Name > ExpQ
 monomorphic :: Name > ExpQ
 data Gen a
 choose :: Random a => (a, a) > Gen a
 oneof :: [Gen a] > Gen a
 frequency :: [(Int, Gen a)] > Gen a
 elements :: [a] > Gen a
 growingElements :: [a] > Gen a
 sized :: (Int > Gen a) > Gen a
 resize :: Int > Gen a > Gen a
 suchThat :: Gen a > (a > Bool) > Gen a
 suchThatMaybe :: Gen a > (a > Bool) > Gen (Maybe a)
 listOf :: Gen a > Gen [a]
 listOf1 :: Gen a > Gen [a]
 vectorOf :: Int > Gen a > Gen [a]
 infiniteListOf :: Gen a > Gen [a]
 vector :: Arbitrary a => Int > Gen [a]
 orderedList :: (Ord a, Arbitrary a) => Gen [a]
 infiniteList :: Arbitrary a => Gen [a]
 generate :: Gen a > IO a
 sample :: Show a => Gen a > IO ()
 sample' :: Gen a > IO [a]
 class Arbitrary a where
 class CoArbitrary a where
 coarbitrary :: a > Gen b > Gen b
 arbitrarySizedIntegral :: Integral a => Gen a
 arbitrarySizedFractional :: Fractional a => Gen a
 arbitrarySizedBoundedIntegral :: (Bounded a, Integral a) => Gen a
 arbitraryBoundedIntegral :: (Bounded a, Integral a) => Gen a
 arbitraryBoundedRandom :: (Bounded a, Random a) => Gen a
 arbitraryBoundedEnum :: (Bounded a, Enum a) => Gen a
 genericShrink :: (Generic a, Typeable a, RecursivelyShrink (Rep a), Subterms (Rep a)) => a > [a]
 subterms :: (Generic a, Typeable a, Subterms (Rep a)) => a > [a]
 recursivelyShrink :: (Generic a, RecursivelyShrink (Rep a)) => a > [a]
 shrinkNothing :: a > [a]
 shrinkList :: (a > [a]) > [a] > [[a]]
 shrinkIntegral :: Integral a => a > [a]
 shrinkRealFrac :: RealFrac a => a > [a]
 shrinkRealFracToInteger :: RealFrac a => a > [a]
 variant :: Integral n => n > Gen a > Gen a
 coarbitraryIntegral :: Integral a => a > Gen b > Gen b
 coarbitraryReal :: Real a => a > Gen b > Gen b
 coarbitraryShow :: Show a => a > Gen b > Gen b
 coarbitraryEnum :: Enum a => a > Gen b > Gen b
 (><) :: (Gen a > Gen a) > (Gen a > Gen a) > Gen a > Gen a
 newtype Blind a = Blind {
 getBlind :: a
 newtype Fixed a = Fixed {
 getFixed :: a
 newtype OrderedList a = Ordered {
 getOrdered :: [a]
 newtype NonEmptyList a = NonEmpty {
 getNonEmpty :: [a]
 newtype Positive a = Positive {
 getPositive :: a
 newtype NonZero a = NonZero {
 getNonZero :: a
 newtype NonNegative a = NonNegative {
 getNonNegative :: a
 newtype Large a = Large {
 getLarge :: a
 newtype Small a = Small {
 getSmall :: a
 data Smart a = Smart Int a
 newtype Shrink2 a = Shrink2 {
 getShrink2 :: a
 data Shrinking s a = Shrinking s a
 class ShrinkState s a where
 shrinkInit :: a > s
 shrinkState :: a > s > [(a, s)]
 data Property
 class Testable prop where
 property :: prop > Property
 exhaustive :: prop > Bool
 forAll :: (Show a, Testable prop) => Gen a > (a > prop) > Property
 forAllShrink :: (Show a, Testable prop) => Gen a > (a > [a]) > (a > prop) > Property
 shrinking :: Testable prop => (a > [a]) > a > (a > prop) > Property
 (==>) :: Testable prop => Bool > prop > Property
 (===) :: (Eq a, Show a) => a > a > Property
 ioProperty :: Testable prop => IO prop > Property
 verbose :: Testable prop => prop > Property
 once :: Testable prop => prop > Property
 within :: Testable prop => Int > prop > Property
 noShrinking :: Testable prop => prop > Property
 (.&.) :: (Testable prop1, Testable prop2) => prop1 > prop2 > Property
 (.&&.) :: (Testable prop1, Testable prop2) => prop1 > prop2 > Property
 conjoin :: Testable prop => [prop] > Property
 (..) :: (Testable prop1, Testable prop2) => prop1 > prop2 > Property
 disjoin :: Testable prop => [prop] > Property
 counterexample :: Testable prop => String > prop > Property
 printTestCase :: Testable prop => String > prop > Property
 whenFail :: Testable prop => IO () > prop > Property
 whenFail' :: Testable prop => IO () > prop > Property
 expectFailure :: Testable prop => prop > Property
 label :: Testable prop => String > prop > Property
 collect :: (Show a, Testable prop) => a > prop > Property
 classify :: Testable prop => Bool > String > prop > Property
 cover :: Testable prop => Bool > Int > String > prop > Property
 data Discard = Discard
 discard :: a
 mapSize :: Testable prop => (Int > Int) > prop > Property
Running tests
quickCheck :: Testable prop => prop > IO ()Source
Tests a property and prints the results to stdout
.
Args specifies arguments to the QuickCheck driver
Args  

