Refill (js_of_ocaml-toplevel.Js_of_ocaml_toplevel

val lexbuf : 
  string ->
  int Stdlib.ref ->
  Stdlib.Format.formatter option ->
  bytes ->
  int ->
  int

lexbuf s p ppf buffer len is a Lexing.from_function-compatible refill function feeding s to the lexer one line at a time, tracking the current position in p. When ppf is Some _, each chunk is echoed to it (this is how the toplevel echoes the phrase being read).

s is normalized so that it ends with ;;: the incremental Toploop.parse_toplevel_phrase otherwise drops a trailing unterminated phrase.