The best way is to create a custom toplevel and map the replies.

"val x : int = 3" is printed using [Toploop.print_out_phrase] which is
a reference, so you can replace by a function adding markers around
the text.

This won't catch error/warning messages. If you also want to
distinguish these, you can redirect the standard formatters. The
toplevel prints replies and error messages on the formatter passed to
[Toploop.loop], which is [Format.std_formatter] by default. So you can
either:
(1) override [Format.std_formatter] callbacks
(2) call [Toploop.loop] with a custom formatter

You can do (1) with [Format.pp_set_all_formatter_output_functions] and
(2) by setting the startup hook:

    Toploop.toplevel_startup_hook := (fun () -> Toploop.loop my_formatter;
exit 0)

Warnings are always printed on [Format.err_formatter] so you'll need
to override its callbacks.

If you can't create a custom toplevel, I guess you can also send the
code to the toplevel at the beginning of the session.



On Mon, Mar 24, 2014 at 1:17 PM, Alan Schmitt <
alan.schmitt@polytechnique.org> wrote:

> Hello,
>
> I'm looking into extending the OCaml support in org-mode, and to do so
> I would like to distinguish what is printed out as replies from the
> toplevel from the data printed from the program.
>
> To give a precise example, I would like to split "foo" from "val x : int
> = 3" in the following interaction:
>
> --8<---------------cut here---------------start------------->8---
> # let x = print_endline "foo"; 3;;
> foo
> val x : int = 3
> --8<---------------cut here---------------end--------------->8---
>
> Is there a way to do it?
>
> Thanks,
>
> Alan
>
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-- 
Jeremie