Re: [Caml-list] Why should I use .mli files?

[Alain Frisch <alain@frisch.fr>]

On 10/30/2012 10:25 PM, Gabriel Scherer wrote:
> There is another related proposal: Alain Frisch's suggestion to make
> compilation units *recursive* modules implicitly, rather than
> non-recursive modules.
>    http://caml.inria.fr/mantis/print_bug_page.php?bug_id=5480
> Using recursive modules induces a definitive amount of complexity, but
> is also more expressive in terms of allowing forward references (the
> top-to-bottom style aspect I mentioned earlier). I think a simpler,
> non-recursive proposition is a more reasonable choice. Alain described
> a compromise in use at Lexifi, where compilation units are
> *type-checked* are recursive modules but *compiled* as non-recursive
> modules. That's probably close in expressivity to this proposal (but
> harder, I think, to explain to the newcomer).

Type-checking compilation units as recursive modules addresses several 
issues at the same time:

  - Mutual recursion between type-level components of different kinds 
(e.g. a data type declaration and a class type).  The current solution 
is to introduce a local recursive module.

  - Avoid duplication of type declarations between the interface and the 
implementation (at least for structural definitions; a type-include 
feature, as described in the Mantis ticket, would also give a solution 
for data types).


In addition, compiling units as recursive modules (not only for 
type-checking) would address the common need of forward references and 
the less common need of allowing recursion between, say, a class 
definition and a function.  The current work-around is to use references 
to break the recursion, but this is quite an ugly solution (the nice 
thing with it, though, is that it also works for allowing recursion 
between several compilation units).


Unfortunately, the type-checking and compilation of recursive modules 
does not seem to be a mastered domain yet, and it sounds dangerous to 
rely on them for defining the semantics of normal compilation units, at 
least for now (this is Xavier's argument, and I tend to agree). 
Hopefully, some more research will give a better understanding of 
recursive modules...



Alain