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README - PXP, the XML parser for O'Caml
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Abstract
==============================================================================
PXP is a validating parser for XML-1.0 which has been written entirely in
Objective Caml.
PXP means "Polymorphic XML parser" and emphasizes its most useful property:
that the API is polymorphic and can be configured such that different objects
are used to store different types of elements.
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Download
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You can download PXP as gzip'ed tarball [1]. The parser needs the Netstring [2]
package (0.9.3). Note that PXP requires O'Caml 3.00.
Information about the latest development version is available here [3].
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User's Manual
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The manual is included in the distribution both as Postscript document and
bunch of HTML files. An online version can be found here [4].
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Author, Credits, Copying
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PXP has been written by Gerd Stolpmann [5]; it contains contributions by
Claudio Sacerdoti Coen. You may copy it as you like, you may use it even for
commercial purposes as long as the license conditions are respected, see the
file LICENSE coming with the distribution. It allows almost everything.
Thanks also to Alain Frisch and Haruo Hosoya for discussions and bug reports.
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Description
==============================================================================
PXP is a validating XML parser for O'Caml [6]. It strictly complies to the
XML-1.0 [7] standard.
The parser is simple to call, usually only one statement (function call) is
sufficient to parse an XML document and to represent it as object tree.
Once the document is parsed, it can be accessed using a class interface. The
interface allows arbitrary access including transformations. One of the
features of the document representation is its polymorphic nature; it is simple
to add custom methods to the document classes. Furthermore, the parser can be
configured such that different XML elements are represented by objects created
from different classes. This is a very powerful feature, because it simplifies
the structure of programs processing XML documents.
Note that the class interface does not comply to the DOM standard. It was not a
development goal to realize a standard API (industrial developers can this much
better than I); however, the API is powerful enough to be considered as
equivalent with DOM. More important, the interface is compatible with the XML
information model required by many XML-related standards.
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Detailed feature list
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- The XML instance is validated against the DTD; any violation of a validation
constraint leads to the rejection of the instance. The validator has been
carefully implemented, and conforms strictly to the standard. If needed, it
is also possible to run the parser in a well-formedness mode.
- If possible, the validator applies a deterministic finite automaton to
validate the content models. This ensures that validation can always be
performed in linear time. However, in the case that the content models are
not deterministic, the parser uses a backtracking algorithm which can be
much slower. - It is also possible to reject non-deterministic content
models.
- In particular, the validator also checks the complicated rules whether
parentheses are properly nested with respect to entities, and whether the
standalone declaration is satisfied. On demand, it is checked whether the
IDREF attributes only refer to existing nodes.
- Entity references are automatically resolved while the XML text is being
scanned. It is not possible to recognize in the object tree where a
referenced entity begins or ends; the object tree only represents the
logical structure.
- External entities are loaded using a configurable resolver infrastructure.
It is possible to connect the parser with an arbitrary XML source.
- The parser can read XML text encoded in a variety of character sets.
Independent of this, it is possible to choose the encoding of the internal
representation of the tree nodes; the parser automatically converts the
input text to this encoding. Currently, the parser supports UTF-8 and
ISO-8859-1 as internal encodings.
- The interface of the parser has been designed such that it is best
integrated into the language O'Caml. The first goal was simplicity of usage
which is achieved by many convenience methods and functions, and by allowing
the user to select which parts of the XML text are actually represented in
the tree. For example, it is possible to store processing instructions as
tree nodes, but the parser can also be configured such that these
instructions are put into hashtables. The information model is compatible
with the requirements of XML-related standards such as XPath.
- In particular, the node tree can optionally contain or leave out processing
instructions and comments. It is also possible to generate a "super root"
object which is the parent of the root element. The attributes of elements
are normally not stored as nodes, but it is possible to get them wrapped
into nodes.
- The powerful type system of O'Caml makes it possible that the parser
supports polymorphism based on the element types, i.e. it can be configured
that the parser selects different classes to represent different element
types. Note that no generator is needed for this feature.
- There is also an interface for DTDs; you can parse and access sequences of
declarations. The declarations are fully represented as recursive O'Caml
values.
- Since PXP 1.1, the parser supports namespaces. This has been implemented
using a technique called "prefix normalization", i.e. while parsing the
namespace prefixes are changed (in a configurable way) such that they become
unique in the whole document (or document domain). This is again a solution
that is different from other parsers, but it allows a very convenient style
of processing namespaces while sticking strictly to the XML standard.
Another advantage of this solution is that DTDs can refer to namespaces in a
transparent way, i.e. it is possible to validate a document against a DTD
that uses different namespace prefixes for the same namespaces.
