Re: [Caml-list] "with module" surprises

[Yaron Minsky <yminsky@gmail.com>]

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I agree that specializing modules is a reasonable thing to do in general.
But that's not what I think the "with" syntax is usually for.  It's usually
for adding sharing constraints, and this kind of modification of a module is
not the same thing as adding a constraint.  Note that the following code
fails, as I think it should:

  module type S' = sig end
  with type t = int

Do you by any chance have a use-case that you think benefits from these
semantics?

y

On Mon, May 9, 2011 at 11:12 AM, Markus Mottl <markus.mottl@gmail.com>wrote:

> The current semantics seems to make sense to me.  E.g.
>
>  module type M' = sig type t end
>
> specifies that M' needs a type t.  It doesn't say that a module
> matching this signature needs to keep t abstract.  You can hence
> specialize this signature using "with" to e.g. require that it be an
> "int".
>
> Module constraints work similarly.  If a signature is empty, this
> doesn't mean that a module matching it must not contain anything,
> rather the opposite: any module can match it.  You can again
> specialize the signature using "with" to require further entries.  The
> module passed to "with" only needs to match the first signature, which
> is trivially true in this case.  Its own (possibly inferred) signature
> will then specialize the previous signature, potentially adding more
> entries.
>
> A maybe more intuitive way to think about this is following: in OCaml
> you can only make things more strict, never less strict.  An empty
> signature is less strict (can be matched by more modules) than a
> non-empty one.  Hence extending it is the right "direction".
>
> Markus
>
> On Mon, May 9, 2011 at 10:27, Yaron Minsky <yminsky@janestreet.com> wrote:
> > I've gotten bitten recently by the semantics of "with module", and after
> > getting an explanation about how this seems to work in OCaml, I'm now
> > deeply confused.  Here's the example I was shown:
> >
> >  module M = struct
> >    let x = 13
> >  end
> >
> >  module type S = sig
> >    module M' : sig end
> >  end
> >  with module M' = M
> >
> > The inferred types for this will be:
> >
> >  module M : sig val x : int end
> >  module type S = sig module M' : sig val x : int end end
> >
> > Whereas I would have expected this:
> >
> >  module M : sig val x : int end
> >  module type S = sig module M' : sig end end
> >
> > In other words, the "with module" constraint has added new structure to
> > the signature S, rather than just adding constraints.  This strikes me
> > as deeply strange, and indeed, has caused a bunch of head-scratching
> > here when using "with module".  Is this a bug?  Or is this really the
> > desired semantics.  My understanding is that in SML, "with module"
> > simply adds in a bunch of type-level sharing constraints.  From that
> > point of view, this behavior is pretty surprising.
> >
> > Not only that, it's what the OCaml manual says.  From section 6.10.4
> >
> >  The constraint [module module-path = extended-module-path] adds type
> >  equations to all type components of the sub-structure denoted by
> >  [module-path], making them equivalent to the corresponding type
> >  components of the structure denoted by [extended-module-path].
> >
> > y
> >
> > --
> > Yaron Minsky
> >
> > --
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> >
>
>
>
> --
> Markus Mottl        http://www.ocaml.info        markus.mottl@gmail.com
>
>
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>

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