Intro to QuickCheck

age.hs

data Age     = Age Int
             deriving (Show)

instance Arbitrary Age where
  arbitrary  = Age `liftM` choose (0, 100)

fibs.hs

fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

person.hs

type Name    = String
data Person  = Person Name Age
             deriving (Show)

instance Arbitrary Person where
  arbitrary  = liftM2 Person arbitrary arbitrary

person.sample

> sample (arbitrary :: Gen Person)
Person "" (Age 93)
Person "_J" (Age 73)
Person "\vw\ENQ" (Age 84)
Person "\148Sm\197" (Age 45)
Person "S\SYN\246" (Age 75)
Person "rI=\246\&6*Fv" (Age 87)
Person "\b:\FSB\227\DC1\187!\SIJs" (Age 77)
Person "" (Age 68)
Person "\200KC\f\f\222D-s\148P,\140" (Age 29)
Person "$)\171.\n" (Age 91)
Person "\teP\DEL5A\251\252\DC4 V" (Age 37)

prop_Fibonacci.hs

smallNonNegativeIntegers = choose (0, 500)

prop_Fibonacci =
  forAll smallNonNegativeIntegers $ \n ->
    let x = fibs !! (n)
        y = fibs !! (n+1)
        z = fibs !! (n+2)
    in x + y == z

prop_Idempotent.hs

prop_Idempotent xs =
  sort (sort xs) == sort xs

prop_Idempotent.out

> quickCheck prop_Idempotent
+++ OK, passed 100 tests.

prop_Idempotent2.hs

prop_Idempotent xs =
  collect (length xs) $
    sort (sort xs) == sort xs

prop_Idempotent2.out

> quickCheck prop_Idempotent
+++ OK, passed 100 tests.
 9% 14
 7% 0
 5% 11
 4% 4
 4% 21
 4% 10
...

prop_Idempotent3.hs

prop_Idempotent xs =
  classify (length xs < 2) "trivial" $
    sort (sort xs) == sort xs

prop_Idempotent3.out

> quickCheck prop_Idempotent
+++ OK, passed 100 tests (8% trivial).

prop_SquareCube.hs

prop_Square x =
  x^2 >= x

prop_CubeOfPositive (NonNegative x) =
  x^3 >= x

shrink.out

> shrink [1,2,3]
[[],[2,3],[1,3],[1,2],[0,2,3],[1,0,3],[1,1,3],[1,2,0],[1,2,2]]