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A bi-directional XML parser


xml_parse(?Chars, ?Document)
Parse or generate XML documents
xml_parse(+Controls, ?Chars, ?Document)
Parse or generate XML documents
Pretty-prints XMLDocument on the current output stream
xml_subterm(+XMLTerm, ?Subterm)
Unifies Subterm with a sub-term of Term.


Note for ECLiPSe users

This code creates and accepts character lists rather than ECLiPSe strings. To convert between character lists and (UTF8 or ASCII) strings, use the ECLiPSe built-in string_list/3. For example, to parse a UTF-8 encoded XML file, use the following code:

xml_parse_file(File, Document) :-
	open(File, read, Stream),
	read_string(Stream, end_of_file, _, Utf8String),
	string_list(Utf8String, Chars, utf8),
	xml_parse(Chars, Document).

Most of the subsequent text is taken literally from http://www.john.fletcher.dial.pipex.com/xml.pl.shtml.


This program is offered free of charge, as unsupported source code. You may use it, copy it, distribute it, modify it or sell it without restriction.

We hope that it will be useful to you, but it is provided "as is" without any warranty express or implied, including but not limited to the warranty of non-infringement and the implied warranties of merchantability and fitness for a particular purpose.

Binding Time Limited will not be liable for any damages suffered by you as a result of using the Program. In no event will Binding Time Limited be liable for any special, indirect or consequential damages or lost profits even if Binding Time Limited has been advised of the possibility of their occurrence. Binding Time Limited will not be liable for any third party claims against you.

$Log: xml_comments.ecl,v $
Revision  2006/09/23 01:45:21  snovello
Cisco initial import

Revision 1.1  2003/03/31 13:58:02  js10
Upgraded to latest version from John Fletcher's web site

Revision 1.2  2002/03/26 22:56:55  js10
Added John Fletcher's public domain XML parser/generator

Revision 1.1  2002/03/26 22:50:07  js10
Added John Fletcher's public domain XML parser/generator

Revision 1.1  2002-01-31 21:04:45+00  john
Updated Copyright statements.

Revision 1.0  2001-10-17 20:46:24+01  john
Initial revision


xml.pl is a module for parsing XML with Prolog, which provides Prolog applications with a simple interface to XML documents. We have used it successfully in a number of applications.

It supports a subset of XML suitable for XML Data and Worldwide Web applications. It is neither as strict nor as comprehensive as the XML 1.0 Specification mandates.

It is not as strict, because, while the specification must eliminate ambiguities, not all errors need to be regarded as faults, and some reasonable examples of real XML usage would have to be rejected if they were.

It is not as comprehensive, because, where the XML specification makes provision for more or less complete DTDs to be provided as part of a document, xml.pl supports the local definition of ENTITIES only.

We have placed the code, and a small Windows application which embodies it, into the public domain, to encourage the use of Prolog with XML.

We hope that they will be useful to you, but they are not supported, and Binding Time Limited accept NO LIABILITY WHATSOEVER in respect of their use.


Three predicates are exported by the module: xml_parse/[2,3], xml_subterm/2 and xml_pp/1.

xml_parse( {+Controls}, +?Chars, ?+Document ) parses Chars, a list of character codes, to/from a data structure of the form xml(<attributes>, <content>) , where:

<attributes> is a list of <name>=<data> attributes from the (possibly implicit) XML signature of the document.

<content> is a (possibly empty) list comprising occurrences of :

pcdata(<data>) Text
comment(<string>) An xml comment;
namespace(<URI>,<prefix>,<element>) a Namespace
element(<tag>, <attributes>, <content>) <tag>..</tag> encloses <content> or <tag /> if empty
instructions(<name>, <data>) A PI <? <name> <data> ?>
cdata(<data>) <![CDATA[ <string> ]]>
doctype(<tag>, <doctype id>) DTD <!DOCTYPE .. >

The conversions are not completely symmetrical, in that weaker XML is accepted than can be generated. Specifically, in-bound (Chars -> Document) parsing does not require strictly well-formed XML. If Chars does not represent well-formed XML, Document is instantiated to the term malformed(<attributes>, <content>) .


The <content> of a malformed/2 structure can include:

unparsed( <string> ) Text which has not been parsed
out_of_context( <tag> ) <tag> is not closed

in addition to the parsed term types.

Out-bound (Document -> Chars) parsing does require that Document defines well-formed XML. If an error is detected a 'domain' exception is raised.

The domain exception will attempt to identify the particular sub-term in error and the message will show a list of its ancestor elements in the form <tag>{(id)}* where <id> is the value of any attribute named id.

