{
 (* Revised extract of program by Peter Sestoft                        *) 

 (* WHILElex.lex: lexer specification for a simple WHILE language      *)

 open Lexing WHILEpar;

 exception LexicalError of string * int * int (* (message, loc1, loc2) *)

 fun lexerError lexbuf s = 
     raise LexicalError (s, getLexemeStart lexbuf, getLexemeEnd lexbuf);

 (* Scan keywords as identifiers and use this function to distinguish them. *)
 (* If the set of keywords is large, use an auxiliary hashtable.            *)

 fun keyword s =
     case s of
         "true"        => TRUE
       | "false"       => FALSE
       | "if"          => IF
       | "THEN"        => THEN
       | "ELSE"        => ELSE
       | "while"       => WHILE
       | "do"          => DO
       | _             => VAR s;
}

rule Token = 
  parse
    [` ` `\t` `\n` `\r`]  { Token lexbuf }
  | [`0`-`9`]+            { case Int.fromString (getLexeme lexbuf) of
                               NONE   => lexerError lexbuf "internal error"
                             | SOME i => INT i
                          }  
  | [`a`-`z``A`-`Z`][`a`-`z``A`-`Z``0`-`9`]*
                          { keyword (getLexeme lexbuf) }
  | `+`                   { PLUS  } 
  | `-`                   { MINUS } 
  | `*`                   { TIMES }
  | `&`                   { AND   }
  | `!`                   { NEG   } 
  | `=`                   { EQ    } 
  | `<`                   { LT    }
  | `(`                   { LPAR  } 
  | `)`                   { RPAR  } 
  | `;`                   { SEMI  }
  | ":="                  { ASS   }
  | eof                   { EOF   }
  | _                     { lexerError lexbuf "Illegal symbol in input" }
;