Toplevel Integration

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 directives (#directory and #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";;

This will print directives that will load without sealing it behind foo.mli. This is particularly useful for peeking and prodding at a module’s internals.

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 ();
   | exception e ->
     always ();
     raise e

 let use_output command =
   let fn = Filename.temp_file "ocaml" "" in
     ~always:(fun () -> try Sys.remove fn with Sys_error _ -> ())
     (fun () ->
         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" 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"