SWI-Prolog SGML/XML parser
Jan Wielemaker
VU University Amsterdam
University of Amsterdam
The Netherlands
E-mail: J.Wielemaker@vu.nl
Abstract
Markup languages are an increasingly important method for data-representation and exchange. This article documents the package `library(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.

# Table of Contents

## 1 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 xml2pl, an SP interface to SWI-Prolog written by Anjo Anjewierden.

## 2 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 `library(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 `sgml.pl` as a starting point for dealing with data from other sources than files, such as SWI-Prolog resources, network-sockets, character strings, etc. The first example below loads an HTML file.

    <!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>

    ?- load_html('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.'
                                  ])
                        ])
              ])
    ].

The document is represented as a list, each element being an atom to represent `CDATA` or a term `element(Name, Attributes, Content)`. Entities (e.g. `&lt;`) are expanded and included in the atom representing the element content or attribute value.1Up to SWI-Prolog 5.4.x, Prolog could not represent wide characters and entities that did not fit in the Prolog characters set were emitted as a term `number(+Code)`. With the introduction of wide characters in the 5.5 branch this is no longer needed.

### 2.1 `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.

load_sgml(+Source, -ListOfContent, :Options)
  Calls load_structure/3 with the given Options, using the default option `dialect(sgml)`
load_xml(+Source, -ListOfContent, :Options)
  Calls load_structure/3 with the given Options, using the default option `dialect(xml)`
load_html(+Source, -ListOfContent, :Options)
  Calls load_structure/3 with the given Options, using the default options `dialect(HTMLDialect)`, where HTMLDialect is `html4` or `html5` (default), depending on the Prolog flag `html_dialect`. Both imply the option `shorttag(false)`. The option `dtd(DTD)` is passed, where DTD is the HTML DTD as obtained using `dtd(html, DTD)`. See dtd/2.

## 3 Predicate Reference

### 3.1 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.

load_structure(+Source, -ListOfContent, +Options)
  Parse Source and return the resulting structure in ListOfContent. Source is either a term of the format `stream(StreamHandle)` or a file-name. 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 ListOfContent consists of the following types:

  Atom
    Atoms are used to represent `CDATA`. Note this is possible in SWI-Prolog, as there is no length-limit on atoms and atom garbage collection is provided.
  element(Name, ListAttributes, ListOfContent)
    Name is the name of the element. Using SGML, which is case-insensitive, all element names are returned as lowercase atoms.

    ListOfAttributes is a list of Name=Value pairs for attributes. Attributes of type `CDATA` are returned literal. Multi-valued attributes (`NAMES`, etc.) are returned as a list of atoms. Handling attributes of the types `NUMBER` and `NUMBERS` depends on the setting of the `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. ListOfContent defines the content for the element.

  sdata(Text)
    If an entity with declared content-type `SDATA` is encountered, this term is returned holding the data in Text.
  ndata(Text)
    If an entity with declared content-type `NDATA` is encountered, this term is returned holding the data in Text.
  pi(Text)
    If a processing instruction is encountered (`<?...?>`), Text holds the text of the processing instruction. Please note that the `<?xml ...?>` instruction is handled internally.

  The Options list controls the conversion process. Currently defined options are below. Other options are passed to sgml_parse/2.

