We ran into an example (in SML) during the development of MLWorks.  I can
only remember it vaguely, but there were two large module hierarchies.
These hierarchies were in logically different parts of the system.  A new
development built on top of these hierarchies meant that we needed to make
one element from the bottom of each hierarchy be mutually recursive with
each other.  I suspect that since we were using a fully functorised style,
what we wanted to do was to make two functors mutually recursive.  In
practice we had to combine the module hierarchies to make the types and
values mutually recursive at the bottom of the new hierarchy.

I wish I could remember what the example was, because it was a clear case
where the ML module system was too inflexible to meet the pragmatic needs.
Imagine how we would have been stuck if the two module hierarchies were
libraries from different suppliers, especially if they were only distributed
in compiled form.

Dave. 
  

-----Original Message-----
From: Claudio Russo [mailto:crusso@microsoft.com]
Sent: Thursday, January 11, 2001 10:33
To: Alain Frisch
Cc: Brian Rogoff; Caml list
Subject: RE: first class, recursive, mixin modules (was: RE: first class
modules)