False positive your program contains effect handlers warning?

[johnridesabike]

Describe the bug

I recently tested a project with OCaml 5 and JSOO 5 and it printed this warning:

Warning: your program contains effect handlers; you should probably run js_of_ocaml with option '--enable=effects'

I'm almost certain that this is incorrect. None of the project's code has changed since using OCaml 4.14, and it only uses a few small dependencies. I opened the JavaScript output and saw that the Effect module functions are included, but do not seem to be called anywhere.

Expected behavior

The repository for my project: https://github.com/johnridesabike/acutis

Expected behavior:

• Run dune build
• Everything builds with no warnings.

Actual behavior: the above warning prints.

Versions

ocamlc: 5.0.0
js_of_ocaml: 5.0.1

[hhugo]

This is due to the use of separate compilation. When compiling the stdlib to JS, we don't know yet whether the Effect module will be used or not later. Also, as tracked by #943, with separate compilation, we unconditionally link all modules of libraries.

In the short term, you can build with --profile release which disable separate compilation of JS. or change the compilation mode in the env stanza (see https://dune.readthedocs.io/en/stable/jsoo.html?highlight=separate#separate-compilation).

Once #943 is fixed, we should be able to improve the situation and only warn during the linking phase.

[johnridesabike]

Thanks for the information! I can confirm that --profile release does indeed build without the warning.

It may be useful to include that information in the documentation and/or readme. I can imagine that other people may see the warning and incorrectly assume that they need to use --enable=effects when they actually do not.

[hhugo]

This was fixed in #1378. We still need to wait for a dune release including ocaml/dune#6832 and then change the default value for auto-link in jsoo.