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Intro_getting_started



In the following sections we'll explain how to solve a basic task in PXP, namely to parse a file and to represent it in memory, followed by paragraphs on variations of this task, because not everybody will be happy with the basic solution.

Parse a file and represent it as tree

The basic piece of code to parse "filename.xml" is:

let config = Pxp_types.default_config
let spec = Pxp_tree_parser.default_spec
let source = Pxp_types.from_file "filename.xml"
let doc = Pxp_tree_parser.parse_document_entity config source spec

As you can see, a some defaults are loaded (Pxp_types.default_config, and Pxp_tree_parser.default_spec). These defaults have these effects (as far as being important for an introduction):

  • The parsed document is represented in ISO-8859-1. The file can be encoded differently, however, and if so, it is automatically recoded to ISO-8859-1.
  • The generated tree only has nodes for elements and character data sections, but not for comments, and processing instructions.
  • The top-most node of the tree, doc#root, is the top-most element.
  • No namespace processing is performed.
XML does not know the concept of file names. All files (or other resources) are named by so-called ID's. Although we can pass here a file name to from_file, it is immediately converted into a SYSTEM ID which is essentially a URL of the form file:///dir1/.../dirN/filename.xml. This ID can be processed - especially it is now clear how to treat releative SYSTEM ID's that occur in the parsed document. For instance, if another file is included by "filename.xml", and the SYSTEM ID is "parts/part1.xml", the usual rules for resolving relative URL's say that the effective file to read is file:///dir1/.../dirN/parts/part1.xml. Relative SYSTEM ID's are resolved relative to the URL of the file where the entity reference occurs that leads to the inclusion of the other file (this is comparable to how hyperlinks in HTML are treated).

Note that we make here some assumptions about the file system of the computer. Pxp_reader.make_file_url has to deal with character encodings of file names. It assumes UTF-8 by default. By passing arguments to this function, other assumptions about the encoding of file names can be made. Unfortunately, there is no portable way of determining the character encoding the system uses for file names (see the hyperlinks at the end of this section).

The returned doc object is of type Pxp_document.document. This type is used for all regular documents that exist independently. The root of the node tree is returned by doc#root which is a . See Intro_trees for more about the tree representation.

The call Pxp_tree_parser.parse_document_entity does not only parse, but it also validates the document. This works only if there is a DTD, and the document conforms to the DTD. There is a weaker criterion for formal correctness called well-formedness. See below how to only the check for well-formedness while parsing without doing the whole validation.

Links about the file name encoding problem:

Compiling and linking

It is strongly recommended to compile and link with the help of ocamlfind. For (byte) compiling use one of

  • ocamlfind ocamlc -package pxp-engine -c file.ml
  • ocamlfind ocamlc -package pxp -c file.ml
The package pxp-engine refers to the core library while pxp refers to an extended version including the various lexers. For compiling, there is no big difference between the two because the lexers are usually not directly invoked. However, at link time you need these lexers. You can choose between using the pre-defined package pxp and a manually selected combination of pxp-engine with some lexer packages. So for linking e.g. use one of:

  • ocamlfind ocamlc -package pxp -linkpkg -o executable ... to get the standard selection of lexers
  • ocamlfind ocamlc -package pxp-engine,pxp-lex-iso88591,pxp-ulex-utf8 -linkpkg -o executable ... to get lexers for ISO-8859-1 and UTF-8
There is a special lexer for every choice of encoding for the internal representation of XML. If you e.g. choose to represent the document as UTF-8 there must be a lexer capable of handling UTF-8. The package pxp includes a standard set of lexers, including UTF-8 and many encodings of the ISO-8859 series. For more about encodings, see below Encodings.

Variations

Catching and printing exceptions

The relevant exceptions are defined in Pxp_types. You can catch these exceptions (as thrown by the parser) as in:

try ...
with
  | Pxp_types.Validation_error _
  | Pxp_types.WF_error _
  | Pxp_types.Namespace_error _
  | Pxp_types.Error _
  | Pxp_types.At(_,_) as error ->
      print_endline ("PXP error " ^ Pxp_types.string_of_exn error)
 

There are more exceptions, but these are usually caught within PXP and converted to one of the mentioned exceptions.

