Validity and validity-based testing

Why?

to make writing correct software cheaper.

• Free generators
• Free shrinking
• Cheap properties

Cheap properties

• Property Combinators:

specify "inverse functions" $ inverseFunctions (+1) (-1)
specify "equivalent functions" $ equivalent ((+ 1) . (- 1)) id
specify "transitivity" $ transitivity ((>) :: Int -> Int -> Bool)
specify "symmetry" $ symmetry ((==) :: Int -> Int -> Bool)

• Test suite combinators:

ordSpec @Int

  Ord Int
    (<=) :: Int -> Int -> Bool
      is reflexive for "even Int"'s
        +++ OK, passed 100 tests.
      is antisymmetric for "even Int"'s
        +++ OK, passed 100 tests.
      is transitive for "even Int"'s
        +++ OK, passed 100 tests.
      is equivalent to (\a b -> compare a b /= GT) for "even Int"'s
        +++ OK, passed 100 tests.
    (>=) :: Int -> Int -> Bool
      is reflexive for "even Int"'s
        +++ OK, passed 100 tests.
      is antisymmetric for "even Int"'s
        +++ OK, passed 100 tests.
      is transitive for "even Int"'s
        +++ OK, passed 100 tests.
      is equivalent to (\a b -> compare a b /= LT) for "even Int"'s
        +++ OK, passed 100 tests.
    (<) :: Int -> Int -> Bool
      is antireflexive for "even Int"'s
        +++ OK, passed 100 tests.
      is transitive for "even Int"'s
        +++ OK, passed 100 tests.
      is equivalent to (\a b -> compare a b == LT) for "even Int"'s
        +++ OK, passed 100 tests.
    (>) :: Int -> Int -> Bool
      is antireflexive for "even Int"'s
        +++ OK, passed 100 tests.
      is transitive for "even Int"'s
        +++ OK, passed 100 tests.
      is equivalent to (\a b -> compare a b == GT) for "even Int"'s
        +++ OK, passed 100 tests.

Free generators and shrinking

data MyType = MyType
  { myRational :: Rational
  , myBool :: Bool
  } deriving (Show, Eq, Generic)

instance Validity MyType -- Implementation is derived via Generic
instance GenValid MyType -- Default implementation via Generic

genValid :: Gen MyType -- Free generator
shrinkValid :: MyType -> [MyType] -- Free _valid_ shrinking function

Cachix cache

There is a cachix cache for this project.

To use it, use cachix use validity or add the appropriate details to your nixos configuration.