OCaml provides a small REPL to use the language interactively. We
generally call this tool a toplevel. The compiler distribution comes
with a small REPL called simply
ocaml, and the community has
developed enhanced versions such as UTop.
It’s possible to load Dune projects in any toplevel. To do that, simply execute the following in your toplevel:
# #use_output "dune ocaml top";;
dune ocaml top is a Dune command that builds all the libraries in the
current directory and subdirectories and outputs the relevant toplevel
#load) to make the various modules
available in the toplevel.
Additionally, if some of the libraries are PPX rewriters, the phrases you type in the toplevel will be rewritten with these PPX rewriters.
This command became available with Dune 2.5.0.
It’s also possible to load individual modules (since dune 3.6.0) for interactive development. Use the following dune command:
# #use_output "dune ocaml top-module foo.ml";;
This will print directives that will load
foo.ml without sealing it behind
foo.mli. This is particularly useful for peeking and prodding at a module’s
Note that the
#use_output directive has only been available since OCaml 4.11. You
can add the following snippet to your
~/.ocamlinit file to make it available
in older versions of OCaml:
#directory "+compiler-libs" let try_finally ~always f = match f () with | x -> always (); x | exception e -> always (); raise e let use_output command = let fn = Filename.temp_file "ocaml" "_toploop.ml" in try_finally ~always:(fun () -> try Sys.remove fn with Sys_error _ -> ()) (fun () -> match Printf.ksprintf Sys.command "%s > %s" command (Filename.quote fn) with | 0 -> ignore (Toploop.use_file Format.std_formatter fn : bool) | n -> Format.printf "Command exited with code %d.@." n) let () = let name = "use_output" in if not (Hashtbl.mem Toploop.directive_table name) then Hashtbl.add Toploop.directive_table name (Toploop.Directive_string use_output) ;; #remove_directory "+compiler-libs"