from the haskell quickcheck library, code explained at https://www.youtube.com/watch?v=6COvD8oynmI (min 35-40)

quickcheck.hs

quickCheck :: Testable a => a -> IO ()

class Testable a where
    test :: a -> RandSupply -> Bool

class Arbitrary a where
    arby :: RandSupply -> a

instance Testable Bool where
    test b r = b

instance (Arbitrary a, Testable b)
        => Testable (a -> b) where
    test f r = test (f (arby r1)) r2
        where (r1, r2) = split r

variadic_str.hs

class MyString r where
    mystr :: String -> r

instance MyString String where
    mystr = id

instance (MyString r) => MyString (String -> r) where
    mystr x y = mystr (x ++ y)