  dtd(?DTD)
    Reference to a DTD object. If specified, the `<!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 section 3.5.
  dialect(+Dialect)
    Specify the parsing dialect. Supported are `sgml` (default), `html4`, `html5`, `html` (same as `html4`, `xhtml`, `xhtml5`, `xml` and `xmlns`. See the option `dialect` of set_sgml_parser/2 for details.
  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 ``/``.
  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 `xml:space`. See section 3.2.
  number(+NumberMode)
    Determines how attributes of type `NUMBER` and `NUMBERS` are handled. If `token` (default) they are passed as an atom. If `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 ``-`` sign is not allowed and `1` is different from `01`. For this reason the default is to handle numeric attributes as tokens. If conversion to integer is enabled, negative values are silently accepted.
  case_sensitive_attributes(+Boolean)
    Treat attribute values as case sensitive. The default is `true` for XML and `false` for SGML and HTML dialects.
  case_preserving_attributes(+Boolean)
    Treat attribute values as case insensitive but do not alter their case. The default is `false`. Setting this option sets the `case_sensitive_attributes` to the same value. This option was added to support HTML quasi quotations and most likely has little value in other contexts.
  system_entities(+Boolean)
    Define whether SYSTEM entities are expanded. The default is `false`.
  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 `false`, only the attributes occurring in the source are emitted.
  entity(+Name, +Value)
    Defines (overwrites) an entity definition. At the moment, only `CDATA` entities can be specified with this construct. Multiple entity options are allowed.
  file(+Name)
    Sets the name of the file on which errors are reported. Sets the linenumber to 1.
  line(+Line)
    Sets the starting line-number for reporting errors.
  max_memory(+Max)
    Sets the maximum buffer size in bytes available for input data and CDATA output. If this limit is reached a resource error is raised. Using `max_memory(0)` (the default) means no resource limit will be enforced.
  cdata(+Representation)
    Specify the representation of cdata elements. Supported are `atom` (default), and `string`. The choice is not obvious. Strings are allocated on the Prolog stacks and subject to normal stack garbage collection. They are quicker to create and avoid memory fragmentation. But, multiple copies of the same string are stored multiple times, while the text is shared if atoms are used. Strings are also useful for security sensitive information as they are invisible to other threads and cannot be enumerated using, e.g., current_atom/1. Finally, using strings allows for resource usage limits using the global stack limit (see set_prolog_stack/2).
  attribute_value(+Representation)
    Specify the representation of attribute values. Supported are `atom` (default), and `string`. See above for the advantages and disadvantages of using strings.
  keep_prefix(+Boolean)
    If `true`, xmlns namespaces with prefixes are returned as `ns(Prefix, URI)` terms. If `false` (default), the prefix is ignored and the xmlns namespace is returned as just the URI.

### 3.2 Handling white-space

SGML2PL has four modes for handling white-space. The initial mode can be switched using the `space(SpaceMode)` option to load_structure/3 and set_sgml_parser/2. In XML mode, the mode is further controlled by the `xml:space` attribute, which may be specified both in the DTD and in the document. The defined modes are:

space(sgml)
  In SGML, newlines at the start and end of an element are removed.2In 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.
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.
space(default)
  In addition to `sgml` space-mode, all consequtive white-space is reduced to a single space-character. This mode canonicalises all white space.
space(remove)
  In addition to `default`, all leading and trailing white-space is removed from `CDATA` objects. If, as a result, the `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 `default` and `preserve`.

      Consider adjacent <b>bold</b> <ul>and</ul> <it>italic</it> words.

### 3.3 XML documents

The parser can operate in two modes: `sgml` mode and `xml` mode, as defined by the `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.

    <?xml ... ?>

Currently switching to XML mode implies:

- XML empty elements
  The construct `<element [attribute...] />` is recognised as an empty element.

- Predefined entities
  The following entitities are predefined: `lt` (`<`), `gt` (`>`), `amp` (`&`), `apos` (`'`) and `quot` (`"`).

- Case sensitivity
  In XML mode, names are treated case-sensitive, except for the DTD reserved names (i.e. `ELEMENT`, etc.).

- Character classes
  In XML mode, underscores (`_`) and colon (`:`) are allowed in names.

- White-space handling
  White space mode is set to `preserve`. In addition to setting white-space handling at the toplevel the XML reserved attribute `xml:space` is honoured. It may appear both in the document and the DTD. The `remove` extension is honoured as `xml:space` value. For example, the DTD statement below ensures that the `pre` element preserves space, regardless of the default processing mode.

