maybe-monads.hs

import Control.Applicative (Applicative(..))
import Control.Monad       (liftM, ap)

-- MyMaybe is to play with Maybes
data MyMaybe x = MyJust x | MyNothing
    deriving Show

-- Monad MyMabye

instance Functor MyMaybe where
    fmap = liftM

instance Applicative MyMaybe where
    pure x = MyJust x
    (<*>) = ap

instance Monad MyMaybe where
    (>>=) (MyJust x) f = f x
    (>>=) MyNothing f = MyNothing


data Term = Con Int | Div Term Term

eval :: Term -> MyMaybe Int
eval (Con a) = return a
eval (Div t1 t2) = eval t1 >>= (\x -> eval t2 >>= (\y -> return (x `div` y)))