Specifying recursive modules?

Specifying recursive modules?

[Jérémie Lumbroso]

Hello,

I'd always thought of separating specification and definition as
simply not possible in OCaml, but OCaml's reference manual (3.10,
which is, as far as I can tell, the most recent version of the
documentation) seems to contradict my assumption:

http://caml.inria.fr/pub/docs/manual-ocaml/manual021.html#htoc100

It says that recursive specifications can be written as:

  <code>
  module rec <mod-name> : <mod-type> { and ... }
  </code>

What does this mean? When I attempt to use this feature in the
toplevel, it results in an error:

  <toplevel>
  # module rec Tmp : sig
      type t = Stop | Next of Tmp.t
    end**;;**
  Syntax error
  </toplevel>

On the off-chance that OCaml might explicitly need an "and", I also
tried adding a second dummy module to the definition, but no dice ...
When I try to use this feature in a .mli/.ml file coupling, I get the
same (syntax) error. Have I misunderstood the usage? Or has this been
removed since its introduction (and the documentation not updated
accordingly)? Or is this a bug?

Jérémie

Re: [Caml-list] Specifying recursive modules?

[Martin Jambon]

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On Wed, 20 Aug 2008, Jérémie Lumbroso wrote:

> Hello,
>
> I'd always thought of separating specification and definition as
> simply not possible in OCaml, but OCaml's reference manual (3.10,
> which is, as far as I can tell, the most recent version of the
> documentation) seems to contradict my assumption:
>
> http://caml.inria.fr/pub/docs/manual-ocaml/manual021.html#htoc100
>
> It says that recursive specifications can be written as:
>
>  <code>
>  module rec <mod-name> : <mod-type> { and ... }
>  </code>
>
> What does this mean? When I attempt to use this feature in the
> toplevel, it results in an error:
>
>  <toplevel>
>  # module rec Tmp : sig
>      type t = Stop | Next of Tmp.t
>    end**;;**
>  Syntax error
>  </toplevel>

This is a signature item, i.e. goes directly in a .mli file or
in the definition of a module signature:

module type X =
sig

   module rec A :
   sig
     type t = A of B.t
   end
   and B :
   sig
     type t = B of A.t
   end

end


Martin


> On the off-chance that OCaml might explicitly need an "and", I also
> tried adding a second dummy module to the definition, but no dice ...
> When I try to use this feature in a .mli/.ml file coupling, I get the
> same (syntax) error. Have I misunderstood the usage? Or has this been
> removed since its introduction (and the documentation not updated
> accordingly)? Or is this a bug?
>
> Jérémie
>
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