\documentclass[11pt]{article}
\usepackage{times}
\usepackage{pl}
\usepackage{html}
\sloppy
\makeindex
\onefile
\htmloutput{html} % Output directory
\htmlmainfile{index} % Main document file
\bodycolor{white} % Page colour
\begin{document}
\title{SWI-Prolog SGML/XML parser}
\author{Jan Wielemaker \\
HCS, \\
University of Amsterdam \\
The Netherlands \\
E-mail: \email{J.Wielemaker@uva.nl}}
\maketitle
\begin{abstract}
Markup languages are an increasingly important method for
data-representation and exchange. This article documents the package
\pllib{sgml}, a foreign library for SWI-Prolog to parse SGML
and XML documents, returning information on both the document and the
document's DTD. The parser is designed to be small, fast and flexible.
\end{abstract}
\pagebreak
\tableofcontents
\vfill
\vfill
\newpage
\section{Introduction}
Markup languages have recently regained popularity for two reasons. One
is document exchange, which is largely based on HTML, an instance of
SGML, and the other is for data exchange between programs, which is
often based on XML, which can be considered a simplified and
rationalised version of SGML.
James Clark's SP parser is a flexible SGML and XML parser. Unfortunately
it has some drawbacks. It is very big, not very fast, cannot work under
event-driven input and is generally hard to program beyond the scope of
the well designed generic interface. The generic interface however does
not provide access to the DTD, does not allow for flexible handling of
input or parsing the DTD independently of a document instance.
The parser described in this document is small (less than 100 kBytes
executable on a Pentium), fast (between 2 and 5 times faster than SP),
provides access to the DTD, and provides flexible input handling.
The document output is equal to the output produced by \jargon{xml2pl},
an SP interface to SWI-Prolog written by Anjo Anjewierden.
\section{Bluffer's Guide}
This package allows you to parse SGML, XML and HTML data into a Prolog
data structure. The high-level interface defined in \pllib{sgml}
provides access at the file-level, while the low-level interface defined
in the foreign module works with Prolog streams. Please use the source
of \file{sgml.pl} as a starting point for dealing with data from
other sources than files, such as SWI-Prolog resources, network-sockets,
character strings, \emph{etc.} The first example below loads an HTML file.
\begin{code}
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<html>
<head>
<title>Demo</title>
</head>
<body>
<h1 align=center>This is a demo</title>
Paragraphs in HTML need not be closed.
This is called `omitted-tag' handling.
</body>
</html>
\end{code}
\begin{code}
?- load_html_file('test.html', Term),
pretty_print(Term).
[ element(html,
[],
[ element(head,
[],
[ element(title,
[],
[ 'Demo'
])
]),
element(body,
[],
[ '\n',
element(h1,
[ align = center
],
[ 'This is a demo'
]),
'\n\n',
element(p,
[],
[ 'Paragraphs in HTML need not be closed.\n'
]),
element(p,
[],
[ 'This is called `omitted-tag\' handling.'
])
])
])
].
\end{code}
The document is represented as a list, each element being an atom to
represent \const{CDATA} or a term \term{element}{Name, Attributes, Content}.
Entities (e.g. \verb$<$) are expanded and included in the
atom representing the element content or attribute value.%
\footnote{Up to SWI-Prolog 5.4.x, Prolog could not represent
\jargon{wide} characters and entities that did not fit in
the Prolog characters set were emitted as a term
\term{number}{+Code}. With the introduction of wide
characters in the 5.5 branch this is no longer needed.}
\subsection{`Goodies' Predicates}
These predicates are for basic use of the library, converting entire and
self-contained files in SGML, HTML, or XML into a structured term. They
are based on load_structure/3.
\begin{description}
\predicate{load_sgml_file}{2}{+File, -ListOfContent}
Same as \term{load_structure}{File, ListOfContent, [dialect(sgml)]}.
\predicate{load_xml_file}{2}{+File, -ListOfContent}
Same as \term{load_structure(File, ListOfContent, [dialect(xml)]}.
\predicate{load_html_file}{2}{+File, -Content}
Load \arg{File} and parse as HTML. Implemented as below. Note that
load_html_file/2 re-uses a cached DTD object as defined by dtd/2. As DTD
objects may be corrupted while loading errornous documents sharing is
undesirable if the documents are not known to be correct. See dtd/2 for
details.
\begin{code}
load_html_file(File, Term) :-
dtd(html, DTD),
load_structure(File, Term,
[ dtd(DTD),
dialect(sgml),
shorttag(false)
]).
\end{code}
\end{description}
\section{Predicate Reference}
\subsection{Loading Structured Documents}
SGML or XML files are loaded through the common predicate
load_structure/3. This is a predicate with many options. For
simplicity a number of commonly used shorthands are provided:
load_sgml_file/2, load_xml_file/2, and
load_html_file/2.
\begin{description}
\predicate{load_structure}{3}{+Source, -ListOfContent, +Options}
Parse \arg{Source} and return the resulting structure in
\arg{ListOfContent}. \arg{Source} is either a term of the format
\term{stream}{StreamHandle} or a file-name. \arg{Options} is a list of
options controlling the conversion process.
A proper XML document contains only a single toplevel element whose name
matches the document type. Nevertheless, a list is returned for
consistency with the representation of element content. The <aref/
ListOfContent/ consists of the following types:
\begin{description}
\termitem{\arg{Atom}}{}
Atoms are used to represent \const{CDATA}. Note
this is possible in SWI-Prolog, as there is no length-limit on atoms and
atom garbage collection is provided.
\termitem{element}{Name, ListAttributes, ListOfContent}
\arg{Name} is the name of the element. Using SGML, which is
case-insensitive, all element names are returned as lowercase atoms.
\arg{ListOfAttributes} is a list of \arg{Name}=\arg{Value} pairs for
attributes. Attributes of type \const{CDATA} are returned literal. Multi-valued
attributes (\const{NAMES}, \emph{etc.}) are returned as a list of atoms.