      <!ATTLIST pre xml:space nmtoken #fixed preserve>

#### 3.3.1 XML Namespaces

Using the dialect `xmlns`, the parser will interpret XML namespaces. In this case, the names of elements are returned as a term of the format

> URL``:``LocalName

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 (`xmlns:<ns>=<url>`) are reported as if `xmlns` were not a defined resource.

In many cases, getting attribute-names as url:name 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 `xmlns` is called as XML namespaces are noticed. It can be used to extend a canonical mapping for later use by the `urlns` call-back. The following illustrates this behaviour. Any namespace containing `rdf-syntax` in its URL or that is used as `rdf` namespace is canonicalised to `rdf`. This implies that any attribute and element name from the RDF namespace appears as `rdf:\bnfmeta{name}`

    :- 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)
                           ]).

The library provides iri_xml_namespace/3 to break down an IRI into its namespace and localname:

[det]iri_xml_namespace(+IRI, -Namespace, -Localname)
  Split an IRI (Unicode URI) into its Namespace (an IRI) and Localname (a Unicode XML name, see xml_name/2). The Localname is defined as the longest last part of the IRI that satisfies the syntax of an XML name. With IRI schemas that are designed to work with XML namespaces, this will typically break the IRI on the last `#` or ``/``. Note however that this can produce unexpected results. E.g., in the example below, one might expect the namespace to be http://example.com/images\#, but an XML name cannot start with a digit.

      ?- iri_xml_namespace('http://example.com/images#12345', NS, L).
      NS = 'http://example.com/images#12345',
      L = ''.

  As we see from the example above, the Localname can be the empty atom. Similarly, Namespace can be the empty atom if IRI is an XML name. Applications will often have to check for either or both these conditions. We decided against failing in these conditions because the application typically wants to know which of the two conditions (empty namespace or empty localname) holds. This predicate is often used for generating RDF/XML from an RDF graph.

[det]iri_xml_namespace(+IRI, -Namespace)
  Same as iri_xml_namespace/3, but avoids creating an atom for the Localname.

### 3.4 DTD-Handling

The DTD (Document Type Definition) 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.

new_dtd(+DocType, -DTD)
  Creates an empty DTD for the named DocType. The returned DTD-reference is an opaque term that can be used in the other predicates of this package.
free_dtd(+DTD)
  Deallocate all resources associated to the DTD. Further use of DTD is invalid.
load_dtd(+DTD, +File)
  Define the DTD by loading the SGML-DTD file File. Same as load_dtd/3 with empty option list.
load_dtd(+DTD, +File, +Options)
  Define the DTD by loading File. Defined options are the `dialect` option from open_dtd/3 and the `encoding` option from open/4. Notably the `dialect` option must match the dialect used for subsequent parsing using this DTD.
open_dtd(+DTD, +Options, -OutStream)
  Open a DTD as an output stream. See load_dtd/2 for an example. Defined options are:
  dialect(Dialect)
    Define the DTD dialect. Default is `sgml`. Using `xml` or `xmlns` processes the DTD case-sensitive.
dtd(+DocType, -DTD)
  Find the DTD representing the indicated 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 `dtd` using the call:

      ...,
      absolute_file_name(dtd(Type),
                         [ extensions([dtd]),
                           access(read)
                         ], DtdFile),
      ...

  Note that DTD objects may be modified while processing errornous documents. For example, loading an SGML document starting with `<?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.