Printing trees in the O'Caml toploop

There are toploop printers for nodes and documents. They are automatically activated when the findlib directive #require "pxp" is used to load PXP into the toploop. Alternatively, one can also do

#install_printer Pxp_document.print_node;;
#install_printer Pxp_document.print_doc;;

For example, the tree <x><y>foo</y></x> would be shown as:

  # tree;;
  _ : ('Pxp_document.node Pxp_document.extension as 'a) Pxp_document.node =
  * T_element "x"
    * T_element "y"
      * T_data "foo"

Parsing in well-formedness mode

In well-formedness mode many checks are not performed regarding the formal integrity of the document. Note that the terms "valid" and "well-formed" are rigidly defined in the XML standard, and that PXP strictly tries to conform to the standard. Especially note that the DOCTYPE clause is not rejected in well-formedness mode and that the declarations are parsed although interpreted differently.

In order to call the parser in well-formedness mode, call one of the "wf" functions, e.g.

let doc = Pxp_tree_parser.parse_wfdocument_entity config source spec

Details. Even in well-formedness mode there is a DTD object. The DTD object is, however, differently treated:

  • All declarations are parsed. However, the declarations of elements, attributes, and notations are not added to the DTD object. The declarations of entities are fully processed. Processing instructions are also not handled in any way differently than when validation is enabled. Note that all this means that you can get syntax errors about ill-formed declarations in well-formedness mode, although the declarations are not further processed.
  • When the parser checks the integrity of elements, attributes or notations it finds in the XML text to parse, it accepts that there is no declaration in the DTD object. This is controlled by a special DTD mode called arbitrary_allowed (see Pxp_dtd.dtd.allow_arbitrary). If enabled as done in well-formedness mode, the DTD reacts specially when a declaration is missing so that the parser knows it has to accept that. Note that, if one added a declaration programmatically to the DTD object, the DTD would find it, and would actually validate against it. Effectively, validation is not disabled in well-formedness mode, only the constraints imposed by the DTD object on the document are weaker. There is in fact a way to add declarations in well-formedness mode to get partly the effects of validation: This is called The mixed mode.
  • It is not checked whether the top-most element is the one declared in the DOCTYPE clause (if that clause exists).
When processing well-formed documents one should be more careful because the parser has not done any checks on the structure of the node tree.

Validating well-formed trees

It is possible to validate a tree later that was originally only parsed in well-formedness mode.

Of course, there is one obvious difficulty. As mentioned in the previous section, the DTD object is incompletely built (declarations of elements, attributes, and notations are ignored), so the DTD object is not suitable for validating the document against it. For validation, however, a complete DTD object is required. The solution is to replace the DTD object by a different one. As the DTD object is referenced from all nodes of the tree, and thus intricately connected with it, the only way to do so is to copy the entire tree. The function Pxp_marshal.relocate_document can be used for this type of copy operation.

We assume here that we can get the replacement DTD from an external file, "file.dtd", and that another constraint is that the root element must be start (as if we had <!DOCTYPE start SYSTEM "file.dtd">). Also doc is the parsed "filename.xml" file as retrieved by

let config = Pxp_types.default_config
let spec = Pxp_tree_parser.default_spec
let source = Pxp_types.from_file "filename.xml"
let doc = Pxp_tree_parser.parse_wfdocument_entity config source spec

Now the validation against a different DTD is done by:

let rdtd_source = Pxp_types.from_file "file.dtd"
let rdtd = Pxp_dtd_parser.parse_dtd_entity config rdtd_source
let () = rdtd # set_root "start"
let vroot = Pxp_marshal.relocate_document doc#root rdtd spec
let () = Pxp_document.validate vroot
let vdoc = new Pxp_document.document config.warner config.encoding
let () = vdoc#init_root vroot doc#raw_root_name

The vdoc document has now the same contents as doc but points to a different DTD, namely rdtd. Also, the validation checks have been performed. A few more comments:

  • We use here the same config for parsing the original document doc and the replacement DTD rdtd. This is not strictly required. However, the encoding of the in-memory representation must be identical (i.e. config.encoding).
  • When you omit rdtd#set_root, any root element is allowed.
  • The entity definitions of the old DTD object are lost.
  • It is of course possible to modify doc before doing the validation, or to validate a doc that is not the result of a parser call but programmatically created.

