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

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

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

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