try a function example

try_finally.re

let try_finally = (~always, f) =>
  switch (f()) {
  | x =>
    always();
    x;
  | exception e =>
    always();
    raise(e);
  };

usage.re

let use_output = command => {
  let fn = Filename.temp_file("ocaml", "_toploop.ml");
  try_finally(
    ~always=
      () =>
        try (Sys.remove(fn)) {
        | Sys_error(_) => ()
        },
    () =>
      switch (
        Printf.ksprintf(Sys.command, "%s > %s", command, Filename.quote(fn))
      ) {
      | 0 => ignore(Toploop.use_file(Format.std_formatter, fn): bool)
      | n => Format.printf("Command exited with code %d.@.", n)
      },
  );
};

let () = {
  let name = "use_output";
  if (!Hashtbl.mem(Toploop.directive_table, name)) {
    Hashtbl.add(
      Toploop.directive_table,
      name,
      Toploop.Directive_string(use_output),
    );
  };
};

https://dune.readthedocs.io/en/stable/toplevel-integration.html#toplevel-integration