import Html exposing (text)
import Text
type Term = Con Int
          | Div Term Term

type M a = Raise Exception | Return a
type alias Exception = String

unit : a -> M a
unit a = Return a

flatMap : M a -> (a -> M b) -> M b
flatMap a k = case a of
  Raise e  -> Raise e
  Return a -> k a

eval : Term -> M Int
eval term = 
  case term of
    Con a -> unit a
    Div t u ->
      flatMap (eval t)(\a ->
        flatMap (eval u)(\b -> 
          if b == 0
            then Raise "divide by zero"
            else unit (a//b)
        )
      )
    

main = text (toString (eval (Div(Div(Con 1972)(Con 2))(Con 0))))