Setting the prompt in an OCaml custom toplevel

Question

In an OCaml custom toplevel, is there a way to programatically set the prompt from # to something else? I would like to be able to change it in response to the user's last one of my custom functions (kinda like in bash how you can set PS1). I can't even find a #directive to change it. Thanks!

Answer 1

In toplevel/toploop.ml:

let prompt =
  if !Clflags.noprompt then ""
  else if !first_line then "# "
  else if Lexer.in_comment () then "* "
  else "  "
in

But wait! The computed prompt is passed to !read_interactive_input, and this reference is exported:

In toplevel/toploop.mli:

(* Hooks for external line editor *)

val read_interactive_input : (string -> string -> int -> int * bool) ref

So it would seem that all you have to do is change the value of Toploop.read_interactive_input from its default to a function that ignores the passed prompt and prints the one you want instead.

The default value for read_interactive_input is:

let read_input_default prompt buffer len =
  output_string Pervasives.stdout prompt; flush Pervasives.stdout;
  let i = ref 0 in
  try
    while true do
      if !i >= len then raise Exit;
      let c = input_char Pervasives.stdin in
      buffer.[!i] <- c;
      incr i;
      if c = '\n' then raise Exit;
    done;
    (!i, false)
  with
  | End_of_file ->
      (!i, true)
  | Exit ->
      (!i, false)

So you can use:

# let my_read_input prompt buffer len =
  output_string Pervasives.stdout  "%%%" ; flush Pervasives.stdout;
  let i = ref 0 in
  try
    while true do
      if !i >= len then raise Exit;
      let c = input_char Pervasives.stdin in
      buffer.[!i] <- c;
      incr i;
      if c = '\n' then raise Exit;
    done;
    (!i, false)
  with
  | End_of_file ->
      (!i, true)
  | Exit ->
      (!i, false)
                                  ;;
val my_read_input : 'a -> string -> int -> int * bool = <fun>
# Toploop.read_interactive_input := my_read_input ;;
- : unit = ()
%%%