[Caml-list] A confusing example with modules and polymorphic variants

[Yaron Minsky <yminsky@janestreet.com>]

Sorry for the previous incomplete message.  Over-eager return key.

We've been running into some troubles with polymorphic variants,
modules, and the value restriction, that we're not quite able to
unravel.  Here's a stripped down version of the problem.

    module type S = sig
      val z : [< `Foo ]
    end

    let f z =
      let module M : S = struct let z = z end in ()

Roughly speaking, we have a module signature S with a value that's a
polymorphic variant type that is not exact (i.e., has some
polymorphism in it.)

The function f then tries to construct a module of that type, by
taking the input value and stuffing it into a locally defined module.
When we do this, however, we're hit by a non-generalizable value
complaint.

    Modules do not match: sig val z : '_a end is not included in S
    Values do not match: val z : '_a is not included in val z : [< `Foo ]
    File "z.ml", line 2, characters 2-19: Expected declaration
    File "z.ml", line 7, characters 8-9: Actual declaration

Any thoughts on why this doesn't work, and how one could get it to?