Handling attributes of the types \const{NUMBER} and \const{NUMBERS} depends on
the setting of the \term{number}{+NumberMode} attribute through
set_sgml_parser/2 or load_structure/3. By
default they are returned as atoms, but automatic conversion to Prolog
integers is supported. \arg{ListOfContent} defines the content for the
element.
\termitem{sdata}{Text}
If an entity with declared content-type \const{SDATA} is encountered, this
term is returned holding the data in \arg{Text}.
\termitem{ndata}{Text}
If an entity with declared content-type \const{NDATA} is encountered, this
term is returned holding the data in \arg{Text}.
\termitem{pi}{Text}
If a processing instruction is encountered (\verb$<?...?>$), <aref/
Text/ holds the text of the processing instruction. Please note that the
\verb$<?xml ...?>$ instruction is handled internally.
\end{description}
The \arg{Options} list controls the conversion process. Currently
defined options are:
\begin{description}
\termitem{dtd}{?DTD}
Reference to a DTD object. If specified, the \verb$<!DOCTYPE ...>$
declaration is ignored and the document is parsed and validated against
the provided DTD. If provided as a variable, the created DTD is
returned. See \secref{implicitdtd}.
\termitem{dialect}{+Dialect}
Specify the parsing dialect. Supported are \const{sgml} (default), \const{xml}
and \const{xmlns}. See \secref{xml} for details on the differences.
\termitem{shorttag}{+Bool}
Define whether SHORTTAG abbreviation is accepted. The default is true
for SGML mode and false for the XML modes. Without SHORTTAG, a <c>/</c>
is accepted with warning as part of an unquoted attribute-value, though
<c>/></c> still closes the element-tag in XML mode. It may be set to
false for parsing HTML documents to allow for unquoted URLs containing
<c>/</c>.
\termitem{space}{+SpaceMode}
Sets the `space-handling-mode' for the initial environment. This mode is
inherited by the other environments, which can override the inherited
value using the XML reserved attribute <elem/xml:space/. See \secref{space}.
\termitem{number}{+NumberMode}
Determines how attributes of type \const{NUMBER} and \const{NUMBERS} are
handled. If \const{token} (default) they are passed as an atom. If
\const{integer} the parser attempts to convert the value to an integer.
If successful, the attribute is passed as a Prolog integer. Otherwise it
is still passed as an atom. Note that SGML defines a numeric attribute
to be a sequence of digits. The \const{-} sign is not allowed and
\exam{1} is different from \exam{01}. For this reason the default is to
handle numeric attributes as tokens. If conversion to integer is
enabled, negative values are silently accepted.
\termitem{defaults}{+Bool}
Determines how default and fixed values from the DTD are used. By
default, defaults are included in the output if they do not appear in
the source. If \const{false}, only the attributes occurring in the source
are emitted.
\termitem{entity}{+Name, +Value}
Defines (overwrites) an entity definition. At the moment, only
\const{CDATA} entities can be specified with this construct. Multiple
entity options are allowed.
\termitem{file}{+Name}
Sets the name of the file on which errors are reported. Sets the
linenumber to 1.
\termitem{line}{+Line}
Sets the starting line-number for reporting errors.
\termitem{max_errors}{+Max}
Sets the maximum number of errors. If this number is reached, an
exception of the format below is raised. The default is 50. Using
\term{max_errors}{-1} makes the parser continue, no matter how many
errors it encounters.
\begin{quote}
\term{error}{limit_exceeded(max_errors, Max), _}
\end{quote}
\end{description}
\end{description}
\subsection{Handling white-space} \label{sec:space}
SGML2PL has four modes for handling white-space. The initial mode can be
switched using the \term{space}{SpaceMode} option to
load_structure/3 and set_sgml_parser/2. In XML
mode, the mode is further controlled by the <elem/xml:space/ attribute,
which may be specified both in the DTD and in the document. The defined
modes are:
\begin{description}
\termitem{space}{sgml}
In SGML, newlines at the start and end of an element are removed.<fn>In
addition, newlines at the end of lines containing only markup should be
deleted. This is not yet implemented.</fn> This is the default mode for
the SGML dialect.
\termitem{space}{preserve}
White space is passed literally to the application. This mode leaves all
white space handling to the application. This is the default mode for
the XML dialect.
\termitem{space}{default}
In addition to \const{sgml} space-mode, all consequtive white-space is
reduced to a single space-character. This mode canonises all white
space.
\termitem{space}{remove}
In addition to \const{default}, all leading and trailing white-space is
removed from \const{CDATA} objects. If, as a result, the \const{CDATA}
becomes empty, nothing is passed to the application. This mode is
especially handy for processing `data-oriented' documents, such as RDF.
It is not suitable for normal text documents. Consider the HTML
fragment below. When processed in this mode, the spaces between the
three modified words are lost. This mode is not part of any standard;
XML 1.0 allows only \const{default} and \const{preserve}.
\begin{code}
Consider adjacent <b>bold</b> <ul>and</ul> <it>italic</it> words.
\end{code}
\end{description}
\subsection{XML documents} \label{sec:xml}
The parser can operate in two modes: \const{sgml} mode and \const{xml} mode, as
defined by the \term{dialect}{Dialect} option. Regardless of this
option, if the first line of the document reads as below, the parser is
switched automatically into XML mode.
\begin{code}
<?xml ... ?>
\end{code}
Currently switching to XML mode implies:
\begin{itemlist}
\item [XML empty elements]
The construct \verb$<element [attribute...] />$ is recognised as
an empty element.
\item [Predefined entities]
The following entitities are predefined: \const{lt} (\verb$<$), \const{gt}
(\verb$>$), \const{amp} (\verb$&$), \const{apos} (\verb$'$)
and \const{quot} (\verb$"$).
