ML RPN Solution

This solution was written by David MacQueen. The code can be found in ~jalewis/451/ml/rpn.sml. It does not handle all the requirements of the assignment, such as reading multiple lines from a file or handling errors.

(* polish.sml *)

structure RevPolish : sig val execute : unit -> int end =
struct

datatype token = INT of int | PLUS | MINUS | TIMES | MOD | DIV

exception ERROR of string
fun error s = raise (ERROR s)

fun token s =
    if Char.isDigit(String.sub(s,0)) orelse 
       (String.sub(s,0) = #"~")
    then case Int.fromString s
	   of SOME n => INT n
	    | NONE => error ("bad token "^s)
    else case s
	   of "+" => PLUS
	    | "-" => MINUS
	    | "*" => TIMES
	    | "DIV" => DIV
	    | "MOD" => MOD
	    | _ => error ("bad token "^s);

fun getStrings () : string list =
    String.tokens Char.isSpace (TextIO.inputLine TextIO.stdIn)

fun getTokens () =
    map token (getStrings())

fun interp (nil: token list, [n]: int list) = n
  | interp ((INT n)::rest,s) = interp(rest,n::s)
  | interp (PLUS::rest,m::n::s) = interp(rest,m+n::s)
  | interp (MINUS::rest,m::n::s) = interp(rest,m-n::s)
  | interp (TIMES::rest,m::n::s) = interp(rest,m*n::s)
  | interp (DIV::rest,m::n::s) = interp(rest,m div n::s)
  | interp (MOD::rest,m::n::s) = interp(rest,m mod n::s)
  | interp _ = error "bad expression"

fun execute () =
    interp(getTokens(),nil)
    handle ERROR s => (print (s^"\n"); 0)

end (* structure RevPolish *)