Removing specific occurrence of "warning: unused value declaration"

Question

I'm trying to use warning decorators ([@*ocaml.warning]) to locally remove a warning about a specific unused function, but I can't find the right syntax (if there is one). I'm using OCaml 4.02.1.

In the code below, I have the dbg function which is not exported and never used, but for which I'd like to silence warning 32 (unused value dbg).

I want to keep warnings activated elsewhere in the code, to avoid accidental mistakes.

I tried putting all kinds of decorators around the function, but the warning still appears:

A.mli:

val f : unit -> unit

A.ml:

let f () = ()

[@@ocaml.warning "-32"]
let dbg () = () [@ocaml.warning "-32"]
[@@ocaml.warning "-32"]

let bla () = ()

ocamlc -w +a a.mli a.ml results in:

File "a.ml", line 4, characters 4-7:
Warning 32: unused value dbg.
File "a.ml", line 7, characters 4-7:
Warning 32: unused value bla.

Note that adding [@@@ocaml.warning "-32"] before let dbg works (it removes the warning), but then I have to add [@@@ocaml.warning "+32"] afterwards to re-enable it, which is not ideal, since it enables warnings even when they were not present in the first place.

For instance, if I then compiled with ocamlc a.mli a.ml, this would add an unwanted warning.

Is there a way to locally disable warning 32?

Answer 1

I believe there is no way to stop individual warning 32 in OCaml 4.02.3.

I had the same trouble at trying to suppress a warning (mine was 39) of one specific toplevel let. The ticket is http://caml.inria.fr/mantis/view.php?id=6677 and http://caml.inria.fr/mantis/view.php?id=6586.

What you can do so far is to declare such variables with _ prefix:

let _dbg () = ()

since variables start with _ is the out of the scope of warning 32. The downside is that there is no tool available to warn if such _ prefixed value is actually used in your code.