Encodings

In PXP, the encoding of the parsed text (the external encoding), and the encoding of the in-memory representation can be distinct. For processing external encodings PXP relies on Ocamlnet. The external encoding is usually indicated in the XML declaration at the beginning of the text, e.g.

<?xml version="1.0" encoding="ISO-8859-2"?>
...

There is also an autorecognition of the external encoding that works for UTF-8 and UTF-16.

It is generally possible to override the external encoding (e.g. because the file has already been converted but the XML declaration was not changed at the same time). Some of the from_* sources allow it to override the encoding directly, e.g. by setting the fixenc argument when calling Pxp_types.from_channel. Note that Pxp_types.from_file does not have this option as this source allows it to read any file. Overriding encodings is, however, only interesting for certain files. A workaround is to combine from_file with a catalog of ID's, and to override the encodings for certain files there. (Catalogs also allow to override external encodings. See below, Specifying sources for examples using catalogs.)

As mentioned, the encoding of the in-memory representation can be distinct from the external encoding. It is required that every character in the document can be represented in the representation encoding. Because of this, the chosen encoding should be a superset of all external encodings that may occur. If you choose UTF-8 for the representation every character can be represented anyway.

You set the representation encoding in the config record, e.g.

let config =
  { Pxp_types.default_config
      with encoding = `Enc_utf8
  }

It is strictly required that only a single encoding is used in a document (and PXP also checks that).

The available encodings for the in-memory representation are a subset of the encodings supported by Ocamlnet. Effectively, UTF-8 is supported and a number of 8-bit encodings as far as they are ASCII- compatible (i.e. extensions of 7 bit ASCII).

For every representation encoding PXP needs a different lexer. PXP already comes with a set of lexers for the supported encodings. However, at link time the user program must ensure that the lexer is linked into the executable. The lexers are available as separate findlib packages:

  • pxp-ulex-utf8: This is the standard lexer for UTF-8
  • pxp-wlex-utf8: This is the old, wlex-based lexer for UTF-8. It is not built when ulex is available.
  • pxp-lex-utf8: This is the old, ocamllex-based lexer for UTF-8. It is slightly faster than pxp-ulex-utf8, but consumes a lot more memory.
  • pxp-lex-*: These are lexers for various 8 bit character sets
For the link command, see above: Compiling and linking.

Event parser (push/pull parsing)

It is sometimes not desirable to represent the parsed XML data as tree. An important reason is that the amount of data would exceed the available memory resources. Another reason may be to combine XML parsing with a custom grammar. In order to support this, PXP can be called as event parser. Basically, PXP emits events (tokens) while parsing certain syntax elements, and the caller of PXP processes these events. This mode can only be used together with well-formedness mode - for validation the tree representation is a prerequisite.

Here we show how to parse "filename.xml" with a pull parser:

let config = Pxp_types.default_config
let source = Pxp_types.from_file "filename.xml"
let entmng = Pxp_ev_parser.create_entity_manager config source
let entry = `Entry_document []
let next = Pxp_ev_parser.create_pull_parser config entry entmng

Now, one can call next() repeatedly to get one event after the other. The events have type Pxp_types.event option.

More about event parsing can be found in Intro_events.

Low-profile trees

When the tree classes in Pxp_document are too much overhead, it is easily possible to define a specially crafted tree data type, and to transform the event-parsed document into such trees. For example, consider this cute definition:

type tree =
  | Element of string * (string * string) list * tree list
  | Data of string

A tree node is either an Element(name,atts,children) or a Data(text) node. Now we event-parse the XML file:

let config = Pxp_types.default_config
let source = Pxp_types.from_file "filename.xml"
let entmng = Pxp_ev_parser.create_entity_manager config source
let entry = `Entry_document []
let next = Pxp_ev_parser.create_pull_parser config entry entmng

