how to run ocamlyacc generated code in utop

Question

I'm a newbie in OCAML. For this example on using ocamlyacc: https://v2.ocaml.org/manual/lexyacc.html#s:lexyacc-example. I can compile and run the calc program. How do I run calc.ml in utop? When I do this in utop

open Parser;;
open Lexer;;
#use "calc.ml";;

I get this error

File "calc.ml", line 5, characters 19-30:
5 |       let result = Parser.main Lexer.token lexbuf in
                       ^^^^^^^^^^^
Error: Unbound value Parser.main

Here are the files from the above website. parser.mly:

%token <int> INT
%token PLUS MINUS TIMES DIV
%token LPAREN RPAREN
%token EOL
%left PLUS MINUS        /* lowest precedence */
%left TIMES DIV         /* medium precedence */
%nonassoc UMINUS        /* highest precedence */
%start main             /* the entry point */
%type <int> main
%%
main:
    expr EOL                { $1 }
;
expr:
    INT                     { $1 }
  | LPAREN expr RPAREN      { $2 }
  | expr PLUS expr          { $1 + $3 }
  | expr MINUS expr         { $1 - $3 }
  | expr TIMES expr         { $1 * $3 }
  | expr DIV expr           { $1 / $3 }
  | MINUS expr %prec UMINUS { - $2 }
;

lexer.mll:

{
open Parser        (* The type token is defined in parser.mli *)
exception Eof
}
rule token = parse
    [' ' '\t']     { token lexbuf }     (* skip blanks *)
  | ['\n' ]        { EOL }
  | ['0'-'9']+ as lxm { INT(int_of_string lxm) }
  | '+'            { PLUS }
  | '-'            { MINUS }
  | '*'            { TIMES }
  | '/'            { DIV }
  | '('            { LPAREN }
  | ')'            { RPAREN }
  | eof            { raise Eof }

calc.ml:

let _ =
  try
    let lexbuf = Lexing.from_channel stdin in
    while true do
      let result = Parser.main Lexer.token lexbuf in
      print_int result; print_newline(); flush stdout
    done
  with Lexer.Eof ->
    exit 0