Hello Ker,

On Wed, 2003-10-08 at 23:22, Ker Lutyn wrote:
> [...]
> In other words, Foo sends message A to Bar. We'd like to be able to
> handle two possibilities:
> 
> (1) Foo sends message A' to Bar, who interprets it as A.
> (2) Foo sends message A to Bar, who interprets it as A'.
> 
> ...where A' is an extension of A.
> 
> To do this it would be necessary to 'extend' a type at one end [...]

This kind of concept is already present (subtyping, versioning,
relaying) in a specification language called Abstract Syntax Notation
One (ASN.1), and, at least, easily supported by one of its encoding rule
called Basic Encoding Rule (BER).

The point of ASN.1 is sharing the same ASN.1 module between the two
possibly hetereogeneous peers, also agreeing on an encoding/decoding
scheme (e.g. BER) and then each peer compile the ASN.1 module into type
definitions and codecs in his application language (obviously, you seems
interested in O'Caml). Then compile and link.

In my PhD thesis I wrote a parser and a checker for ASN.1:1988/90, but
no code generation for O'Caml. I remember that Erlang had such a
facility integrated transparently. It is worth looking this way.

For all information about ASN.1, see the excellent site of France
Telecom R&D at http://asn1.elibel.tm.fr/. By the way, these folks
develop some ASN.1 tools in O'Caml, but do not disclose their source
code in general.


Hope this helps,


Christian

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