\item [Case sensitivity]
In XML mode, names are treated case-sensitive, except for the DTD
reserved names (i.e. \exam{ELEMENT}, \emph{etc.}).
\item [Character classes]
In XML mode, underscores (\verb$_$) and colon (\verb$:$) are
allowed in names.
\item [White-space handling]
White space mode is set to \const{preserve}. In addition to setting
white-space handling at the toplevel the XML reserved attribute
<elem/xml:space/ is honoured. It may appear both in the document and the
DTD. The \const{remove} extension is honoured as <elem/xml:space/ value. For
example, the DTD statement below ensures that the <elem/pre/ element
preserves space, regardless of the default processing mode.
\begin{code}
<!ATTLIST pre xml:space nmtoken #fixed preserve>
\end{code}
\end{itemlist}
\subsubsection{XML Namespaces} \label{sec:xmlns}
Using the \jargon{dialect} \const{xmlns}, the parser will interpret XML
namespaces. In this case, the names of elements are returned as a term
of the format
\begin{quote}
\arg{URL}\const{:}\arg{LocalName}
\end{quote}
If an identifier has no namespace and there is no default namespace it
is returned as a simple atom. If an identifier has a namespace but this
namespace is undeclared, the namespace name rather than the related URL
is returned.
Attributes declaring namespaces ({\tt xmlns:<ns>=<url>}) are reported
as if \const{xmlns} were not a defined resource.
In many cases, getting attribute-names as <xmp>\arg{url}:\arg{name}</xmp>
is not desirable. Such terms are hard to unify and sometimes multiple
URLs may be mapped to the same identifier. This may happen due to poor
version management, poor standardisation or because the the application
doesn't care too much about versions. This package defines two
call-backs that can be set using set_sgml_parser/2 to deal
with this problem.
The call-back \const{xmlns} is called as XML namespaces are noticed.
It can be used to extend a canonical mapping for later use
by the \const{urlns} call-back. The following illustrates this behaviour.
Any namespace containing \const{rdf-syntax} in its URL or that is used as
\const{rdf} namespace is canonised to \const{rdf}. This implies that any
attribute and element name from the RDF namespace appears as
\verb$rdf:<name>$
\begin{code}
:- dynamic
xmlns/3.
on_xmlns(rdf, URL, _Parser) :- !,
asserta(xmlns(URL, rdf, _)).
on_xmlns(_, URL, _Parser) :-
sub_atom(URL, _, _, _, 'rdf-syntax'), !,
asserta(xmlns(URL, rdf, _)).
load_rdf_xml(File, Term) :-
load_structure(File, Term,
[ dialect(xmlns),
call(xmlns, on_xmlns),
call(urlns, xmlns)
]).
\end{code}
\subsection{DTD-Handling}
The DTD (\textbf{D}ocument \textbf{T}ype \textbf{D}efinition) is a
separate entity in sgml2pl, that can be created, freed, defined and
inspected. Like the parser itself, it is filled by opening it as a
Prolog output stream and sending data to it. This section summarises the
predicates for handling the DTD.
\begin{description}
\predicate{new_dtd}{2}{+DocType, -DTD}
Creates an empty DTD for the named \arg{DocType}. The returned
DTD-reference is an opaque term that can be used in the other predicates
of this package.
\predicate{free_dtd}{1}{+DTD}
Deallocate all resources associated to the DTD. Further use of \arg{DTD}
is invalid.
\predicate{load_dtd}{2}{+DTD, +File}
Define the DTD by loading the SGML-DTD file \arg{File}. Same
as load_dtd/3 with empty option list.
\predicate{load_dtd}{3}{+DTD, +File, +Options}
Define the DTD by loading \arg{File}. Defined options are the
\const{dialect} option from open_dtd/3 and the \const{encoding}
option from open/4. Notably the \const{dialect} option must
match the dialect used for subsequent parsing using this DTD.
\predicate{open_dtd}{3}{+DTD, +Options, -OutStream}
Open a DTD as an output stream. See load_dtd/2 for an example.
Defined options are:
\begin{description}
\termitem{dialect}{Dialect}
Define the DTD dialect. Default is \const{sgml}. Using \const{xml} or
\const{xmlns} processes the DTD case-sensitive.
\end{description}
\predicate{dtd}{2}{+DocType, -DTD}
Find the DTD representing the indicated \jargon{doctype}. This predicate
uses a cache of DTD objects. If a doctype has no associated dtd, it
searches for a file using the file search path \exam{dtd} using the call:
\begin{code}
...,
absolute_file_name(dtd(Type),
[ extensions([dtd]),
access(read)
], DtdFile),
...
\end{code}
Note that DTD objects may be modified while processing errornous
documents. For example, loading an SGML document starting with
\verb$<?xml ...?>$ switches the DTD to XML mode and encountering unknown
elements adds these elements to the DTD object. Re-using a DTD object to
parse multiple documents should be restricted to situations where the
documents processed are known to be error-free.
\predicate{dtd_property}{2}{+DTD, ?Property}
This predicate is used to examine the content of a DTD. Property is one
of:
\begin{description}
\termitem{doctype}{DocType}
An atom representing the document-type defined by this DTD.
\termitem{elements}{ListOfElements}
A list of atoms representing the names of the elements in this DTD.
\termitem{element}{Name, Omit, Content}
The DTD contains an element with the given name. \arg{Omit} is a term of
the format \term{omit}{OmitOpen, OmitClose}, where both arguments are
booleans (\const{true} or \const{false} representing whether the open-
or close-tag may be omitted. \arg{Content} is the content-model of the
element represented as a Prolog term. This term takes the following
form:
\begin{description}
\termitem{empty}{}
The element has no content.
\termitem{cdata}{}
The element contains non-parsed character data. All data up to the
matching end-tag is included in the data (\jargon{declared content}).
