-- The three statements below beginning "type" may be omitted
-- but the code is then more fragile.
data Term = Con Int | Div Term Term;
type M a = State -> (a, State);
type State = Int;

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) x = (a, x);
eval (Div t u) x = let (a, y) = eval t x in 
                   let (b, z) = eval u y in (div a b, z+1);
main = print (eval answer 0)

-- > (42,2)