load "Real";
open TextIO Char String Real

exception Eof
exception StackUnderflow
exception BadOperator of string

fun applyOperator operator (s::f::r) = 
    (case operator of
        "+" => f + s
      | "-" => f - s
      | "*" => f * s
      | "/" => f / s
      | _   => raise BadOperator operator)::r
  | applyOperator operator _ = raise StackUnderflow

fun handleToken (token, stack) =
    case fromString token of
        NONE => applyOperator token stack
      | SOME(value) => value::stack

fun handleInput inStack =
    (if not (endOfStream stdIn) then
         let 
             val outStack =
                 foldl handleToken inStack (tokens isSpace (inputLine stdIn))
         in
             if not (null outStack) then 
                 (print (toString (hd outStack)); print "\n")
             else ();
             handleInput outStack
         end
     else raise Eof)
    handle BadOperator operator => (print ("Bad operator: \""
                                           ^ operator ^ "\"\n");
                                    handleInput inStack)
         | StackUnderflow       => (print "Stack underflow\n";
                                    handleInput inStack)

val _ =
    handleInput nil
    handle Eof => ()