Finally, here is a function build_tree that calls the next function to build our low-profile tree:

let rec build_tree() =
  match next() with
    | Some (E_start_tag(name,atts,_,_)) ->
        let children = build_children [] in
        let tree = Element(name,atts,children) in
        skip_rest();
        tree
    | Some (E_error e) ->
        raise e
    | Some _ ->
        build_tree()
    | None ->
        assert false     

and build_node() =
  match next() with
    | Some (E_char_data data) ->
        Some(Data data)
    | Some (E_start_tag(name,atts,_,_)) ->
        let children = build_children [] in
        Some(Element(name,atts,children))
    | Some (E_end_tag(_,_)) ->
        None
    | Some (E_error e) ->
        raise e
    | Some _ ->
        build_node()
    | None ->
        assert false

and build_children l =
  match build_node() with
    | Some n -> build_children (n :: l)
    | None -> List.rev l
    
and skip_rest() =
  match next() with
    | Some E_end_of_stream ->
        ()
    | Some (E_error e) ->
        raise e
    | Some _ ->
        skip_rest()
    | None ->
        assert false

Of course, this all is only reasonable for the well-forermedness mode, as PXP's validation routines depend on the built-in tree representation of Pxp_document.

Choosing the node types to represent

By default, PXP only represents element and data nodes (both in the normal tree representation and in the event stream). It is possible to enable more node types:

  • Comment nodes are created for XML comments. In the tree representation, the node type T_comment is used for them. In the event stream, the event type E_comment is used.
  • Processing instruction nodes are created for processing instructions (PI's) occuring in the normal XML flow (i.e. outside of DTD's). In the tree representation, the T_pinstr node type is used, and in the event stream, the event type E_pinstr is used.
  • The super root node can be put at the top of the tree, so that the top-most element is a child of this node. This can be reasonable especially when comment nodes and PI nodes are also enabled, because when these nodes surround the top-most element they also become children of the super root node. In the tree representation, the T_super_root node type is used, and in the event stream, the event type E_start_super marks the beginning of this node, and E_end_super marks the end of this node.
These node types are enabled in the config record, e.g.

let config =
  { Pxp_types.default_config
      with enable_comment_nodes = true;
           enable_pinstr_nodes = true;
           enable_super_root_node = true 
  }

Note that the "super root node" is sometimes called "root node" in various XML standards giving semantical model of XML. For PXP the name "super root node" is preferred because this node type is not obligatory, and the top-most element node can also be considered as root of the tree.

Controlling whitespace

Depending on the mode, PXP applies some automatic whitespace rules. The user can call functions to reduce whitespace even more.

In validating mode, there are whitespace rules for data nodes and for attributes (the latter below). In this mode it is possible that an element x is declared such that a regular expression describes the permitted children. For instance,

 <!ELEMENT x (y,z)> 

is such a declaration, meaning that x may only have y and z as children, exactly in this order, as in

 <x><y>why</<y><z>zet</z></x> 

XML, however, allows that whitespace is added to make such terms more readable, as in

 
<x>
  <y>why</<y>
  <z>zet</z>
</x> 

The additional whitespace should not, however, appear as children of node x, because it is considered as a purely notational improvement without impact on semantics. By default, PXP does not create data nodes for such notational whitespace. It is possible to disable the suppression of this type of whitespace by setting drop_ignorable_whitespace to false:

  let config =
    { Pxp_types.default_config 
        with drop_ignorable_whitespace = false
    }

In well-formedness mode, there is no such feature because element declarations are ignored.

Note that although in event mode the parser is restricted to well-formedness parsing, it is still possible to get the effect of drop_ignorable_whitespace. See Pxp_event.drop_ignorable_whitespace_filter for how to selectively enable this validation feature.

The other whitespace rules apply to attributes. In all modes line breaks in attribute values are converted to spaces. That means a1 and a2 have identical values:

<x a1="1 2" a2="1\n2" a3="1&#10;2"/>

It is possible to suppress this conversion by using &#10; as line separator, as in a3, which truly includes a line-feed character.

In validating mode only there are more rules because attributes are declared. If the attribute is declared with a list value (IDREFS, ENTITIES, or NMTOKENS), any amount of whitespace can be used to separate the list elements. PXP returns the value as Valuelist l where l is an O'Caml list of strings.

