Clean way to set Lexing.pos_fname field ?

Here is a minimal example showing my problem, it uses just one
silly_lexer.mll file, whose contents are :

{
    let accu=ref[]
    let add_to_accu x=(accu:=x::(!accu));;
}

rule silly_parse = parse
   _ as c
      {  add_to_accu (c,Lexing.lexeme_start_p lexbuf); silly_parse lexbuf}
  | eof { List.rev(!accu) }
  
{

let parse_file fn=
          let _=(accu:=[]) in
          let in_channel=open_in fn in
          let answer=silly_parse (Lexing.from_channel in_channel) in
          close_in in_channel; answer
  
}
  
I execute ocamllex silly_lexer.mll, enter my toplevel and type

        OCaml version 4.02.1

# #use"silly_lexer.ml";;
(...)
# let example=parse_file "silly_lexer.ml";; 
val example : (char * Lexing.position) list =
  [('#', {Lexing.pos_fname = ""; pos_lnum = 1; pos_bol = 0; pos_cnum = 0});
   (' ', {Lexing.pos_fname = ""; pos_lnum = 1; pos_bol = 0; pos_cnum = 1});
   ('1', {Lexing.pos_fname = ""; pos_lnum = 1; pos_bol = 0; pos_cnum = 2});
   (' ', {Lexing.pos_fname = ""; pos_lnum = 1; pos_bol = 0; pos_cnum = 3});
   ('"', {Lexing.pos_fname = ""; pos_lnum = 1; pos_bol = 0; pos_cnum = 4});
   ('s', {Lexing.pos_fname = ""; pos_lnum = 1; pos_bol = 0; pos_cnum = 5});
   
Notice how the Lexing.pos_fname is set to "" rather "silly_lexer.ml".   


 Sure, I know how to manually set this Lexing.pos_fname field, but I can't help
 feeling that this is not the clean way to do things. Why doesn't Ocaml
 automatically set it for me when Lexing.from_channel is called ? What am I missing ?