# Pastebin bkUnHAwg
-- Defines a distinct type in the type system, which has the same
-- runtime representation as Int.
newtype ZeroToSix = ZeroToSix Int

-- Give the ZeroToSix an Arbitrary instance so we can use it in
-- QuickCheck properties.
instance Arbitrary ZeroToSix where
  -- A generator of 'ZeroToSix's.
  arbitrary :: Gen ZeroToSix
  arbitrary = ZeroToSix <$> chooseInt 0 6

  -- `shrink` helps QuickCheck find minimal counterexamples when a
  -- property is violated.
  shrink :: ZeroToSix -> [ZeroToSix]
  shrink (ZeroToSix n) = [ ZeroToSix x | x <- [0..(n-1)] ]

qcProps = testGroup "(checked by QuickCheck)"
  [ QC.testProperty "add7 is commutative" $
      \(ZeroToSix a) (ZeroToSix b) (ZeroToSix c) -> add7 a b c == add7 b c a
  ]