If the tree representation is chosen, the function Pxp_document.strip_whitespace can be called to reduce the amount of whitespace in data nodes.

Checking the ID consistency and looking up nodes by ID

In XML it is possible to identify elements by giving them an ID attribute. The requires a DTD, and could be done with declarations like

  <!ATTLIST x id ID #REQUIRED>

meaning that element x has a mandatory attribute id with the special ID property: Every node must have a unique id value.

In the same context, it is possible to declare attributes as references to other nodes, expressed by denoting the id of the other node:

  <!ATTLIST y r IDREF #IMPLIED>

Here, the (optional) attribute r of y is a reference to another node. It is only allowed to put identifiers into such attributes that also occur in the ID of another node.

By default, PXP does neither check the uniqueness of ID-declared attributes nor the existence of the nodes referenced by IDREF-declared attributes. In tree mode, it is possible to enable that, however.

For that purpose, one has to create an Pxp_tree_parser.index. If passed to the parser function, the parser adds the ID-values of all nodes to the index, and checks whether every ID value is unique. Additionally, when one enables the idref_pass the parser also checks whether IDREF attributes only point to existing nodes. The code:

let config = { Pxp_types.default_config with idref_pass = true }
let spec = Pxp_tree_parser.default_spec
let source = Pxp_types.from_file "filename.xml"
let hash_index = new Pxp_tree_parser.hash_index
let id_index = (hash_index :> _ Pxp_tree_parser.hash_index)
let doc = Pxp_tree_parser.parse_document_entity ~id_index config source spec

The difference between hash_index and id_index is that the former object has one additional method index returning the whole index.

The id_index may also be useful after the document has been parsed. The code processing the parsed documennt can take advantage of it by looking up nodes in it. For example, to find the node identified by "foo", one can call

 id_index # find "foo" 

which either returns this node, or raises Not_found.

Note that the id_index is not automatically updated when the parsed tree is modified.

Finding nodes by element names

As we are at it: PXP does not maintain indexes of any kind. Unlike in other tree representations, there is no index of elements that would help one to quickly find elements by their names. The reason for this omission is that such indexes need to be updated when the tree is modified, and these updates can be quite expensive operations.

The ID index explained in the last section is not automatically updated, and it has only been added to comply fully to the XML standard (which demands ID checking).

Nevertheless, one can easily define indexes of one own (and for the advanced programmer it might be an interesting task to develop an extension module to PXP that generically solves this problem). For instance, here is an index of elements:

  let index = Hashtbl.create 50

  Pxp_document.iter_tree
    ~pre:(fun node ->
             match node with
               | T_element name -> Hashtbl.add index name node
               | _ -> ()
         )
    doc#root

Now, Hashtbl.find can be used to get the last occurrence, and Hashtbl.find_all to get all occurrences.

If it is not worth-while to build an index, one can also call the functions Pxp_document.find_element and Pxp_document.find_all_elements, but these functions rely on linear searching.

Specifying sources

The Pxp_types.source says from where the data to parse comes. The task of the source is more complex as it looks at the first glance, as it not only says from where the initially parsed entity comes, but also from where further entities can be loaded that are referenced and included by the first one.

The mentioned function Pxp_types.from_file allows that all files can be opened as entities, and maps the SYSTEM identifiers to file names. It is very powerful.

There are three more from_* functions:

These three variants differ from from_file in so far as only one entity can be parsed at all (unless one passes alternate resolvers to them). This means it is not possible that the initially parsed entity includes data from another entity. Example code:

 let source = Pxp_types.from_string "<?xml version='1.0'?><foo/>" 

So the source mechanism has these limitations:

  • The Pxp_types.from_file function allows one to read from all files by using SYSTEM URL's of the form file:///path. It is not possible to restrict the file access in any way. There is no support for PUBLIC identifiers.
  • The other functions like Pxp_types.from_string allow one to parse data coming from everywhere, and it is not possible to access any files (as it is not possible to open any further external entity).
There is the Pxp_reader module with a very powerful abstraction called Pxp_reader.resolver. There are resolvers for files, for alternate resources like data channels, and there is the possibility of building more complex resolvers by composing simpler ones.

