`forAll` is a QuickCheck combinator for running a property with a generator chosen explicitly for that property. It turns the generator-and-check pairing back into a `Property`, so custom test data can drive a normal QuickCheck test. section 9.4.1
The capitalization matters: `forAll` is the QuickCheck testing combinator, whereas lowercase `forall` is Haskell syntax whose meaning depends on enabled language extensions. Do not treat them as interchangeable. section 6.2.2 section 9.4.1
Use `forAll` when a property needs inputs with relationships that default generation does not guarantee:
- Build a `Gen` value that produces the required input shape. section 9.4.1
- Pass that generator and a predicate to `forAll`; the resulting value is again a `Property`. section 9.4.1
- Move reusable generation into an `Arbitrary` instance when custom types need generators across many tests. section 9.4.1
Testing alphabet rotation with valid inputs
A rotation property needs a nonempty alphabet and a character drawn from that alphabet. The generator selects an alphabet, bounds the rotation by test size, selects a member character, and returns the triple. `forAll gen prop` then checks that `alphabetRot` returns a character still contained in the alphabet. section 9.4.1
Listing 9.18 Property describing closedness of the alphabet function
prop_alphabetRotClosed :: Property
prop_alphabetRotClosed =
forAll gen prop #1
where
prop :: (Alphabet, Int, Char) -> Bool
prop (alphabet, n, c) = #2
let c' = alphabetRot alphabet n c #2
in c' `elem` alphabet #2
gen :: Gen (Alphabet, Int, Char)
gen = do
size <- getSize #3
alphabet <- arbitrary `suchThat` (not . null) #4
n <- choose (-size, size) #5
c <- elements alphabet #6
return (L.nub alphabet, n, c) #7
The listing places `forAll gen prop` at the property boundary, keeping generation separate from the Boolean check while sharing a tuple-shaped test case. section 9.4.1
