data Term = Con Int | Div Term Term;
data M a = Raise Exception | Return a deriving (Show);
type Exception = String;
answer, err :: Term;
answer = (Div (Div (Con 1972) (Con 2)) (Con 23));
err = (Div (Con 1)(Con 0));
eval :: Term -> M Int;
eval (Con a) = Return a;
eval (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 (quot a b);

main = print((show (eval err)) ++ "; " ++ (show (eval answer)))

-- > "Raise \"divide by zero\"; Return 42"