Very interesting.  So now, looking back at Xavier's POPL'95 paper on
applicative functors, I see what he means by saying it's a fundamental
problem with how applicative functors work in OCaml.  I.e. it's not
just a bug in the typechecker, but in the type system in the original
paper.  In particular, the definition of signature strengthening on
page 7 of that paper includes the following case:

(module x_i : M; S)/p =
    module x_i : (M/p.x); S/p

But I believe this is a mistake, and instead of S/p it should be
  (S{x_i <- p.x})/p

In other words, first replace references to x_i (esp. in functor
applications in types) inside S with references to p.x, and then
proceed with selfification as usual.  Is there some reason this would
not work or would be difficult to implement?

I believe this would eliminate the bugs we're looking at here.  And
I'm sort of surprised that this would be hard to do, but I'm not
familiar with the implementation.

Derek

On 3/21/07, Stephen Weeks <sweeks@sweeks.com> wrote:
> Thanks for the reply Derek.  I also thought it looked like a bug, and
> your explanation makes sense.  However, Markus Mottl pointed me to the
> following  issue in the bug tracker
>
>  http://caml.inria.fr/mantis/view.php?id=3476
>
> The problem there looks very similar, and would seem to indicate that
> the situation is not viewed as a bug (in the sense that it won't be
> fixed).
>