Safe Haskell | None |
---|---|

Language | Haskell2010 |

This module is a generic implementation of the `arbitrary`

method. Example
usage:

data Foo = Foo { _fooX :: X , _fooY :: Y } deriving (Generic) instance Arbitrary Foo where arbitrary = genericArbitrary shrink = genericShrink

This instance can also be derived using DerivingVia language extension

data Foo = Foo { _fooX :: X , _fooY :: Y } deriving (Generic) deriving (Arbitrary) via GenericArbitrary Foo

The generated `arbitrary`

method is equivalent to

Foo`<$>`

arbitrary`<*>`

arbitrary

.

It can also handle a recursive types problem. Assuming a type

data R = R R deriving Generic

there is no instance

instance Arbitrary R where arbitrary = genericArbitrary shrink = genericShrink

If you try to compile this you will get a type level error

• R refers to itself in all constructors

Which means that there is no finite term for `R`

because it is recursive. But,
if you correct the definition of `R`

like this.

data R = R R | F deriving Generic

Then it will compile. And the `arbitrary`

generated will not hang forever, because
it respects the `size`

parameter.

There is a limitation of recursion detection:

data R1 = R1 R2 deriving (Eq, Ord, Show, Generic) deriving anyclass NFData deriving Arbitrary via (GenericArbitrary R1) data R2 = R2 R1 deriving (Eq, Ord, Show, Generic) deriving anyclass NFData deriving Arbitrary via (GenericArbitrary R2)

This code will compile and the `arbitrary`

generated will always hang. Yes,
there is a problem with mutually recursive types.

Now lets see an example of datatype with parameters

data A a = A a deriving (Eq, Ord, Show) deriving anyclass NFData deriving (Generic) instance (Arbitrary a) => Arbitrary (A a) where arbitrary = genericArbitrary shrink = genericShrink

It should work from first glance, but when compile it will throw an error:

• Could not deduce (Test.QuickCheck.Arbitrary.Generic.GArbitrary (A a) (GHC.Generics.D1 ('GHC.Generics.MetaData A ParametersTest "main" 'False) (GHC.Generics.C1 ('GHC.Generics.MetaCons A 'GHC.Generics.PrefixI 'False) (GHC.Generics.S1 ('GHC.Generics.MetaSel 'Nothing 'GHC.Generics.NoSourceUnpackedness 'GHC.Generics.NoSourceStrictness 'GHC.Generics.DecidedLazy) (GHC.Generics.Rec0 a)))) (TypesDiffer (A a) a)) arising from a use of ‘genericArbitrary’

Here the `TypesDiffer`

is a type familty dealing with recursive types and
helping us to eliminate inproper instances. To convince the compiller, that the
`a`

parameter is not an `A a`

we must fix the instance with additional constraint

instance (Arg (A a) a, Arbitrary a) => Arbitrary (A a) where arbitrary = genericArbitrary shrink = genericShrink

Now everything compiles and works as expected.

## Synopsis

- genericArbitrary :: forall a ga some. (Generic a, GArbitrary a ga some, ga ~ Rep a) => Gen a
- newtype GenericArbitrary a = GenericArbitrary {
- unGenericArbitrary :: a

- type Arg self field = TypesDiffer self field ~ 'True
- class Finite self a ~ finite => GArbitrary self a (finite :: Bool)
- class (Finite self a ~ af, Finite self b ~ bf) => FiniteSum self (a :: * -> *) (b :: * -> *) af bf
- class FiniteSumElem self a
- type family Finite self (a :: * -> *) :: Bool where ...
- type family AllFieldsFinal self (a :: * -> *) :: Bool where ...
- type family TypesDiffer a b where ...
- type family ArgumentsCount (a :: * -> *) :: Nat where ...
- type family SumLen a :: Nat where ...
- class Arbitrary a where
- genericShrink :: (Generic a, RecursivelyShrink (Rep a), GSubterms (Rep a) a) => a -> [a]