Result represents the test result
Success  A successful test run 
GaveUp  Given up 
Failure  A failed test run 
 
NoExpectedFailure  A property that should have failed did not 
quickCheckWith :: Testable prop => Args > prop > IO ()Source
Tests a property, using test arguments, and prints the results to stdout
.
quickCheckWithResult :: Testable prop => Args > prop > IO ResultSource
Tests a property, using test arguments, produces a test result, and prints the results to stdout
.
quickCheckResult :: Testable prop => prop > IO ResultSource
Tests a property, produces a test result, and prints the results to stdout
.
Running tests verbosely
verboseCheck :: Testable prop => prop > IO ()Source
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
verboseCheckWith :: Testable prop => Args > prop > IO ()Source
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
.
verboseCheckWithResult :: Testable prop => Args > prop > IO ResultSource
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
.
verboseCheckResult :: Testable prop => prop > IO ResultSource
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
.
Testing all properties in a module
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; 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!
verboseCheckAll :: Q ExpSource
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 []
.
forAllProperties :: Q ExpSource
Test all properties in the current module, using a custom
quickCheck
function. The same caveats as with quickCheckAll
apply.
$
has type forAllProperties
(
.
An example invocation is Property
> IO
Result
) > IO
Bool
$
,
which does the same thing as forAllProperties
quickCheckResult
$
.
quickCheckAll
forAllProperties
has the same issue with scoping as quickCheckAll
:
see the note there about return []
.
Testing polymorphic properties
polyQuickCheck :: Name > ExpQSource
Test a polymorphic property, defaulting all type variables to Integer
.
Invoke as $(
, where polyQuickCheck
'prop)prop
is a property.
Note that just evaluating
in GHCi will seem to
work, but will silently default all type variables to quickCheck
prop()
!
$(
means the same as
polyQuickCheck
'prop)
.
If you want to supply custom arguments to quickCheck
$(monomorphic
'prop)polyQuickCheck
,
you will have to combine quickCheckWith
and monomorphic
yourself.
If you want to use polyQuickCheck
in the same file where you defined the
property, the same scoping problems pop up as in quickCheckAll
:
see the note there about return []
.
polyVerboseCheck :: Name > ExpQSource
Test a polymorphic property, defaulting all type variables to Integer
.
This is just a convenience function that combines verboseCheck
and monomorphic
.
If you want to use polyVerboseCheck
in the same file where you defined the
property, the same scoping problems pop up as in quickCheckAll
:
see the note there about return []
.
monomorphic :: Name > ExpQSource
Monomorphise an arbitrary property by defaulting all type variables to Integer
.
For example, if f
has type
then Ord
a => [a] > [a]$(
has type monomorphic
'f)[
.
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 []
.
Random generation
A generator for values of type a
.
Generator combinators
oneof :: [Gen a] > Gen aSource
Randomly uses one of the given generators. The input list must be nonempty.
frequency :: [(Int, Gen a)] > Gen aSource
Chooses one of the given generators, with a weighted random distribution. The input list must be nonempty.
growingElements :: [a] > Gen aSource
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 nonempty.
sized :: (Int > Gen a) > Gen aSource
Used to construct generators that depend on the size parameter.
resize :: Int > Gen a > Gen aSource
Overrides the size parameter. Returns a generator which uses the given size instead of the runtimesize parameter.
suchThatMaybe :: Gen a > (a > Bool) > Gen (Maybe a)Source
Tries to generate a value that satisfies a predicate.
listOf :: Gen a > Gen [a]Source
Generates a list of random length. The maximum length depends on the size parameter.
listOf1 :: Gen a > Gen [a]Source
Generates a nonempty list of random length. The maximum length depends on the size parameter.
infiniteListOf :: Gen a > Gen [a]Source
