{
 (* Exprlex.lex: lexer specification for the simple expression language *)

 open Lexing Exprpar;

 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 a hashtable instead.               *)

 fun keyword s =
     case s of
         "let"          => LET
       | "in"           => IN
       | "end"          => END
       | _              => NAME s;
}

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