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Code examples
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This distribution contains several examples:
- validate: simply parses a document and prints all error messages
- readme: Defines a DTD for simple "README"-like documents, and offers
conversion to HTML and text files [8].
- xmlforms: This is already a sophisticated application that uses XML as style
sheet language and data storage format. It shows how a Tk user interface can
be configured by an XML style, and how data records can be stored using XML.
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Restrictions and missing features
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The following restrictions apply that are not violations of the standard:
- The attribute "xml:lang" is not supported specially. (The application can
do this.)
- It is currently not possible to check for interoperatibility with SGML.
The following features are also missing:
- There is no support for XPATH or XSLT.
However, I hope that these features will be implemented soon, either by myself
or by contributors (who are invited to do so).
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Release notes
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The Release Notes [9] describe problems and solutions that are detected shortly
before or after the release.
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Recent Changes
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- Changed in 1.1.6:
Updates for O'Caml 3.07
- Changed in 1.1.5:
A packaging error in pxp-wlex has been corrected. (This is the only change.)
- Changed in 1.1.4:
This is a bigger bugfix release that addresses the following problems:
The parser does no longer show very bad performance when large data nodes
without linefeeds are parsed.
Another performance problem with bigger DTDs has been solved, too.
Especially, the XHTML DTD can now be parsed quite quickly.
The interface Pxp_dtd.Entity has been extended, it is now possible to access
more properties of entities than before.
Pxp_marshal has been revised. It is now possible to recode the input or
output stream on the fly in order to change the character encoding.
Furthermore, the functions relocate_subtree and relocate_document allows one
to marshal an XML tree or a document, and to read the marshaled data
immediately to create a copy of the original structure.
Some errors have been fixed in from_file. Especially, this function will no
longer raise the exception Malformed_URL if the current working directory
happens to be "/".
Pxp_document.strip_whitespace implements xml:space now correctly. In
previous versions, xml:space='default' was ignored when it occurred inside
an element with xml:space='preserve'. Now the inner xml:space='default'
overrides the outer xml:space='preserve' as defined in the XML standard.
- Changed in 1.1.3:
This release fixes a single problem ocurring when PXP is compiled with
installed netstring-0.10. (There is not any problem with netstring-0.91.)
- Changed in 1.1.2:
Improved write method for whole documents. It can now also output a
reference to an external DTD.
PXP can be compiled with O'Caml 3.04.
- Changed in 1.1.1:
Minor changes for O'Caml 3.03-alpha. The interfaces have not been modified.
- Changed in 1.1:
The parser supports now namespaces.
Extended/updated Pxp_document interface. There is now a separate class for
every node type. It is now clear which node methods validate and which do
not validate. The node tree can now be simpler modified (insert/delete).
It is now possible to start in well-formedness mode and validate the XML
tree later (as a whole, or partially).
New functions for tree normalization, and whitespace stripping.
The implementation of Pxp_document has been updated, too. There are now
many virtual classes, one class for one task. The attribute representation
has been improved. The overall size of the document tree has been reduced.
The parser is better in counting lines. The option errors_with_line_numbers
could be removed because the parser is now fast enough that it does make
sense to always count lines.
There are now string pools that can save memory in some situations.
New module Pxp_marshal allows marshalling of XML trees over channels
(faster than writing the tree and reparsing it).
For the most important entity functions there is an interface
Pxp_document.Entity.
Although there are many extensions, the parser has been speeded up.
The parser has been divided up into several packages, and the directory
structure of the distribution has been cleaned up.
It is possible to chose among several lexical analyzers. One of them bases
on Alain Frisch's wlex patch, which reduces the size of executables if a
UTF-8 parser is needed.
The parser works under Cygwin.
Of course several bugfixes. Note that most bugs have been added in the
development cycle between 1.0 and 1.1; only very few problems have been
detected in the 1.0 release. I hope that 1.1 has similar quality.
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[1] see http://www.ocaml-programming.de/packages/pxp-1.1.5.tar.gz
[2] see http://www.ocaml-programming.de/packages/documentation/netstring
[3]
see http://www.ocaml-programming.de/packages/documentation/pxp/index_dev.html
[4] see http://www.ocaml-programming.de/packages/documentation/pxp/manual
[5] see mailto:gerd@gerd-stolpmann.de
[6] see http://caml.inria.fr/
[7] see http://www.w3.org/TR/1998/REC-xml-19980210.html
[8] This particular document is an example of this DTD!
[9]
see http://www.ocaml-programming.de/packages/documentation/pxp/index_rel.html
