How to Load a Project in a Toplevel

It is possible to use OCaml code in an interactive way, by typing an expression, which gets evaluated and its result printed. Such a program is called a toplevel, or REPL (Read-Eval-Print Loop).

The compiler distribution comes with a small REPL called simply ocaml, and the community has developed enhanced versions such as UTop.

Building a Specialized UTop Executable

It is possible to generate a specialized version of UTop that embeds the current project. To do so, use the following command:

$ dune utop

The interactive session will start with all the modules loaded.

If some of the libraries are PPX rewriters, the phrases you type in the toplevel will be rewritten with these PPX rewriters. Similarly, PPX derivers defined in the project will be available.

Loading the Project in a Toplevel

It is also 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.

Loading a Single Module in a Toplevel

It’s also possible to load individual modules 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.