type I a = a;
data Term = Con Int | Div Term Term;
answer, err :: Term;
answer = (Div (Div (Con 1972) (Con 2)) (Con 23));
err = (Div (Con 1)(Con 0));

data M a = Raise Exception | Return a;
type Exception = String;
unit :: a -> M a;
unit a = Return a;
(★) :: M a -> (a -> M b) -> M b;
m★k = case m of Raise e -> Raise e; Return a -> k a;
raise :: Exception -> M a;
raise e = Raise e;

eval :: Term -> M Int;
eval (Con a) = Return a;
eval (Div t u) = eval t ★  \a -> eval u ★  \b ->
   if b == 0 then raise "divide by zero";
             else unit (quot a b);

sh :: M Int -> String;
sh (Raise e) = e; sh (Return a) = show a;

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

-- > "42; divide by zero"