At this release, the Controls applying to in-bound (Chars -> Document) parsing are:

extended_characters(<bool>) Use the extended character entities for XHTML (default true)
format(<bool>) Strip layouts when no non-layout character data appears between elements. default true)

For out-bound (Document -> Chars) parsing, the only available option is:

format(<bool>) Indent the element content, (default true)


<tag> An atom naming an element
<data> A "string"
<name> An atom, not naming an element
<URI> An atom giving the URI of a Namespace
<string> A "string": list of character codes.
<doctype id> one of public(<string>, <string>), system(<string>) or local
<bool> one of 'true' or 'false'

xml_subterm( +XMLTerm, ?Subterm ) unifies Subterm with a sub-term of Term. This can be especially useful when trying to test or retrieve a deeply-nested subterm from a document. Note that XMLTerm is a sub-term of itself.

xml_pp( +XMLDocument )"pretty prints" XMLDocument on the current output stream.

Features of xml.pl

The xml/2 data structure has some useful properties.


Using an "abstract" Prolog representation of XML, in which terms represent document "nodes", makes the parser reuseable for any XML application.

In effect, xml.pl encapsulates the application-independent tasks of document parsing and generation, which is essential where documents have components from more than one Namespace.

Same Structure

The Prolog term representing a document has the same structure as the document itself, which makes the correspondence between the literal representation of the Prolog term and the XML source readily apparent.

For example, this simple SVG image:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN" "http://www.w3.org/.../svg10.dtd"
    <!ENTITY redblue "fill: red; stroke: blue; stroke-width: 1">
<svg xmlns="http://www.w3.org/2000/svg" width="500" height="500">
 <circle cx=" 25 " cy=" 25 " r=" 24 " style="&redblue;"/>

... translates into this Prolog term:

xml( [version="1.0", standalone="no"],
    doctype( svg, public( "-//W3C//DTD SVG 1.0//EN", "http://www.w3.org/.../svg10.dtd" ) ),
    namespace( 'http://www.w3.org/2000/svg', "",
        element( svg,
            [width="500", height="500"],
            element( circle,
                [cx="25", cy="25", r="24", style="fill: red; stroke: blue; stroke-width: 1"],
                [] )
            ] )
    ] ).

Efficient Manipulation

Each type of node in an XML document is represented by a different Prolog functor, while data, (PCDATA, CDATA and Attribute Values), are left as "strings", (lists of character codes).

The use of distinct functors for mark-up structures enables the efficient recursive traversal of a document, while leaving the data as strings facilitates application specific parsing of data content (aka Micro-parsing).

For example, to turn every CDATA node into a PCDATA node with tabs expanded into spaces:

cdata_to_pcdata( cdata(CharsWithTabs), pcdata(CharsWithSpaces) ) :-
    tab_expansion( CharsWithTabs, CharsWithSpaces ).
cdata_to_pcdata( xml(Attributes, Content1), xml(Attributes, Content2) ) :-
    cdata_to_pcdata( Content1, Content2 ).
cdata_to_pcdata( namespace(URI,Prefix,Content1), namespace(URI,Prefix,Content2) ) :-
    cdata_to_pcdata( Content1, Content2 ).
cdata_to_pcdata( element(Name,Attrs,Content1), element(Name,Attrs,Content2) ) :-
    cdata_to_pcdata( Content1, Content2 ).
cdata_to_pcdata( [], [] ).
cdata_to_pcdata( [H1|T1], [H2|T2] ) :-
    cdata_to_pcdata( H1, H2 ),
    cdata_to_pcdata( T1, T2 ).
cdata_to_pcdata( pcdata(Chars), pcdata(Chars) ).
cdata_to_pcdata( comment(Chars), comment(Chars) ).
cdata_to_pcdata( instructions(Name, Chars), instructions(Name, Chars) ).
cdata_to_pcdata( doctype(Tag, DoctypeId), doctype(Tag, DoctypeId) ).

The above uses no 'cuts', but will not create any choice points with ground input.


The resolution of entity references and the decomposition of the document into distinct nodes means that the calling application is not concerned with the occasionally messy syntax of XML documents.

For example, the clean separation of namespace nodes means that Namespaces, which are useful in combining specifications developed separately, have similar usefulness in combining applications developed separately.

The source code is available here. Although it is unsupported, please feel free to e-mail queries and suggestions. We will respond as time allows.


Generated from xml.eci on Mon Mar 31 03:17:08 2008