datatype operation = Add | Sub | Mult | Div;

exception BadOperation of string;

fun checkDigit x = if x > 9 then
                       raise BadOperation "too large"
                   else if x < 0 then
                       raise BadOperation "too small"
                   else
                       x;

fun eval Add  x y = checkDigit (x+y)
  | eval Sub  x y = checkDigit (x-y)
  | eval Mult x y = checkDigit (x*y)
  | eval Div  x y = if y = 0 then
                        raise BadOperation "divide by zero"
                    else
                       checkDigit (x div y);

fun eval2 Add x y  = checkDigit (x+y)
  | eval2 Sub x y  = checkDigit (x-y)
  | eval2 Mult x y = checkDigit (x*y)
  | eval2 Div x y  = checkDigit (x div y) handle
                    DivByZero => raise BadOperation "divide by zero";

fun calculate1 oper x y = SOME (eval oper x y) handle
                        (BadOperation msg) => NONE

fun calculate2 oper x y =
    "Answer is: " ^ (Int.toString (eval oper x y)) handle
                        (BadOperation msg) => "Error: " ^ msg;