Please see Pxp_reader and Intro_resolution for deeper explanations. Here are the most important recipes to use this advanced mechanism:

Read from files, and define a catalog of exceptions:

let catalog =
 new Pxp_reader.lookup_id_as_file
  [ System("http://foo.org/our.dtd"), "/usr/share/foo.org/out.dtd";
    Public("-//W3C//DTD XHTML 1.0 Strict//EN",""), "/home/stuff/xhtml_strict.dtd"
  ]
let source = Pxp_types.from_file ~alt:[catalog] "filename.xml"

This allows one to open all local files using the file:///path URL's, but also maps the SYSTEM ID "http://foo.org/our.dtd" and the PUBLIC ID "-//W3C//DTD XHTML 1.0 Strict//EN" to local files.

There is also Pxp_reader.lookup_id_as_string mapping to strings.

Read from files, but restrict access, and map URL's

let resolver =
  new Pxp_reader.rewrite_system_id
    [ "http://foo.org/""file:///usr/share/foo.org";
      "file:///""file:///home/stuff/localxml"
    ]
    (new Pxp_reader.resolve_as_file())
let file_url = Pxp_reader.make_file_url "filename.xml"
let source = ExtID(System((Neturl.string_of_url file_url), resolver)

This allows one to open entities from the whole http://foo.org/ hierarchy, but the data is not downloaded by HTTP, but instead assumed to reside in the local directory hierarchy /usr/share/foo.org. Also, the whole file:/// hierarchy is re-rooted to /home/stuff/localxml. As the URL's are normalized before any access is tried, this scheme provides access protection to other parts of the file system (i.e. one cannot escape from the new root by "..").

In order to combine with a catalog as defined above, use

let resolver =
  new Pxp_reader.combine
    [ catalog;
      new Pxp_reader.rewrite_system_id ...
    ]

Virtual entity hierarchy

Given we have the three identifiers

  • http://virtual.com/f1.xml
  • http://virtual.com/f2.xml
  • http://virtual.com/f3.xml
and these identifiers include each other by using relative SYSTEM ID's, and we have O'Caml strings f1_xml, f2_xml, and f3_xml with the contents, we want to make the virtual.com hierarchy available while parsing from a string s.

let resolver =
  new Pxp_reader.norm_system_id
    (new Pxp_reader.lookup_id_as_string
       [ "http://virtual.com/f1.xml"; f1_xml;
         "http://virtual.com/f2.xml"; f2_xml;
         "http://virtual.com/f3.xml"; f3_xml
       ]
    )
let source = Pxp_types.from_string ~alt:[resolver] s

The trick is Pxp_reader.norm_system_id. This class makes it possible that these three enumerated documents can refer to each other by relative URL. Without the SYSTEM ID normalization, these documents can only be opened when exactly the URL is referenced that is also mentioned in the catalog.

Embedding large constant XML in source code

Sometimes one needs to embed XML files into source code. For small files this is no problem at all, just define them as string literals

let s = "<?xml?> ..."

and parse the strings on demand, using the Pxp_types.from_string source. For larger files, the disadvantage of this approach is that the whole document has to be parsed again for every run of the program. There is an efficient way of avoiding that.

The Pxp_codewriter module provides a function Pxp_codewriter.write_document that takes an already parsed XML tree and writes O'Caml code as output that will create the tree again when executed. This can be used as follows:

  • Write a helper application generate that parses the XML file with the required configuration options and that outputs the O'Caml code for this file using Pxp_codewriter
  • In the real program that needs to operate on the XML document reconstruct the document by running the generated code. Use the same configuration options as in generate
There is also Pxp_marshal for marshalling XML trees. The codewriter module uses it.

Using the preprocessor to create XML trees

One way of creating XML trees programmatically is to call the create_* functions in Pxp_document, e.g. Pxp_document.create_element_node. However, this looks ugly, e.g. for creating <x><y>foo</y></x> one ends up with

let tree =
  Pxp_document.create_element_node spec dtd "x" []
let y =
  Pxp_document.create_element_node spec dtd "y" []
let data =
  Pxp_document.create_data_node spec dtd "foo"
# append_node data;
tree # append_node y

It is easier to use the PXP preprocessor, a camlp4 extension of the O'Caml syntax. It simplifies the above code to (line breaks are optional):

  let tree =
    <:pxp_tree<
      <x>
        <y>
          "foo"
    >>

For more about the preprocessor, see Intro_preprocessor.