# Main

genericArbitrary :: forall a ga some. (Generic a, GArbitrary a ga some, ga ~ Rep a) => Gen a Source #

newtype GenericArbitrary a Source #

Newtype for `DerivingVia`

Usage:

data Foo = Foo { _fooX :: X , _fooY :: Y } deriving (Generic) deriving (Arbitrary) via GenericArbitrary Foo

*Since: 1.0.0*

#### Instances

Eq a => Eq (GenericArbitrary a) Source # | |

Defined in Test.QuickCheck.Arbitrary.Generic (==) :: GenericArbitrary a -> GenericArbitrary a -> Bool # (/=) :: GenericArbitrary a -> GenericArbitrary a -> Bool # | |

Show a => Show (GenericArbitrary a) Source # | |

Defined in Test.QuickCheck.Arbitrary.Generic showsPrec :: Int -> GenericArbitrary a -> ShowS # show :: GenericArbitrary a -> String # showList :: [GenericArbitrary a] -> ShowS # | |

(Generic a, GArbitrary a (Rep a) some, RecursivelyShrink (Rep a), GSubterms (Rep a) a) => Arbitrary (GenericArbitrary a) Source # | |

Defined in Test.QuickCheck.Arbitrary.Generic arbitrary :: Gen (GenericArbitrary a) # shrink :: GenericArbitrary a -> [GenericArbitrary a] # |

type Arg self field = TypesDiffer self field ~ 'True Source #

Constraint helper for types with parameters

Usage:

data A a = A a deriving (Generic) instance (Arg (A a) a, Arbitrary a) => Arbitrary (A a) where arbitrary = genericArbitrary shrink = genericShrink

*Since: 1.0.0*

# Internal

class Finite self a ~ finite => GArbitrary self a (finite :: Bool) Source #

Generic arbitrary.

Parameters are:
* self: the ADT we generating instance for
* a: some part of the `Rep self`
* finite: Is `a`

finite? Infinite type has no finite values (like Stream)

gArbitrary

#### Instances

GArbitrary self (U1 :: Type -> Type) 'True Source # | Unit type instance |

Defined in Test.QuickCheck.Arbitrary.Generic gArbitrary :: Proxy self -> Gen (U1 x) | |

(GArbitrary self a af, GArbitrary self b bf, KnownNat (SumLen a), KnownNat (SumLen b), (af || bf) ~ some) => GArbitrary self (a :+: b) some Source # | Any sum inside of declaration |

Defined in Test.QuickCheck.Arbitrary.Generic gArbitrary :: Proxy self -> Gen ((a :+: b) x) | |

(GArbitrary self a af, GArbitrary self b bf, (af && bf) ~ some) => GArbitrary self (a :*: b) some Source # | Product |

Defined in Test.QuickCheck.Arbitrary.Generic gArbitrary :: Proxy self -> Gen ((a :*: b) x) | |

(Arbitrary t, Finite self (K1 R t :: Type -> Type) ~ some) => GArbitrary self (K1 R t :: Type -> Type) some Source # | Data of the constructor field |

Defined in Test.QuickCheck.Arbitrary.Generic gArbitrary :: Proxy self -> Gen (K1 R t x) | |

(TypeError ('ShowType self :<>: 'Text " refers to itself in all constructors") :: Constraint, (Finite self a || Finite self b) ~ 'False) => GArbitrary self (M1 D t (a :+: b)) 'False Source # | |

Defined in Test.QuickCheck.Arbitrary.Generic | |

(FiniteSum self a b af bf, GArbitrary self (a :+: b) 'True) => GArbitrary self (M1 D t (a :+: b)) 'True Source # | ADT declaration with multiple constructors |

Defined in Test.QuickCheck.Arbitrary.Generic | |

(TypeError ('ShowType self :<>: 'Text " refers to itself in all constructors") :: Constraint, AllFieldsFinal self f ~ 'False) => GArbitrary self (M1 D t (M1 C c f)) 'False Source # | |

