Loading OCaml modules not in the current directory

Question

I'm writing a large OCaml project. I wrote a file foo.ml, which works perfectly. In a subdirectory of foo.ml's directory, there is a file bar.ml.

bar.ml references code in foo.ml, so its opening line is:

open Foo

This gives me an error at compile time:

Unbound module Foo.

What can I do to fix this without changing the location of foo.ml?

Answer 1

The easy path is to use one of OCaml build system like ocamlbuild or oasis. Another option would be jbuilder but jbuilder is quite opiniated about file organization and does not allow for the kind of subdirectory structure that you are asking for.

The more explicit path comes with a warning: OCaml build process is complicated with many moving parts that can be hard to deal with.

After this customary warning, when looking for modules, OCaml compiler first looks for module in the current compilation environment, then looks for compiled interface ".cmi" files in the directories specified by the "-I" option flags (plus the current directory and the standard library directory).

Thus in order to compile your bar.ml file, you will need to add the parent directory in the list of included directories with the -I .. option.

After all this, you will discover that during the linking phase, all object files (i.e. .cmo or .cmx) need to be listed in a topological order compatible with the dependency graph of your project.

Consequently, let me repeat my advice: use a proper build system.