Generates an infinite list.
Generators which use Arbitrary
orderedList :: (Ord a, Arbitrary a) => Gen [a]Source
Generates an ordered list of a given length.
infiniteList :: Arbitrary a => Gen [a]Source
Generate an infinite list.
Running a generator
Generator debugging
Arbitrary and CoArbitrary classes
Random generation and shrinking of values.
A generator for values of the given type.
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.
Most implementations of shrink
should try at least three things:
 Shrink a term to any of its immediate subterms.
 Recursively apply
shrink
to all immediate subterms.  Typespecific 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 shrinkx
,l
andr
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;
note that these only work on GHC 7.2 and above.
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
as this shrinks shrink
x = Nil:genericShrink
xNil
to Nil
, and shrinking will go into an
infinite loop.
If all this leaves you bewildered, you might try
to begin with,
after deriving shrink
= genericShrink
Generic
and Typeable
for your type. However, if your data type has any
special invariants, you will need to check that genericShrink
can't break those invariants.
class CoArbitrary a whereSource
Used for random generation of functions.
coarbitrary :: a > Gen b > Gen bSource
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)
Helper functions for implementing arbitrary
arbitrarySizedIntegral :: Integral a => Gen aSource
Generates an integral number. The number can be positive or negative and its maximum absolute value depends on the size parameter.
arbitrarySizedFractional :: Fractional a => Gen aSource
Generates a fractional number. The number can be positive or negative and its maximum absolute value depends on the size parameter.
arbitrarySizedBoundedIntegral :: (Bounded a, Integral a) => Gen aSource
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.
arbitraryBoundedIntegral :: (Bounded a, Integral a) => Gen aSource
Generates an integral number. The number is chosen uniformly from
the entire range of the type. You may want to use
arbitrarySizedBoundedIntegral
instead.
arbitraryBoundedRandom :: (Bounded a, Random a) => Gen aSource
Generates an element of a bounded type. The element is chosen from the entire range of the type.
arbitraryBoundedEnum :: (Bounded a, Enum a) => Gen aSource
Generates an element of a bounded enumeration.
Helper functions for implementing shrink
genericShrink :: (Generic a, Typeable a, RecursivelyShrink (Rep a), Subterms (Rep a)) => a > [a]Source
Shrink a term to any of its immediate subterms, and also recursively shrink all subterms.
subterms :: (Generic a, Typeable a, Subterms (Rep a)) => a > [a]Source
All immediate subterms of a term.
recursivelyShrink :: (Generic a, RecursivelyShrink (Rep a)) => a > [a]Source
Recursively shrink all immediate subterms.
shrinkNothing :: a > [a]Source
Returns no shrinking alternatives.
shrinkList :: (a > [a]) > [a] > [[a]]Source
Shrink a list of values given a shrinking function for individual values.
shrinkIntegral :: Integral a => a > [a]Source
Shrink an integral number.
shrinkRealFrac :: RealFrac a => a > [a]Source
Shrink a fraction.
shrinkRealFracToInteger :: RealFrac a => a > [a]Source
Shrink a fraction, but only shrink to integral values.
Helper functions for implementing coarbitrary
coarbitraryIntegral :: Integral a => a > Gen b > Gen bSource
A coarbitrary
implementation for integral numbers.
coarbitraryReal :: Real a => a > Gen b > Gen bSource
A coarbitrary
implementation for real numbers.
coarbitraryShow :: Show a => a > Gen b > Gen bSource
coarbitrary
helper for lazy people :).
coarbitraryEnum :: Enum a => a > Gen b > Gen bSource
A coarbitrary
implementation for enums.
(><) :: (Gen a > Gen a) > (Gen a > Gen a) > Gen a > Gen aSource
Deprecated: Use ordinary function composition instead
Combine two generator perturbing functions, for example the
results of calls to variant
or coarbitrary
.
Typelevel modifiers for changing generator behavior
Blind x
: as x, but x does not have to be in the Show
class.
Fixed x
: as x, but will not be shrunk.
newtype OrderedList a Source
Ordered xs
: guarantees that xs is ordered.
Ordered  