\termitem{rcdata}{}
As \const{cdata}, but entity-references are expanded.
\termitem{any}{}
The element may contain any number of any element from the DTD in
any order.
\termitem{\#pcdata}{}
The element contains parsed character data .
\termitem{\arg{element}} An element with this name.
\termitem{*}{SubModel}
0 or more appearances.
\termitem{?}{SubModel}
0 or one appearance.
\termitem{+}{SubModel}
1 or more appearances.
\termitem{,}{SubModel1, SubModel2}
\arg{SubModel1} followed by \arg{SubModel2}.
\termitem{\&}{SubModel1, SubModel2}
\arg{SubModel1} and \arg{SubModel2} in any order.
\termitem{\chr{|}}{SubModel1, SubModel2}
\arg{SubModel1} or \arg{SubModel2}.
\end{description}
\termitem{attributes}{Element, ListOfAttributes}
\arg{ListOfAttributes} is a list of atoms representing the attributes
of the element \arg{Element}.
\termitem{attribute}{Element, Attribute, Type, Default}
Query an element. \arg{Type} is one of \const{cdata}, \const{entity},
\const{id}, \const{idref}, \const{name}, \const{nmtoken},
\const{notation}, \const{number} or \const{nutoken}. For DTD types that
allow for a list, the notation \term{list}{Type} is used. Finally, the
DTD construct \verb$(a|b|...)$ is mapped to the term
\term{nameof}{ListOfValues}.
\arg{Default} describes the sgml default. It is one \const{required},
\const{current}, \const{conref} or \const{implied}. If a real default is
present, it is one of \term{default}{Value} or \term{fixed}{Value}.
\termitem{entities}{ListOfEntities}
\arg{ListOfEntities} is a list of atoms representing the names of the
defined entities.
\termitem{entity}{Name, Value}
\arg{Name} is the name of an entity with given value. Value is one of
\begin{description}
\termitem{\arg{Atom}}{}
If the value is atomic, it represents the literal value of the entity.
\termitem{system}{Url}
\arg{Url} is the URL of the system external entity.
\termitem{public}{Id, Url}
For external public entities, \arg{Id} is the identifier. If an URL is
provided this is returned in \arg{Url}. Otherwise this argument is
unbound.
\end{description}
\termitem{notations}{ListOfNotations}
Returns a list holding the names of all \const{NOTATION} declarations.
\termitem{notation}{Name, Decl}
Unify \arg{Decl} with a list if \term{system}{+File} and/or
\term{public}{+PublicId}.
\end{description}
\end{description}
\subsubsection{The DOCTYPE declaration}
As this parser allows for processing partial documents and process the
DTD separately, the DOCTYPE declaration plays a special role.
If a document has no DOCTYPE declaraction, the parser returns a list
holding all elements and CDATA found. If the document has a DOCTYPE
declaraction, the parser will open the element defined in the DOCTYPE as
soon as the first real data is encountered.
\subsection{Extracting a DTD} \label{sec:implicitdtd}
Some documents have no DTD. One of the neat facilities of this library
is that it builds a DTD while parsing a document with an <jargon/
implicit/ DTD. The resulting DTD contains all elements encountered in
the document. For each element the content model is a disjunction of
elements and possibly \verb$#PCDATA$ that can be repeated. Thus, if we
found element <elem/y/ and CDATA in element <elem/x/, the model is:
\begin{code}
<!ELEMENT x - - (y|#PCDATA)*>
\end{code}
Any encountered attribute is added to the attribute list with the type
\const{CDATA} and default \const{\#IMPLIED}.
The example below extracts the elements used in an unknown XML document.
\begin{code}
elements_in_xml_document(File, Elements) :-
load_structure(File, _,
[ dialect(xml),
dtd(DTD)
]),
dtd_property(DTD, elements(Elements)),
free_dtd(DTD).
\end{code}
\subsection{Parsing Primitives}
\begin{description}
\predicate{new_sgml_parser}{2}{-Parser, +Options}
Creates a new parser. A parser can be used one or multiple times for
parsing documents or parts thereof. It may be bound to a DTD or the DTD
may be left implicit, in which case it is created from the document
prologue or parsing is performed without a DTD. Options:
\begin{description}
\termitem{dtd}{?DTD}
If specified with an initialised DTD, this DTD is used for parsing the
document, regardless of the document prologue. If specified using as a
variable, a reference to the created DTD is returned. This DTD may be
created from the document prologue or build implicitely from the
document's content.
\end{description}
\predicate{free_sgml_parser}{1}{+Parser}
Destroy all resources related to the parser. This does not destroy the
DTD if the parser was created using the \term{dtd}{DTD} option.
\predicate{set_sgml_parser}{2}{+Parser, +Option}
Sets attributes to the parser. Currently defined attributes:
\begin{description}
\termitem{file}{File}
Sets the file for reporting errors and warnings. Sets the line to 1.
\termitem{line}{Line}
Sets the current line. Useful if the stream is not at the start of the
(file) object for generating proper line-numbers.
\termitem{charpos}{Offset}
Sets the current character location. See also the \term{file}{File}
option.
\termitem{dialect}{Dialect}
Set the markup dialect. Known dialects:
\begin{description}
\termitem{sgml}{}
The default dialect is to process as SGML. This implies markup is
case-insensitive and standard SGML abbreviation is allowed (abreviated
attributes and omitted tags).
\termitem{xml}{}
This dialect is selected automatically if the processing instruction
\verb$<?xml ...>$ is encountered. See \secref{xml} for details.
\termitem{xmlns}{}
Process file as XML file with namespace support. See \secref{xmlns} for
details. See also the \verb$qualify_attributes$ option below.
\end{description}
\termitem{qualify_attributes}{Boolean}
How to handle unqualified attribute (i.e. without an explicit namespace)
in XML namespace (\const{xmlns}) mode. Default and standard compliant is
not to qualify such elements. If \const{true}, such attributes are
qualified with the namespace of the element they appear in. This option
is for backward compatibility as this is the behaviour of older
versions. In addition, the namespace document suggests unqualified
attributes are often interpreted in the namespace of their element.
