--Experiment with type system and monads

main = do
          print (show (eval question 0)) -- the zero is the initial state

data Term = Con Int | Div Term Term deriving Show

question=Div(Div (Con 18) (Con 2)) (Con 3)

type M a = State -> (a,State)
type State = Int

rtn :: a -> M a
rtn a = \x -> (a,x)

(>>>=) :: M a -> (a->M b) -> M b
m >>>= k = \x -> let (a,y)=m x
                     (b,z)=k a y
                 in (b,z)

tick :: M ()
tick  = \x -> ((),x+1)

eval :: Term -> M Int
eval (Con a) = rtn a
eval (Div t u) = eval(t) >>>= \a -> eval(u) >>>= \b -> tick >>>= \y -> rtn(a `div` b)