Functor OrderedList  
Eq a => Eq (OrderedList a)  
Ord a => Ord (OrderedList a)  
Read a => Read (OrderedList a)  
Show a => Show (OrderedList a)  
(Ord a, Arbitrary a) => Arbitrary (OrderedList a) 
newtype NonEmptyList a Source
NonEmpty xs
: guarantees that xs is nonempty.
NonEmpty  

Functor NonEmptyList  
Eq a => Eq (NonEmptyList a)  
Ord a => Ord (NonEmptyList a)  
Read a => Read (NonEmptyList a)  
Show a => Show (NonEmptyList a)  
Arbitrary a => Arbitrary (NonEmptyList a) 
Positive x
: guarantees that x > 0
.
Positive  

Functor Positive  
Enum a => Enum (Positive a)  
Eq a => Eq (Positive a)  
Integral a => Integral (Positive a)  
(Ord a, Num a) => Num (Positive a)  
Ord a => Ord (Positive a)  
Read a => Read (Positive a)  
Real a => Real (Positive a)  
Show a => Show (Positive a)  
(Num a, Ord a, Arbitrary a) => Arbitrary (Positive a) 
NonZero x
: guarantees that x /= 0
.
NonZero  

newtype NonNegative a Source
NonNegative x
: guarantees that x >= 0
.
Functor NonNegative  
Enum a => Enum (NonNegative a)  
Eq a => Eq (NonNegative a)  
Integral a => Integral (NonNegative a)  
(Ord a, Num a) => Num (NonNegative a)  
Ord a => Ord (NonNegative a)  
Read a => Read (NonNegative a)  
Real a => Real (NonNegative a)  
Show a => Show (NonNegative a)  
(Num a, Ord a, Arbitrary a) => Arbitrary (NonNegative a) 
Large x
: by default, QuickCheck generates Int
s drawn from a small
range. Large Int
gives you values drawn from the entire range instead.
Small x
: generates values of x
drawn from a small range.
The opposite of Large
.
Smart _ x
: tries a different order when shrinking.
Shrink2 x
: allows 2 shrinking steps at the same time when shrinking x
Shrink2  