\termitem{space}{SpaceMode}
Define the initial handling of white-space in PCDATA. This attribute is
described in \secref{space}.
\termitem{number}{NumberMode}
If \const{token} (default), attributes of type number are passed as a
Prolog atom. If \const{integer}, such attributes are translated into
Prolog integers. If the conversion fails (e.g. due to overflow) a
warning is issued and the value is passed as an atom.
\termitem{encoding}{Encoding}
Set the initial encoding. The default initial encoding for XML documents is
UTF-8 and for SGML documents ISO-8859-1. XML documents may change the
encoding using the <xmp>encoding=</xmp> attribute in the header. Explicit
use of this option is only required to parse non-conforming documents.
Currently accepted values are \const{iso-8859-1} and \const{utf-8}.
\termitem{doctype}{Element}
Defines the toplevel element expected. If a \verb$<!DOCTYPE$
declaration has been parsed, the default is the defined doctype. The
parser can be instructed to accept the first element encountered as the
toplevel using <xmp>doctype(_)</xmp>. This feature is especially
useful when parsing part of a document (see the \const{parse} option to
sgml_parse/2.
\end{description}
\predicate{get_sgml_parser}{2}{+Parser, -Option}
Retrieve infomation on the current status of the parser. Notably useful
if the parser is used in the call-back mode. Currently defined options:
\begin{description}
\termitem{file}{-File}
Current file-name. Note that this may be different from the provided
file if an external entity is being loaded.
\termitem{line}{-Line}
Line-offset from where the parser started its processing in the file-object.
\termitem{charpos}{-CharPos}
Offset from where the parser started its processing in the file-object.
See \secref{indexaccess}.
\termitem{charpos}{-Start, -End}
Character offsets of the start and end of the source processed causing the
current call-back. Used in \program{PceEmacs} to for colouring
text in SGML and XML modes.
\termitem{source}{-Stream}
Prolog stream being processed. May be used in the \const{on_begin}, \emph{etc.}
callbacks from sgml_parse/2.
\termitem{dialect}{-Dialect}
Return the current dialect used by the parser (\const{sgml}, \const{xml} or \const{xmlns}).
\termitem{event_class}{-Class}
The \jargon{event class} can be requested in call-back events. It
denotes the cause of the event, providing useful information for syntax
highlighting. Defined values are:
\begin{description}
\termitem{explicit}{}
The code generating this event is explicitely present in the
document.
\termitem{omitted}{}
The current event is caused by the insertion of an omitted tag.
This may be a normal event in SGML mode or an error in XML mode.
\termitem{shorttag}{}
The current event (\const{begin} or \const{end}) is caused by an
element written down using the \jargon{shorttag} notation
(\verb$<tag/value/>$.
\termitem{shortref}{}
The current event is caused by the expansion of a
\jargon{shortref}. This allows for highlighting shortref strings
in the source-text.
\end{description}
\termitem{doctype}{-Element}
Return the defined document-type (= toplevel element). See also
set_sgml_parser/2.
\termitem{dtd}{-DTD}
Return the currently used DTD. See dtd_property/2 for obtaining information
on the DTD such as element and attribute properties.
\termitem{context}{-StackOfElements}
Returns the stack of currently open elements as a list. The head of this
list is the current element. This can be used to determine the context
of, for example, CDATA events in call-back mode. The elements
are passed as atoms. Currently no access to the attributes is provided.
\termitem{allowed}{-Elements}
Determines which elements may be inserted at the current location. This
information is returned as a list of element-names. If character data is
allowed in the current location, \const{\#pcdata} is part of
\arg{Elements}. If no element is open, the \jargon{doctype} is returned.
This option is intended to support syntax-sensitive editors. Such an
editor should load the DTD, find an appropriate starting point and then
feed all data between the starting point and the caret into the parser.
Next it can use this option to determine the elements allowed at this
point. Below is a code fragment illustrating this use given a parser
with loaded DTD, an input stream and a start-location.
\begin{code}
...,
seek(In, Start, bof, _),
set_sgml_parser(Parser, charpos(Start)),
set_sgml_parser(Parser, doctype(_)),
Len is Caret - Start,
sgml_parse(Parser,
[ source(In),
content_length(Len),
parse(input) % do not complete document
]),
get_sgml_parser(Parser, allowed(Allowed)),
...
\end{code}
\end{description}
\predicate{sgml_parse}{2}{+Parser, +Options}
Parse an XML file. The parser can operate in two input and two output
modes. Output is either a structured term as described with
load_structure/2 or call-backs on predefined events. The
first is especially suitable for manipulating not-too-large documents,
while the latter provides a primitive means for handling very large
documents.
Input is a stream. A full description of the option-list is below.
\begin{description}
\termitem{document}{+Term}
A variable that will be unified with a list describing the content of
the document (see load_structure/2).
\termitem{source}{+Stream}
An input stream that is read. This option <em/must/ be given.
\termitem{content_length}{+Characters}
Stop parsing after \arg{Characters}. This option is useful to parse
input embedded in <em/envelopes/, such as the HTTP protocol.
\termitem{parse}{Unit}
Defines how much of the input is parsed. This option is used to parse
only parts of a file.
\begin{description}
\termitem{file}{}
Default. Parse everything upto the end of the input.
\termitem{element}{}
The parser stops after reading the first element. Using
\term{source}{Stream}, this implies reading is stopped as soon
as the element is complete, and another call may be issued on the same
stream to read the next element.
\termitem{content}{}
The value \const{content} is like \const{element} but assumes the
element has already been opened. It may be used in a call-back from
\term{call}{\const{on_begin}, Pred} to parse individual elements after
validating their headers.
