| Safe Haskell | Trustworthy |
|---|---|
| Language | Haskell98 |
Test.QuickCheck.Arbitrary
Contents
Description
Type classes for random generation of values.
Note: the contents of this module are re-exported by Test.QuickCheck. You do not need to import it directly.
Synopsis
- class Arbitrary a where
- class CoArbitrary a where
- coarbitrary :: a -> Gen b -> Gen b
- class Arbitrary1 f where
- liftArbitrary :: Gen a -> Gen (f a)
- liftShrink :: (a -> [a]) -> f a -> [f a]
- arbitrary1 :: (Arbitrary1 f, Arbitrary a) => Gen (f a)
- shrink1 :: (Arbitrary1 f, Arbitrary a) => f a -> [f a]
- class Arbitrary2 f where
- liftArbitrary2 :: Gen a -> Gen b -> Gen (f a b)
- liftShrink2 :: (a -> [a]) -> (b -> [b]) -> f a b -> [f a b]
- arbitrary2 :: (Arbitrary2 f, Arbitrary a, Arbitrary b) => Gen (f a b)
- shrink2 :: (Arbitrary2 f, Arbitrary a, Arbitrary b) => f a b -> [f a b]
- applyArbitrary2 :: (Arbitrary a, Arbitrary b) => (a -> b -> r) -> Gen r
- applyArbitrary3 :: (Arbitrary a, Arbitrary b, Arbitrary c) => (a -> b -> c -> r) -> Gen r
- applyArbitrary4 :: (Arbitrary a, Arbitrary b, Arbitrary c, Arbitrary d) => (a -> b -> c -> d -> r) -> Gen r
- arbitrarySizedIntegral :: Integral a => Gen a
- arbitrarySizedNatural :: Integral a => Gen a
- arbitraryBoundedIntegral :: (Bounded a, Integral a) => Gen a
- arbitrarySizedBoundedIntegral :: (Bounded a, Integral a) => Gen a
- arbitrarySizedFractional :: Fractional a => Gen a
- arbitraryBoundedRandom :: (Bounded a, Random a) => Gen a
- arbitraryBoundedEnum :: (Bounded a, Enum a) => Gen a
- arbitraryUnicodeChar :: Gen Char
- arbitraryASCIIChar :: Gen Char
- arbitraryPrintableChar :: Gen Char
- genericShrink :: (Generic a, RecursivelyShrink (Rep a), GSubterms (Rep a) a) => a -> [a]
- subterms :: (Generic a, GSubterms (Rep a) a) => a -> [a]
- recursivelyShrink :: (Generic a, RecursivelyShrink (Rep a)) => a -> [a]
- genericCoarbitrary :: (Generic a, GCoArbitrary (Rep a)) => a -> Gen b -> Gen b
- shrinkNothing :: a -> [a]
- shrinkList :: (a -> [a]) -> [a] -> [[a]]
- shrinkMap :: Arbitrary a => (a -> b) -> (b -> a) -> b -> [b]
- shrinkMapBy :: (a -> b) -> (b -> a) -> (a -> [a]) -> b -> [b]
- shrinkIntegral :: Integral a => a -> [a]
- shrinkRealFrac :: RealFrac a => a -> [a]
- shrinkDecimal :: RealFrac a => a -> [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
- vector :: Arbitrary a => Int -> Gen [a]
- orderedList :: (Ord a, Arbitrary a) => Gen [a]
- infiniteList :: Arbitrary a => Gen [a]
Arbitrary and CoArbitrary classes
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.
Minimal complete definition
Methods
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!
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 = , but by customising
the behaviour of genericShrinkshrink you can often get simpler counterexamples.
Most implementations of shrink should try at least three things:
- Shrink a term to any of its immediate subterms.
You can use
subtermsto do this. - Recursively apply
shrinkto all immediate subterms. You can userecursivelyShrinkto do this. - 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 shrinkx,landrin 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 xNil to Nil, and shrinking will go into an
infinite loop.
If all this leaves you bewildered, you might try shrink = genericShrinkGeneric for your type. However, if your data type has any
special invariants, you will need to check that genericShrink can't break those invariants.
Instances
class CoArbitrary a where #
Used for random generation of functions.
You should consider using 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).
Minimal complete definition
Nothing
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 [] =variant0 coarbitrary (x:xs) =variant1 . coarbitrary (x,xs)
coarbitrary :: (Generic a, GCoArbitrary (Rep a)) => 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 [] =variant0 coarbitrary (x:xs) =variant1 . coarbitrary (x,xs)
Instances
Unary and Binary classes
class Arbitrary1 f where #
Lifting of the Arbitrary class to unary type constructors.
Minimal complete definition
Instances
arbitrary1 :: (Arbitrary1 f, Arbitrary a) => Gen (f a) #
shrink1 :: (Arbitrary1 f, Arbitrary a) => f a -> [f a] #
class Arbitrary2 f where #
Lifting of the Arbitrary class to binary type constructors.
