Bug in the OCaml manual for open! syntax?

The explanation of the open! syntax seems to be incomplete. The manual states:

Since OCaml 4.01, open statements shadowing an existing identifier (which is later used) trigger the warning 44. Adding a ! character after the open keyword indicates that such a shadowing is intentional and should not trigger the warning.

Looking at the manual, we see:

Warning 44: Open statement shadows an already defined identifier.

However, in testing, I’ve found that adding open! will not only disable warning 44, but also warning 33.

Warning 33: Unused open statement.

I’ve constructed a minimal example which shows the description about open! to be incomplete:

(* main.ml *)

let foo = "abc"

module Bar = struct
  let foo = "xyz"

open Bar

let () = print_endline foo

Compiling with

$ ocamlc -w +33+44 -c main.ml

will trigger warning 44 because module Bar contains a value foo which is shadowed, and then used.

And changing

let () = print_endline foo


let () = print_endline "foo"

will trigger warning 33 because the Bar module is no longer used.

However, if you change

open Bar


open! Bar

then neither warning will be triggered regardless of whether foo or "foo" is used.

I have confirmed this to be the case with compiler versions 4.03.0, 4.04.1, and 4.05.0

1 Like

The documentation and the current behavior of the compiler are indeed at odds, however there was no general consensus on which one is erroneous the last time the issue was discussed, see https://caml.inria.fr/mantis/view.php?id=6638 and https://github.com/ocaml/ocaml/pull/1110 .