RE: [Caml-list] Lazy modules

["David Allsopp" <dra-news@metastack.com>]

Dario Teixeira wrote:
> Hi,
> 
> > AFAIK local modules is a syntax extension not a compiler extension - I
> > expect (not looked at it) that the syntax extension simply alpha
> > renames all the local module declarations to make them unique and puts
> > them globally... a very useful extension but no expressive power
> > added.
> 
> But if that were true, wouldn't the functor instantiation happen at
> initialisation time, thus preventing the delayed instantiation that is
> key for this solution to work?

Yup, somewhat embarrassingly I seemed to be barking in the wrong forest there, let alone up the wrong tree!!

> > I believe that the module system due for OCaml 3.12.0 will allow this
> > kind of runtime application of functors as modules are first class
> > values.
> 
> Again, I'm under the impression that functor application can already
> (with 3.11 at least) occur at runtime when local modules are used.
> (Or are you talking about different things?).  For example:
> 
> # module Foo (S: sig end) = struct let () = print_endline "foo!" end;;
> module Foo : functor (S : sig  end) -> sig  end
> 
> # let hello () = let module F = Foo (struct end) in ();; val hello :
> unit -> unit = <fun>
> 
> # hello ();;
> foo!
> - : unit = ()
> 
> 
> > Hope that's helpful - the inability to do what you're wanting to do is
> > one of the reasons that I've never delved deeply into the module
> > system - powerful as it may be, it didn't seem to be able to help
> > performing a simple task (similar to yours) that I wanted it to do (I
> > have also in the past wanted to exactly what you're doing - i.e. a
> > module of loaded configuration values).
> 
> Yep, Alain confirmed that modules as first-class values are indeed
> coming for 3.12.  Ocaml's module system just got more interesting...

Definitely!


David