  The DTD `html` is handled seperately. The Prolog flag `html_dialect` specifies the default html dialect, which is either `html4` or `html5` (default).3Note that HTML5 has no DTD. The loaded DTD is an informal DTD that includes most of the HTML5 extensions (http://www.cs.tut.fi/~jkorpela/html5-dtd.html). In addition, the parser sets the `dialect` flag of the DTD object. This is used by the parser to accept HTML extensions. Next, the corresponding DTD is loaded.

dtd_property(+DTD, ?Property)
  This predicate is used to examine the content of a DTD. Property is one of:
  doctype(DocType)
    An atom representing the document-type defined by this DTD.
  elements(ListOfElements)
    A list of atoms representing the names of the elements in this DTD.
  element(Name, Omit, Content)
    The DTD contains an element with the given name. Omit is a term of the format `omit(OmitOpen, OmitClose)`, where both arguments are booleans (`true` or `false` representing whether the open- or close-tag may be omitted. Content is the content-model of the element represented as a Prolog term. This term takes the following form:
    empty
      The element has no content.
    cdata
      The element contains non-parsed character data. All data up to the matching end-tag is included in the data (declared content).
    rcdata
      As `cdata`, but entity-references are expanded.
    any
      The element may contain any number of any element from the DTD in any order.
    #pcdata
      The element contains parsed character data .
    element(element)
      n element with this name.
    `*`(SubModel)
      0 or more appearances.
    `?`(SubModel)
      0 or one appearance.
    `+`(SubModel)
      1 or more appearances.
    `,`(SubModel1, SubModel2)
      SubModel1 followed by SubModel2.
    &(SubModel1, SubModel2)
      SubModel1 and SubModel2 in any order.
    ``|``(SubModel1, SubModel2)
      SubModel1 or SubModel2.
  attributes(Element, ListOfAttributes)
    ListOfAttributes is a list of atoms representing the attributes of the element Element.
  attribute(Element, Attribute, Type, Default)
    Query an element. Type is one of `cdata`, `entity`, `id`, `idref`, `name`, `nmtoken`, `notation`, `number` or `nutoken`. For DTD types that allow for a list, the notation `list(Type)` is used. Finally, the DTD construct `(a|b|...)` is mapped to the term `nameof(ListOfValues)`.

    Default describes the sgml default. It is one `required`, `current`, `conref` or `implied`. If a real default is present, it is one of `default(Value)` or `fixed(Value)`.

  entities(ListOfEntities)
    ListOfEntities is a list of atoms representing the names of the defined entities.
  entity(Name, Value)
    Name is the name of an entity with given value. Value is one of
    Atom
      If the value is atomic, it represents the literal value of the entity.
    system(Url)
      Url is the URL of the system external entity.
    public(Id, Url)
      For external public entities, Id is the identifier. If an URL is provided this is returned in Url. Otherwise this argument is unbound.
  notations(ListOfNotations)
    Returns a list holding the names of all `NOTATION` declarations.
  notation(Name, Decl)
    Unify Decl with a list if `system(+File)` and/or `public(+PublicId)`.

#### 3.4.1 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.

### 3.5 Extracting a DTD

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 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 `#PCDATA` that can be repeated. Thus, if we found element `y` and CDATA in element `x`, the model is:

    <!ELEMENT x - - (y|#PCDATA)*>

Any encountered attribute is added to the attribute list with the type `CDATA` and default `#IMPLIED`.

The example below extracts the elements used in an unknown XML document.

    elements_in_xml_document(File, Elements) :-
            load_structure(File, _,
                           [ dialect(xml),
                             dtd(DTD)
                           ]),
            dtd_property(DTD, elements(Elements)),
            free_dtd(DTD).

