[Ocaml-pxp-users] [repost] Creating a simple document with namespaces

Mon May 16 05:45:18 PDT 2005

Am Montag, den 16.05.2005, 12:44 +0100 schrieb Richard Jones:
> My last query wasn't perhaps focused enough to generate any responses.
> I'm finding the PXP API to be very hard to understand, even to do
> apparently simple things.

Well, it is validation-centric. That explains a lot of its complexity.

> I want to create and print out an XML document with namespaces.  I
> don't need validation or DTDs at this point.

Keep in mind that PXP always needs a DTD object, even if validation is
disabled.

> Here is my test program:
> 
> ----------------------------------------------------------------------
> (* Namespace test.
>  * Compile with:
>  * ocamlc -I +pcre -I +equeue -I +netstring -I +netclient -I +pxp-lex-iso88591 -I +pxp-engine str.cma unix.cma pcre.cma equeue.cma netstring.cma netclient.cma pxp_engine.cma pxp_lex_iso88591.cma pxp_lex_link_iso88591.cmo test.ml -o test
>  *)
> 
> let dtd =
>   new Pxp_dtd.dtd Pxp_types.default_config.Pxp_types.warner
>     Pxp_types.default_config.Pxp_types.encoding
> 
> (* Helper functions for creating element and data nodes. *)
> 
> let create_element, create_data =
>   let dtd =
>     let collect_warnings = Pxp_types.default_config.Pxp_types.warner in
>     let rep_encoding = Pxp_types.default_config.Pxp_types.encoding in
>     new Pxp_dtd.dtd collect_warnings rep_encoding in
> 
>   let exemplar = new Pxp_document.namespace_element_impl
>     Pxp_tree_parser.default_extension in
> 
>   let create_element name =
>     exemplar#create_element ~valcheck:false
>       dtd (Pxp_document.T_element name) []
>   in
>   let create_data data =
>     exemplar#create_data dtd data

This does not work, because [exemplar] is an element, not a data node.
You need something like

let data_exemplar = new data_impl Pxp_tree_parser.default_extension in
...
let create_data data = 
  data_exemplar # create_data dtd data

Furthermore, I recommend not to call the _methods_ create_xxx but the
_functions_ create_xxx_node. The functions have a much nicer interface. 
You need a so-called specification to use them, but this is simply a
container for all your exemplars, e.g.

let spec = make_spec_from_alist 
              ~data_exemplar:...
              ~default_element_exemplar:...
              () in
...
create_element_node spec dtd name []
...
create_data_node spec dtd data

You can also use Pxp_tree_parser.default_spec for a reasonable default.

Btw, you should always use the same DTD object for all your nodes.

>   in
>   create_element, create_data
> 
> let () =
>   let env = create_element "Envelope" in
>   let body = create_element "Body" in
>   env#set_nodes [body];
>   let call = create_element "hello" in
>   body#set_nodes [call];
> 
>   let params =
>     let id = create_element "id" in
>     id#set_nodes [create_data "1"];
>     let p1 = create_element "p1" in
>     p1#set_nodes [create_data "2"];
>     [ id; p1 ] in
> 
>   call#set_nodes params;
> 
>   let buf = Buffer.create 1024 in
>   env#write (`Out_buffer buf) `Enc_utf8;
>   let data = Buffer.contents buf in
> 
>   print_endline ("XML request = \n" ^ data);
> ----------------------------------------------------------------------

Another hint for people who want to create documents by programs: The
PXP preprocessor simplifies this a lot. For example, this program can be
shortened to:

let spec = Pxp_tree_parser.default_spec;;
let dtd = Pxp_dtd.create_dtd `Enc_iso88591;;

let env =
  <:pxp_tree<
    <Envelope>
      [ <body>
          [ <hello>
              [ <id>[ "1" ] <p1>[ "2" ] ]
          ]
      ]
  >>

The preprocessor supports the dynamic creation of nodes, too (with
non-constant names and contents). See the description in
doc/PREPROCESSOR in the PXP tarball.

Gerd
-- 
------------------------------------------------------------
Gerd Stolpmann * Viktoriastr. 45 * 64293 Darmstadt * Germany 
gerd at gerd-stolpmann.de          http://www.gerd-stolpmann.de
------------------------------------------------------------