\termitem{declaration}{}
This may be used to stop the parser after reading the first
declaration. This is especially useful to parse only the \exam{doctype}
declaration.
\termitem{input}{}
This option is intended to be used in conjunction with the
\term{allowed}{Elements} option of get_sgml_parser/2.
It disables the parser's default to complete the parse-tree by closing
all open elements.
\end{description}
\termitem{max_errors}{+MaxErrors}
Set the maximum number of errors. If this number is exceeded further
writes to the stream will yield an I/O error exception. Printing of
errors is suppressed after reaching this value. The default is 100.
\termitem{syntax_errors}{+ErrorMode}
Defines how syntax errors are handled.
\begin{description}
\termitem{quiet}{}
Suppress all messages.
\termitem{print}{}
Default. Pass messages to <pref builtin>print_message/2.
\termitem{style}{}
Print dubious input such as attempts for redefinitions in the DTD
using <pref builtin>print_message/2 with severity
\const{informational}.
\end{description}
\termitem{call}{+Event, :PredicateName}
Issue call-backs on the specified events. \arg{PredicateName} is the
name of the predicate to call on this event, possibly prefixed with a
module identifier. If the handler throws an exception, parsing is stopped
and sgml_parse/2 re-throws the exception. The defined events are:
\begin{description}
\termitem{begin}{}
An open-tag has been parsed. The named handler is called with three
arguments: \term{\arg{Handler}}{+Tag, +Attributes, +Parser}.
\termitem{end}{}
A close-tag has been parsed. The named handler is called with two
arguments: \term{\arg{Handler}}{+Tag, +Parser}.
\termitem{cdata}{}
CDATA has been parsed. The named handler is called with two arguments:
\term{Handler}{+CDATA, +Parser}, where CDATA is an atom
representing the data.
\termitem{pi}{}
A processing instruction has been parsed. The named handler is called
with two arguments: \term{\arg{Handler}}{+Text, +Parser}, where
\arg{Text} is the text of the processing instruction.
\termitem{decl}{}
A declaration (\verb$<!...>$) has been read. The named handler is
called with two arguments: \term{\arg{Handler}}{+Text, +Parser},
where \arg{Text} is the text of the declaration with comments removed.
This option is expecially useful for highlighting declarations and comments in
editor support, where the location of the declaration is extracted using
get_sgml_parser/2.
\termitem{error}{}
An error has been encountered. the named handler is called with three
arguments: \term{\arg{Handler}}{+Severity, +Message, +Parser}, where
\arg{Severity} is one of \const{warning} or \const{error} and
\arg{Message} is an atom representing the diagnostic message. The
location of the error can be determined using get_sgml_parser/2
If this option is present, errors and warnings are not reported using
print_message/3
\termitem{xmlns}{}
When parsing an in \const{xmlns} mode, a new namespace declaraction is
pushed on the environment. The named handler is called with three
arguments: \term{\arg{Handler}}{+NameSpace, +URL, +Parser}.
See \secref{xmlns} for details.
\termitem{urlns}{}
When parsing an in \const{xmlns} mode, this predicate can be used to map a
url into either a canonical URL for this namespace or another internal
identifier. See \secref{xmlns} for details.
\end{description}
\end{description}
\end{description}
\subsubsection{Partial Parsing}
In some cases, part of a document needs to be parsed. One option is to
use load_structure/2 or one of its variations and extract
the desired elements from the returned structure. This is a clean
solution, especially on small and medium-sized documents. It however is
unsuitable for parsing really big documents. Such documents can only be
handled with the call-back output interface realised by the
\term{call}{Event, Action} option of sgml_parse/2.
Event-driven processing is not very natural in Prolog.
The SGML2PL library allows for a mixed approach. Consider the case where
we want to process all descriptions from RDF elements in a document. The
code below calls <xmp>process_rdf_description(Element)</xmp> on each element
that is directly inside an RDF element.
\begin{code}
:- dynamic
in_rdf/0.
load_rdf(File) :-
retractall(in_rdf),
open(File, read, In),
new_sgml_parser(Parser, []),
set_sgml_parser(Parser, file(File)),
set_sgml_parser(Parser, dialect(xml)),
sgml_parse(Parser,
[ source(In),
call(begin, on_begin),
call(end, on_end)
]),
close(In).
on_end('RDF', _) :-
retractall(in_rdf).
on_begin('RDF', _, _) :-
assert(in_rdf).
on_begin(Tag, Attr, Parser) :-
in_rdf, !,
sgml_parse(Parser,
[ document(Content),
parse(content)
]),
process_rdf_description(element(Tag, Attr, Content)).
\end{code}
\subsection{Type checking}
\begin{description}
\predicate{xml_is_dom}{1}{@{Term}}
True if \arg{Term} is an SGML/XML term as produced by one of the above
predciates and acceptable by xml_write/3 and friends.
\end{description}
\section{Stream encoding issues} \label{sec:encoding}
The parser can deal with ISO Latin-1 and UTF-8 encoded files, doing
decoding based on the encoding argument provided to
set_sgml_parser/2 or, for XML, based on the \const{encoding}
attribute of the XML header. The parser reads from SWI-Prolog streams,
which also provide encoding handling. Therefore, there are two modes
for parsing. If the SWI-Prolog stream has encoding \const{octet} (which
is the default for binary streams), the decoder of the SGML parser will
be used and positions reported by the parser are octet offsets in the
stream. In other cases, the Prolog stream decoder is used and offsets
are character code counts.
\section{Processing Indexed Files} \label{sec:indexaccess}
In some cases applications wish to process small portions of large
SGML, XML or RDF files. For example, the \emph{OpenDirectory} project
by Netscape has produced a 90MB RDF file representing the main index.
