FB.hs

-- We don't really care about the types pt, pf, rt and rf here,
-- and we want to get an Int back when both predicates are false.
type FizzBuzz = FizzResult Int () () () ()

-- Helper function to format predicates to the form mkFizzBuzz needs.
boolToEither :: Bool -> Either () ()
boolToEither True = Right ()
boolToEither False = Left ()

fizzbuzz :: Int -> FizzBuzz
fizzbuzz =
  let divisibleBy' d x = boolToEither (divisibleBy d x)
  in mkFizzBuzz (divisibleBy' 3) (divisibleBy' 5)