How to make OCaml compiler report unused module aliases?

Ecosystem
,
1

Hi,

I wonder if it is possible to make the OCaml compiler to report unused module aliases?

For example, in the following file (test.ml), I want to enable ocamlc to report the warnings that two module aliases S and L are unused.

module S = String
module L = List

let foo () =
  let a = 1 in
  let b = 2 in
  a + b
;;

The only relevant warning options that I can find from ocamlc -warn-help are 31 (31 [module-linked-twice] A module is linked twice in the same executable) and 60 ([unused-module] Unused module declaration).

However, when running the following command, ocamlc doesn’t report any warning.

$ ocamlc -w +31+60+66 test.ml

# no warning is reported

Does anyone know how to enable ocamlc to report such warnings that module S = String and module L = List are unused?

Thank you for spending your time to read my question!

2

Hello,

You need to enable warning 60 and add an empty interface file for it to trigger.

Cheers,
Nicolas

2 Likes
3

@nojb: Thanks a lot! This is a great tip.

I create an empty file test.mli and now ocamlc can report the unused module aliases:

$ ocamlc -w +60 test.mli test.ml

File "test.ml", line 2, characters 0-15:
2 | module L = List
    ^^^^^^^^^^^^^^^
Warning 60 [unused-module]: unused module L.
4

@nojb: Do you know if there is an option to enable this warning for the whole project of many files?

I just realized that the above solution requires me to create empty interfaces for all files in my project, and it’s quite laborious…

Btw, I’m using Dune in my build system.

1 Like
5

Yes, it is enugh to add

(env (_ (flags :standard -w +60)))

in the top-level dune file.

For executables, you can set (executables_implicit_empty_intf true) in your dune-project file to automatically generate an empty interface whenever one does not exist already (see Stanza Reference — Dune documentation).

For libraries, there will be an analogous option in 3.0, see https://github.com/ocaml/dune/pull/4955.

Cheers,
Nicolas

6 Likes
6

@nojb: Thanks a lot for the update! I was able to use those options with the newest Dune 3.0-alpha built from source code.

FYI, the latest dune 2.9.1 from Opam doesn’t support (executables_implicit_empty_intf true) which I think is the syntax of (lang dune 3.0)