Shrinking _ x
: allows for maintaining a state during shrinking.
Shrinking s a 
class ShrinkState s a whereSource
shrinkInit :: a > sSource
shrinkState :: a > s > [(a, s)]Source
Properties
The type of properties.
Backwards combatibility note: in older versions of QuickCheck
Property
was a type synonym for
, so you could mix
and match property combinators and Gen
Prop
Gen
monad operations. Code
that does this will no longer typecheck.
However, it is easy to fix: because of the Testable
typeclass, any
combinator that expects a Property
will also accept a
.
If you have a Gen
Property
Property
where you need a
, simply wrap
the property combinator inside a Gen
a
return
to get a
, and
all should be well.
Gen
Property
class Testable prop whereSource
The class of things which can be tested, i.e. turned into a property.
Property combinators
forAll :: (Show a, Testable prop) => Gen a > (a > prop) > PropertySource
Explicit universal quantification: uses an explicitly given test case generator.
forAllShrink :: (Show a, Testable prop) => Gen a > (a > [a]) > (a > prop) > PropertySource
Like forAll
, but tries to shrink the argument for failing test cases.
:: Testable prop  
=> (a > [a]) 

> a  The original argument 
> (a > prop)  
> Property 
Shrinks the argument to property if it fails. Shrinking is done automatically for most types. This is only needed when you want to override the default behavior.
(==>) :: Testable prop => Bool > prop > PropertySource
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.
(===) :: (Eq a, Show a) => a > a > PropertySource
Like ==
, but prints a counterexample when it fails.
ioProperty :: Testable prop => IO prop > PropertySource
Do I/O inside a property. This can obviously lead to unrepeatable testcases, so use with care.
For more advanced monadic testing you may want to look at Test.QuickCheck.Monadic.
Controlling property execution
verbose :: Testable prop => prop > PropertySource
Prints out the generated testcase every time the property is tested.
Only variables quantified over inside the verbose
are printed.
once :: Testable prop => prop > PropertySource
Modifies a property so that it only will be tested once.
within :: Testable prop => Int > prop > PropertySource
Considers a property failed if it does not complete within the given number of microseconds.
noShrinking :: Testable prop => prop > PropertySource
Disables shrinking for a property altogether.
Conjunction and disjunction
(.&.) :: (Testable prop1, Testable prop2) => prop1 > prop2 > PropertySource
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.
(.&&.) :: (Testable prop1, Testable prop2) => prop1 > prop2 > PropertySource
Conjunction: p1
.&&.
p2
passes if both p1
and p2
pass.
(..) :: (Testable prop1, Testable prop2) => prop1 > prop2 > PropertySource
Disjunction: p1
..
p2
passes unless p1
and p2
simultaneously fail.
What to do on failure
counterexample :: Testable prop => String > prop > PropertySource
Adds the given string to the counterexample.
printTestCase :: Testable prop => String > prop > PropertySource
Deprecated: Use counterexample instead
Adds the given string to the counterexample.
whenFail :: Testable prop => IO () > prop > PropertySource
Performs an IO
action after the last failure of a property.
whenFail' :: Testable prop => IO () > prop > PropertySource
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.
expectFailure :: Testable prop => prop > PropertySource
Indicates that a property is supposed to fail. QuickCheck will report an error if it does not fail.
Analysing test distribution
label :: Testable prop => String > prop > PropertySource
Attaches a label to a property. This is used for reporting test case distribution.
collect :: (Show a, Testable prop) => a > prop > PropertySource
Labels a property with a value:
collect x = label (show x)
:: Testable prop  
=> Bool 

> String  Label. 
> prop  
> Property 
Conditionally labels test case.
:: Testable prop  
=> Bool 

> Int  The required percentage (0100) of test cases. 
> String  Label for the test case class. 
> prop  
> Property 
Checks that at least the given proportion of the test cases belong to the given class.
Miscellaneous
If a property returns Discard
, the current test case is discarded,
the same as if a precondition was false.
A special exception that makes QuickCheck discard the test case.
Normally you should use ==>
, but if for some reason this isn't
possible (e.g. you are deep inside a generator), use discard
instead.