Namespaces

PXP support namespaces, but

  • this has to be enabled explicitly, and
  • the way of processing namespaces is different from what parsers do that output DOM trees
How to enable namespace processing. Depending on the mode different things have to be done. In any case a namespace manager is required, and it has to be made available to PXP in the config record:

let m = Pxp_dtd.create_namespace_manager()

let config =
  { Pxp_types.default_config
      with enable_namespace_processing = Some m
  }

In event mode, this is already enough. In tree mode, you also need to direct PXP that it uses the special namespace-enabled node classes:

let spec = Pxp_tree_parser.default_namespace_spec

Of course, PXP can also parse namespace directives when namespace processing is off. However, all the namespace-specific node methods do not work like Pxp_document.node.namespace_uri.

Prefix normalization. PXP implements a technique called prefix normalization when processing namespaces. The namespace prefix is the part before the colon in element and attribute names like prefix:localname. The prefix is changed in the document so every namespace is uniquely identified by a prefix. Note that this means that the elements and attributes may be renamed by the parser.

For details how the prefix normalization works, see Intro_namespaces. Namespace processing can also be combined with event-oriented parsing, see Events and namespaces.

Specifying which classes implement nodes - the mysterious spec parameter

For the tree representation PXP defines a set of classes implementing the various node types. These classes, such as element_impl, are all defined in Pxp_document.

It is now possible to instruct PXP to use different classes. In the last section we have already seen an example of this, because for namespace-enabled parsing a different set of node classes is used:

let spec = Pxp_tree_parser.default_namespace_spec

The mysterious spec parameter controls which class it uses for which node type. In the source code of Pxp_tree_parser, we find

let default_spec =
  make_spec_from_mapping
    ~super_root_exemplar:      (new super_root_impl default_extension)
    ~comment_exemplar:         (new comment_impl default_extension)
    ~default_pinstr_exemplar:  (new pinstr_impl default_extension)
    ~data_exemplar:            (new data_impl default_extension)
    ~default_element_exemplar: (new element_impl default_extension)
    ~element_mapping:          (Hashtbl.create 1)
    ()


let default_namespace_spec =
  make_spec_from_mapping
    ~super_root_exemplar:      (new super_root_impl default_extension)
    ~comment_exemplar:         (new comment_impl default_extension)
    ~default_pinstr_exemplar:  (new pinstr_impl default_extension)
    ~data_exemplar:            (new data_impl default_extension)
    ~default_element_exemplar: (new namespace_element_impl default_extension)
    ~element_mapping:          (Hashtbl.create 1)
    ()

The function Pxp_document.make_spec_from_mapping creates a spec from a set of constructors. In the namespace version of spec, the only difference is that a special implementation for element nodes is used.

One can also use this mechanism to let the parser create trees made of customized classes. Note, however, that it is not possible to simply create new classes by inherting from a predefined classes and then adding new methods. The problem is that the typing constraints of PXP do not allow that users add methods directly to node classes. However, there is a special extension mechanism built-in, and one can use it to add new methods indirectly to nodes. This means these methods do not appear directly in the class type of nodes, but in the class type of the node extension. See Intro_extensions for more about this.

What PXP cannot do for you

Although PXP has a long list of features, there are some types of parsing XML it is not designed for:

  • It is not possible to leave entities unresolved in the text. Whenever there is an &entity; or %entity; PXP replaces it with the definition of that entity. It is an error if the entity turns out to be undefined, and parsing is stopped with an exception.
  • It is not possible to figure out notational details of the XML text, such as where CDATA sections are used
  • It is not possible to parse a syntactically wrong document as much as possible, and to return the parseable parts. PXP either parses the document completely, or it fails completely.
Effectively, this makes it hard to use PXP for XML editing, but otherwise does not limit its uses.


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