The parser described here can process this document as a unit, but
loading takes 85 seconds on a Pentium-II 450 and the resulting term
requires about 70MB global stack. One option is to process the entire
document and output it as a Prolog fact-base of RDF triplets, but in
many cases this is undesirable. Another example is a large SGML file
containing online documentation. The application normally wishes to
provide only small portions at a time to the user. Loading the entire
document into memory is then undesirable.
Using the \term{parse}{element} option, we open a file, seek
(using <pref builtin>seek/4) to the position of the element and
read the desired element.
The index can be built using the call-back interface of
sgml_parse/2. For example, the following code makes an
index of the \file{ structure.rdf} file of the OpenDirectory
project:
\begin{code}
:- dynamic
location/3. % Id, File, Offset
rdf_index(File) :-
retractall(location(_,_)),
open(File, read, In, [type(binary)]),
new_sgml_parser(Parser, []),
set_sgml_parser(Parser, file(File)),
set_sgml_parser(Parser, dialect(xml)),
sgml_parse(Parser,
[ source(In),
call(begin, index_on_begin)
]),
close(In).
index_on_begin(_Element, Attributes, Parser) :-
memberchk('r:id'=Id, Attributes),
get_sgml_parser(Parser, charpos(Offset)),
get_sgml_parser(Parser, file(File)),
assert(location(Id, File, Offset)).
\end{code}
The following code extracts the RDF element with required id:
\begin{code}
rdf_element(Id, Term) :-
location(Id, File, Offset),
load_structure(File, Term,
[ dialect(xml),
offset(Offset),
parse(element)
]).
\end{code}
\section{External entities}
While processing an SGML document the document may refer to external
data. This occurs in three places: external parameter entities, normal
external entities and the \const{DOCTYPE} declaration. The current version
of this tool deals rather primitively with external data. External
entities can only be loaded from a file and the mapping between the
entity names and the file is done using a \jargon{catalog} file in a
format compatible with that used by James Clark's SP Parser,
based on the SGML Open (now OASIS) specification.
Catalog files can be specified using two primitives: the predicate
sgml_register_catalog_file/2 or the environment variable
\env{SGML_CATALOG_FILES} (compatible with the SP package).
\begin{description}
\predicate{sgml_register_catalog_file}{2}{+File, +Location}
Register the indicated \arg{File} as a catalog file. \arg{Location} is
either \const{start} or \const{end} and defines whether the catalog is
considered first or last. This predicate has no effect if \arg{File} is
already part of the catalog.
If no files are registered using this predicate, the first query on the
catalog examines \env{SGML_CATALOG_FILES} and fills the catalog with
all files in this path.
\end{description}
Two types of lines are used by this package.
\begin{quote}
\const{DOCTYPE} \arg{doctype} \arg{file} \\
\const{PUBLIC} \exam{"}\arg{Id}\exam{"} \arg{file}
\end{quote}
The specified \arg{file} path is taken relative to the location of the
catolog file. For the \const{DOCTYPE} declaraction, \pllib{sgml} first
makes an attempt to resolve the \const{SYSTEM} or \const{PUBLIC}
identifier. If this fails it tries to resolve the \arg{doctype} using
the provided catalog files.
Strictly speaking, \pllib{sgml} breaks the rules for XML,
where system identifiers must be Universal Resource Indicators, not
local file names. Simple uses of relative URIs will work correctly under
UNIX and Windows.
In the future we will design a call-back mechanism for locating and
processing external entities, so Prolog-based file-location and Prolog
resources can be used to store external entities.
\section{Writing markup}
\subsection{Writing documents}
The library \pllib{sgml_write} provides the inverse of the parser,
converting the parser's output back into a file. This process is fairly
simple for XML, but due to the power of the SGML DTD it is much harder
to achieve a reasonable generic result for SGML.
These predicates can write the output in two encoding schemas depending
on the encoding of the \arg{Stream}. In UTF-8 mode, all characters are
encoded using UTF-8 sequences. In ISO Latin-1 mode, characters outside
the ISO Latin-1 range are represented using a named character entity if
provided by the DTD or a numeric character entity.
\begin{description}
\predicate{xml_write}{3}{+Stream, +Term, +Options}
Write the XML header with encoding information and the content of
the document as represented by \arg{Term} to \arg{Stream}. This
predicate deals with XML with or without namespaces. If namespace
identifiers are not provided they are generated. This predicate
defines the following \arg{Options}
\begin{description}
\termitem{dtd}{DTD}
Specify the DTD. In SGML documents the DTD is required to distinguish
between elements that are declared empty in the DTD and elements that
just happen to have no content. Further optimisation (shortref, omitted
tags, etc.) could be considered in the future. The DTD is also used to
find the declared named character entities.
\termitem{doctype}{Doctype}
Document type to include in the header. When omitted it is taken from
the outer element.
\termitem{header}{Bool}
If \arg{Bool} is \const{false}, the XML header is suppressed. Useful for
embedding in other XML streams.
\termitem{layout}{Bool}
Do/do not emit layout characters to make the output readable, Default is
to emit layout. With layout enabled, elements only containing other
elements are written using increasing indentation. This introduces
(depending on the mode and defined whitespace handling) CDATA sequences
with only layout between elements when read back in. If \const{false}, no
layout characters are added. As this mode does not need to analyse the
document it is faster and guarantees correct output when read back.
Unfortunately the output is hardly human readable and causes problems
with many editors.
\termitem{indent}{Integer}
Set the initial element indentation. It more than zero, the indent
is written before the document.
\termitem{nsmap}{Map}
Set the initial namespace map. \arg{Map} is a list of
\arg{Name} = \arg{URI}. This option, together with \const{header} and
\const{ident} is added to use xml_write/3 to generate XML
that is embedded in a larger XML document.
