QuickCheck common typeclasses.

This library provides QuickCheck properties to ensure that typeclass instances adhere to the set of laws that they are supposed to. There are other libraries that do similar things, such as `genvalidity-hspec` and checkers. This library differs from other solutions by not introducing any new typeclasses that the user needs to learn.

Note: on GHC < 8.5, this library uses the higher-kinded typeclasses (Data.Functor.Classes.Show1, Data.Functor.Classes.Eq1, Data.Functor.Classes.Ord1, etc.), but on GHC >= 8.5, it uses `-XQuantifiedConstraints` to express these constraints more cleanly.

quickcheck-classes

This library provides sets of properties that should hold for common typeclasses, along with three (3) simple functions that you can use to test them.

lawsCheck:

A convenience function for testing properties in GHCi. For example, at GHCi:

>>> lawsCheck (monoidLaws (Proxy :: Proxy Ordering))
Monoid: Associative +++ OK, passed 100 tests.
Monoid: Left Identity +++ OK, passed 100 tests.
Monoid: Right Identity +++ OK, passed 100 tests.

Assuming that the Arbitrary instance for Ordering is good, we now have confidence that the Monoid instance for Ordering satisfies the monoid laws.

lawsCheckMany:

A convenience function for checking multiple typeclass instances of multiple types. Consider the following Haskell source file:

import Data.Proxy (Proxy(..))
import Data.Map (Map)
import Data.Set (Set)

-- A 'Proxy' for 'Set' 'Int'. 
setInt :: Proxy (Set Int)
setInt = Proxy

-- A 'Proxy' for 'Map' 'Int' 'Int'.
mapInt :: Proxy (Map Int Int)
mapInt = Proxy

myLaws :: Proxy a -> [Laws]
myLaws p = [eqLaws p, monoidLaws p]

namedTests :: [(String, [Laws])]
namedTests =
  [ ("Set Int", myLaws setInt)
  , ("Map Int Int", myLaws mapInt)
  ]

Now, in GHCi:

>>> lawsCheckMany namedTests

Testing properties for common typeclasses
-------------
-- Set Int --
-------------

Eq: Transitive +++ OK, passed 100 tests.
Eq: Symmetric +++ OK, passed 100 tests.
Eq: Reflexive +++ OK, passed 100 tests.
Monoid: Associative +++ OK, passed 100 tests.
Monoid: Left Identity +++ OK, passed 100 tests.
Monoid: Right Identity +++ OK, passed 100 tests.
Monoid: Concatenation +++ OK, passed 100 tests.

-----------------
-- Map Int Int --
-----------------

Eq: Transitive +++ OK, passed 100 tests.
Eq: Symmetric +++ OK, passed 100 tests.
Eq: Reflexive +++ OK, passed 100 tests.
Monoid: Associative +++ OK, passed 100 tests.
Monoid: Left Identity +++ OK, passed 100 tests.
Monoid: Right Identity +++ OK, passed 100 tests.
Monoid: Concatenation +++ OK, passed 100 tests.

lawsCheckOne

A convenience function that allows one to check many typeclass instances of the same type.

For example, in GHCi:

>>> lawsCheckOne (Proxy :: Proxy Word) [jsonLaws, showReadLaws]
ToJSON/FromJSON: Encoding Equals Value +++ OK, passed 100 tests.
ToJSON/FromJSON: Partial Isomorphism +++ OK, passed 100 tests.
Show/Read: Partial Isomorphism +++ OK, passed 100 tests.