gistfile1.hs

{-# LANGUAGE FlexibleInstances, IncoherentInstances #-}

import Control.Monad

data Id a = Id a
          deriving Show

instance Monad Id where
  (Id x) >>= f = f x
  return = Id


class Zero a where
  isZero :: a -> Bool
  isZero _ = False

instance Zero (Id (Id a)) where
  isZero _ = False

instance Zero (Id a) where
  isZero _ = True

class Number a where
  numValue :: a -> Int

instance Number (Id a) where
  numValue x
    | isZero x = 0
    | otherwise = numValue $ join x