Minimal complete definition
Methods
liftArbitrary2 :: Gen a -> Gen b -> Gen (f a b) #
liftShrink2 :: (a -> [a]) -> (b -> [b]) -> f a b -> [f a b] #
Instances
| Arbitrary2 Either # | |
Defined in Test.QuickCheck.Arbitrary Methods liftArbitrary2 :: Gen a -> Gen b -> Gen (Either a b) # liftShrink2 :: (a -> [a]) -> (b -> [b]) -> Either a b -> [Either a b] # | |
| Arbitrary2 (,) # | |
Defined in Test.QuickCheck.Arbitrary Methods liftArbitrary2 :: Gen a -> Gen b -> Gen (a, b) # liftShrink2 :: (a -> [a]) -> (b -> [b]) -> (a, b) -> [(a, b)] # | |
| Arbitrary2 (Const :: Type -> Type -> Type) # | |
Defined in Test.QuickCheck.Arbitrary Methods liftArbitrary2 :: Gen a -> Gen b -> Gen (Const a b) # liftShrink2 :: (a -> [a]) -> (b -> [b]) -> Const a b -> [Const a b] # | |
| Arbitrary2 (Constant :: Type -> Type -> Type) # | |
Defined in Test.QuickCheck.Arbitrary Methods liftArbitrary2 :: Gen a -> Gen b -> Gen (Constant a b) # liftShrink2 :: (a -> [a]) -> (b -> [b]) -> Constant a b -> [Constant a b] # | |
arbitrary2 :: (Arbitrary2 f, Arbitrary a, Arbitrary b) => Gen (f a b) #
shrink2 :: (Arbitrary2 f, Arbitrary a, Arbitrary b) => f a b -> [f a b] #
Helper functions for implementing arbitrary
applyArbitrary2 :: (Arbitrary a, Arbitrary b) => (a -> b -> r) -> Gen r #
Apply a binary function to random arguments.
applyArbitrary3 :: (Arbitrary a, Arbitrary b, Arbitrary c) => (a -> b -> c -> r) -> Gen r #
Apply a ternary function to random arguments.
applyArbitrary4 :: (Arbitrary a, Arbitrary b, Arbitrary c, Arbitrary d) => (a -> b -> c -> d -> r) -> Gen r #
Apply a function of arity 4 to random arguments.
arbitrarySizedIntegral :: 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.
arbitrarySizedNatural :: Integral a => Gen a #
Generates a natural number. The number's maximum value depends on the size parameter.
arbitraryBoundedIntegral :: (Bounded a, Integral a) => Gen a #
Generates an integral number. The number is chosen uniformly from
the entire range of the type. You may want to use
arbitrarySizedBoundedIntegral instead.
arbitrarySizedBoundedIntegral :: (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.
arbitrarySizedFractional :: Fractional a => Gen a #
Generates a fractional number. The number can be positive or negative and its maximum absolute value depends on the size parameter.
arbitraryBoundedRandom :: (Bounded a, Random a) => Gen a #
Generates an element of a bounded type. The element is chosen from the entire range of the type.
arbitraryBoundedEnum :: (Bounded a, Enum a) => Gen a #
Generates an element of a bounded enumeration.
Generators for various kinds of character
arbitraryUnicodeChar :: Gen Char #
Generates any Unicode character (but not a surrogate)
arbitraryASCIIChar :: Gen Char #
Generates a random ASCII character (0-127).
arbitraryPrintableChar :: Gen Char #
Generates a printable Unicode character.
Helper functions for implementing shrink
genericShrink :: (Generic a, RecursivelyShrink (Rep a), GSubterms (Rep a) a) => a -> [a] #
Shrink a term to any of its immediate subterms, and also recursively shrink all subterms.
recursivelyShrink :: (Generic a, RecursivelyShrink (Rep a)) => a -> [a] #
Recursively shrink all immediate subterms.
genericCoarbitrary :: (Generic a, GCoArbitrary (Rep a)) => a -> Gen b -> Gen b #
Generic CoArbitrary implementation.
shrinkNothing :: a -> [a] #
Returns no shrinking alternatives.
shrinkList :: (a -> [a]) -> [a] -> [[a]] #
Shrink a list of values given a shrinking function for individual values.
shrinkMap :: 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
shrinkMapBy :: (a -> b) -> (b -> a) -> (a -> [a]) -> b -> [b] #
Non-overloaded version of shrinkMap.
shrinkIntegral :: Integral a => a -> [a] #
Shrink an integral number.
shrinkRealFrac :: RealFrac a => a -> [a] #
Shrink a fraction, preferring numbers with smaller
numerators or denominators. See also shrinkDecimal.
shrinkDecimal :: RealFrac a => a -> [a] #
Shrink a real number, preferring numbers with shorter
decimal representations. See also shrinkRealFrac.
Helper functions for implementing coarbitrary
coarbitraryIntegral :: Integral a => a -> Gen b -> Gen b #
A coarbitrary implementation for integral numbers.
coarbitraryReal :: Real a => a -> Gen b -> Gen b #
A coarbitrary implementation for real numbers.
coarbitraryShow :: Show a => a -> Gen b -> Gen b #
coarbitrary helper for lazy people :-).
coarbitraryEnum :: Enum a => a -> Gen b -> Gen b #
A coarbitrary implementation for enums.
(><) :: (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.
Generators which use arbitrary
orderedList :: (Ord a, Arbitrary a) => Gen [a] #
Generates an ordered list.
infiniteList :: Arbitrary a => Gen [a] #
Generates an infinite list.