\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} Demo

This is a demo Paragraphs in HTML need not be closed. This is called `omitted-tag' handling. \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 $), $ 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$$ 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 / is accepted with warning as part of an unquoted attribute-value, though /> still closes the element-tag in XML mode. It may be set to false for parsing HTML documents to allow for unquoted URLs containing /. \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 In addition, newlines at the end of lines containing only markup should be deleted. This is not yet implemented. 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 bold
and
italic 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} \end{code} Currently switching to XML mode implies: \begin{itemlist} \item [XML empty elements] The construct \verb$$ 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 \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:=}) are reported as if \const{xmlns} were not a defined resource. In many cases, getting attribute-names as 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$rdf:$ \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$$ 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 \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$$ 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 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(_). 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$$. \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 print_message/2. \termitem{style}{} Print dubious input such as attempts for redefinitions in the DTD using 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 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 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$$ (default, \term{net}{true}) or \verb$$ (\term{net}{false}). For SGML, this applies to empty elements, so you get \verb$$ (if foo is declared to be \const{EMPTY} in the DTD), \verb$$ (default, \term{net}{false}) or \verb$/ can be emitted as \verb$xxx$ (default, \term{net}{false} or \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$$. 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$content$, which can also be written as \verb$content$. Empty start tags (\verb$<>$), unclosed start tags (\verb$) and unclosed end tags (