\termitem{net}{Bool}
Use/do not use \jargon{Null End Tags}. For XML, this applies only to
empty elements, so you get \verb$<foo/>$ (default,
\term{net}{true}) or \verb$<foo></foo>$
(\term{net}{false}). For SGML, this applies to empty elements, so
you get \verb$<foo>$ (if foo is declared to be \const{EMPTY} in the DTD),
\verb$<foo></foo>$ (default, \term{net}{false}) or
\verb$<foo//$ (\term{net}{true}). In SGML code, short character
content not containing <c>/</c> can be emitted as \verb$<b>xxx</b>$
(default, \term{net}{false} or \verb$<b/xxx/$ (\term{net}{true})
\end{description}
\predicate{sgml_write}{3}{+Stream, +Term, +Options}
Write the SGML \const{DOCTYPE} header and the content of the document as
represented by \arg{Term} to \arg{Stream}. The \arg{Options} are
described with xml_write/3.
\predicate{html_write}{3}{+Stream, +Term, +Options}
Same as sgml_write/3, but passes the HTML DTD as obtained
from dtd/2. The \arg{Options} are described with
xml_write/3.
\end{description}
\subsection{Simplify quoting}
The \pllib{sgml} package is a parser. Output is generally
much easier achieved directly from Prolog. Nevertheless, it contains a
few building blocks for emitting markup data. The quote funtions return
a version of the input text into one that contains entities for
characters that need to be escaped. These are the XML meta characters
and the characters that cannot be expressed by the document encoding.
Therefore these predicates accept an \arg{encoding} argument. Accepted
values are \const{ascii}, \const{iso_latin_1}, \const{utf8} and
\const{unicode}. Versions with two arguments are provided for backward
compatibility, making the safe \const{ascii} encoding assumption.
\begin{description}
\predicate{xml_quote_attribute}{3}{+In, -Quoted, +Encoding}
Map the characters that may not appear in XML attributes to entities.
Currently these are \verb$<>&"$.%
\footnote{Older versions also mapped \texttt{'} to
\texttt{\'}.}
Characters that cannot represented in \arg{Encoding} are mapped to XML
character entities.
\predicate{xml_quote_attribute}{2}{+In, -Quoted}
Backward compatibility version for xml_quote_attribute/3.
Assumes \const{ascii} encoding.
\predicate{xml_quote_cdata}{3}{+In, -Quoted, +Encoding}
Very similar to xml_quote_attribute/3, but does not quote the
single- and double-quotes.
\predicate{xml_quote_cdata}{2}{+In, -Quoted}
Backward compatibility version for xml_quote_cdata/3.
Assumes \const{ascii} encoding.
\predicate{xml_name}{2}{+In, +Encoding}
Succeed if \arg{In} is an atom or string that satisfies the rules for
a valid XML element or attribute name. As with the other predicates in
this group, if \arg{Encoding} cannot represent one of the characters, this
function fails. It uses a hard-coded table for ASCII-range characters and
iswalpha()/iswalnum() for the first and remaining characters of the name.
\predicate{xml_name}{1}{+In}
Backward compatibility version for xml_name/2. Assumes \const{ascii}
encoding.
\end{description}
\section{Unsupported features}
The current parser is rather limited. While it is able to deal with many
serious documents, it omits several less-used features of SGML and XML.
Known missing SGML features include
\begin{itemlist}
\item [NOTATION on entities]
Though notation is parsed, notation attributes on external entity
declarations are not handed to the user.
\item [NOTATION attributes]
SGML notations may have attributes, declared using
\verb$<!ATTLIST #NOTATION name attributes>$. Those data attributes
are provided when you declare an external CDATA, NDATA, or SDATA entity.
XML does not include external CDATA, NDATA, or SDATA entities,
nor any of the other uses to which data attributes are put in SGML,
so it doesn't include data attributes for notations either.
Sgml2pl does not support this feature and is unlikely to;
you should be aware that SGML documents using this feature cannot
be converted faithfully to XML.
\item [SHORTTAG]
The SGML SHORTTAG syntax is only partially implemented. Currently,
\verb$<tag/content/$ is a valid abbreviation for
\verb$<tag>content</tag>$, which can also be written as
\verb$<tag>content</>$.
Empty start tags (\verb$<>$), unclosed start tags
(\verb$<a<b</verb>) and unclosed end tags (<verb></a<b$) are not
supported.
\item [SGML declaration]
The `SGML declaration' is fixed, though most of the parameters are
handled through indirections in the implementation.
\item [The DATATAG feature]
It is regarded as superseeded by SHORTREF, which is supported.
(SP does not support it either.)
\item [The RANK feature]
It is regarded as obsolete.
\item [The LINK feature]
It is regarded as too complicated.
\item [The CONCUR feature]
Concurrent markup allows a document to be tagged according to more than
one DTD at the same time. It is not supported.
\end{itemlist}
In XML mode the parser recognises SGML constructs that are not allowed
in XML. Also various extensions of XML over SGML are not yet realised.
In particular, XInclude is not implemented because the designers of
XInclude can't make up their minds whether to base it on elements or
attributes yet, let alone details.
\section{Installation}
\subsection{Unix systems}
Installation on Unix system uses the commonly found \program{configure},
<\program{make} and \program{make install} sequence. SWI-Prolog
should be installed before building this package. If SWI-Prolog is not
installed as \program{pl}, the environment variable \env{PL} must be set
to the name of the SWI-Prolog executable. Installation is now
accomplished using:
\begin{code}
% ./configure
% make
% make install
\end{code}
This installs the foreign libraries in \file{$PLBASE/lib/$PLARCH} and
the Prolog library files in \file{$PLBASE/library}, where \file{$PLBASE}
refers to the SWI-Prolog `home-directory'.
\section{Acknowledgements}
The Prolog representation for parsed documents is based on the
SWI-Prolog interface to SP by Anjo Anjewierden.
Richard O'Keefe has put a lot of effort testing and providing bug
reports consisting of an illustrative example and explanation of the
standard. He also made many suggestions for improving this document.
\printindex
\end{document}