Defined in Test.QuickCheck.Arbitrary.Generic | |

GArbitrary self f some => GArbitrary self (M1 S t f) some Source # | Constructor field meta information |

Defined in Test.QuickCheck.Arbitrary.Generic gArbitrary :: Proxy self -> Gen (M1 S t f x) | |

(GArbitrary self f some, KnownNat (ArgumentsCount f), AllFieldsFinal self f ~ some) => GArbitrary self (M1 C c f) some Source # | The constructor meta information |

Defined in Test.QuickCheck.Arbitrary.Generic gArbitrary :: Proxy self -> Gen (M1 C c f x) | |

GArbitrary self (M1 C c f) 'True => GArbitrary self (M1 D t (M1 C c f)) 'True Source # | |

Defined in Test.QuickCheck.Arbitrary.Generic |

class (Finite self a ~ af, Finite self b ~ bf) => FiniteSum self (a :: * -> *) (b :: * -> *) af bf Source #

finiteSum

#### Instances

(FiniteSumElem self b, Finite self a ~ 'False, Finite self b ~ 'True) => FiniteSum self a b 'False 'True Source # | |

Defined in Test.QuickCheck.Arbitrary.Generic | |

(FiniteSumElem self a, Finite self a ~ 'True, Finite self b ~ 'False) => FiniteSum self a b 'True 'False Source # | |

Defined in Test.QuickCheck.Arbitrary.Generic | |

(FiniteSumElem self a, FiniteSumElem self b, Finite self a ~ 'True, Finite self b ~ 'True) => FiniteSum self a b 'True 'True Source # | |

Defined in Test.QuickCheck.Arbitrary.Generic |

class FiniteSumElem self a Source #

finiteElem

#### Instances

FiniteSum self a b af bf => FiniteSumElem (self :: Type) (a :+: b :: Type -> Type) Source # | |

Defined in Test.QuickCheck.Arbitrary.Generic finiteElem :: forall (p :: k). Proxy self -> [Gen ((a :+: b) p)] | |

GArbitrary self (M1 C c f) 'True => FiniteSumElem (self :: Type) (M1 C c f :: Type -> Type) Source # | |

Defined in Test.QuickCheck.Arbitrary.Generic finiteElem :: forall (p :: k). Proxy self -> [Gen (M1 C c f p)] |

type family Finite self (a :: * -> *) :: Bool where ... Source #

Finite self U1 = 'True | |

Finite self (K1 R field) = TypesDiffer self field | |

Finite self (a :*: b) = Finite self a && Finite self b | |

Finite self (M1 D t f) = Finite self f | |

Finite self (a :+: b) = Finite self a || Finite self b | |

Finite self (M1 C c f) = AllFieldsFinal self f | |

Finite self (M1 S s f) = Finite self f |

type family AllFieldsFinal self (a :: * -> *) :: Bool where ... Source #

AllFieldsFinal self U1 = 'True | |

AllFieldsFinal self (a :*: b) = AllFieldsFinal self a && AllFieldsFinal self b | |

AllFieldsFinal self (M1 S t (K1 R field)) = TypesDiffer self field |

type family TypesDiffer a b where ... Source #

TypesDiffer a a = 'False | |

TypesDiffer a b = 'True |

type family ArgumentsCount (a :: * -> *) :: Nat where ... Source #

ArgumentsCount U1 = 1 | |

ArgumentsCount (M1 S s f) = 1 | |

ArgumentsCount (a :*: b) = ArgumentsCount a + ArgumentsCount b |

type family SumLen a :: Nat where ... Source #

Calculates count of constructors encoded by particular `:+:`

.
Internal use only.

# Reexports

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.

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 `genericShrink`

`shrink`

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
`subterms`

to do this. - Recursively apply
`shrink`

to all immediate subterms. You can use`recursivelyShrink`

to 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 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

as this shrinks `shrink`

x = Nil:`genericShrink`

x`Nil`

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`

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

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.