On Sun, 27 Apr 2008, Richard Jones wrote:

> I'm trying to add named patterns to my bitmatch syntax extension.  The
> idea would be you could write (exact syntax isn't nailed down yet):
>
>  let p = BITMATCH { a : 4; b : 4 } ;;
>
>  bitmatch bs with
>  | { p } -> (a, b)
>
>  let f a b = BITSTRING p
>
> This is analogous to micmatch's Named regular expressions feature:
>
>  http://martin.jambon.free.fr/micmatch-manual.html#htoc5
> eg:
>  RE phone = digit{3} '-' digit{4}
>
> Reading the code to micmatch, these are implemented by saving the
> camlp4 AST into a Hashtbl, so the example above would create a hash
> entry ("phone" -> abstract syntax tree of (digit{3} '-' digit{4})).
> At the point of use of the named RE, the AST is substituted.

Yes. It's a global table that ignores everything about module boundaries.


> Of course micmatch's scheme only works if the named RE appears in the
> same compilation unit as the substitution.  There is no way that I can
> see to save these named expressions across compilation units.  In
> other words this is not allowed:
>
>  --- my_regexps_lib.ml -----
>  RE phone = digit{3} '-' digit{4}
>
>  --- my_regexps_lib.mli -----
>  val phone : Micmatch.regexp
>
>  --- another file -----
>  open My_regexps_lib
>  (* ... and use 'phone' *)
>
> I think this limits the usefulness of named expressions, but at the
> same time I don't know how one would go about implementing
> cross-module named expressions.  Is it even possible?  Presumably if
> it could be done at all, we'd have to save the camlp4 AST
> representation into the output file (*.cmo).  It would be easy enough
> to marshal the AST into a string at the point of definition.  I don't
> quite see how it can be accessed & unmarshalled at the point of use
> however.
>
> Any insights here gratefully accepted!

I had some ideas on the subject:

   http://caml.inria.fr/pub/ml-archives/caml-list/2007/01/6f2e2f9db39543e92806742ddc10fa5f.en.html

Nothing clear comes out of this...


Martin

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