### 3.6 Parsing Primitives

new_sgml_parser(-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:
  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.
free_sgml_parser(+Parser)
  Destroy all resources related to the parser. This does not destroy the DTD if the parser was created using the `dtd(DTD)` option.
set_sgml_parser(+Parser, +Option)
  Sets attributes to the parser. Currently defined attributes:
  file(File)
    Sets the file for reporting errors and warnings. Sets the line to 1.
  line(Line)
    Sets the current line. Useful if the stream is not at the start of the (file) object for generating proper line-numbers.
  linepos(LinePos)
    Sets notion of the current column in the source line.
  charpos(Offset)
    Sets the current character location. See also the `file(File)` option.
  position(Position)
    Set source location from a stream position term as obtained using `stream_property(Stream, position(Position))`.
  dialect(Dialect)
    Set the markup dialect. Known dialects:
    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).
    html
    html4
      This is the same as `sgml`, but implies `shorttag(false)` and accepts XML empty element declarations (e.g., `<img src="..."/>`).
    html5
      In addition to `html`, accept attributes named `data-` without warning. This value initialises the charset to UTF-8.
    xhtml
    xhtml5
      These document types are processed as `xml`. Dialect `xhtml5` accepts attributes named `data-` without warning.
    xml
      This dialect is selected automatically if the processing instruction `<?xml ...>` is encountered. See section 3.3 for details.
    xmlns
      Process file as XML file with namespace support. See section 3.3.1 for details. See also the `qualify_attributes` option below.
  xmlns(+URI)
    Set the default namespace of the outer environment. This option is provided to process partial XML content with proper namespace resolution.
  xmlns(+NS, +URI)
    Specify a namespace for the outer environment. This option is provided to process partial XML content with proper namespace resolution.
  qualify_attributes(Boolean)
    How to handle unqualified attribute (i.e. without an explicit namespace) in XML namespace (`xmlns`) mode. Default and standard compliant is not to qualify such elements. If `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.
  space(SpaceMode)
    Define the initial handling of white-space in PCDATA. This attribute is described in section 3.2.
  number(NumberMode)
    If `token` (default), attributes of type number are passed as a Prolog atom. If `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.
  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 `encoding=` attribute in the header. Explicit use of this option is only required to parse non-conforming documents. Currently accepted values are `iso-8859-1` and `utf-8`.
  doctype(Element)
    Defines the toplevel element expected. If a `<!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 `doctype(_)`. This feature is especially useful when parsing part of a document (see the `parse` option to sgml_parse/2.
get_sgml_parser(+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:
  file(-File)
    Current file-name. Note that this may be different from the provided file if an external entity is being loaded.
  line(-Line)
    Line-offset from where the parser started its processing in the file-object.
  charpos(-CharPos)
    Offset from where the parser started its processing in the file-object. See section 6.
  charpos(-Start, -End)
    Character offsets of the start and end of the source processed causing the current call-back. Used in PceEmacs to for colouring text in SGML and XML modes.
  source(-Stream)
    Prolog stream being processed. May be used in the `on_begin`, etc. callbacks from sgml_parse/2.
  dialect(-Dialect)
    Return the current dialect used by the parser (`sgml`, `html`, `html5`, `xhtml`, `xhtml5`, `xml` or `xmlns`).
  event_class(-Class)
    The 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:
    explicit
      The code generating this event is explicitely present in the document.
    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.
    shorttag
      The current event (`begin` or `end`) is caused by an element written down using the shorttag notation (`<tag/value/>`.
    shortref
      The current event is caused by the expansion of a shortref. This allows for highlighting shortref strings in the source-text.
  doctype(-Element)
    Return the defined document-type (= toplevel element). See also set_sgml_parser/2.
  dtd(-DTD)
    Return the currently used DTD. See dtd_property/2 for obtaining information on the DTD such as element and attribute properties.
  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.
  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, `#pcdata` is part of Elements. If no element is open, the 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.

                ...,
                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)),
                ...

sgml_parse(+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.

  document(-Term)
    A variable that will be unified with a list describing the content of the document (see load_structure/2).
  source(+Stream)
    An input stream that is read. This option must be given.
  content_length(+Characters)
    Stop parsing after Characters. This option is useful to parse input embedded in envelopes, such as the HTTP protocol.
  parse(Unit)
    Defines how much of the input is parsed. This option is used to parse only parts of a file.
    file
      Default. Parse everything upto the end of the input.
    element
      The parser stops after reading the first element. Using `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.
    content
      The value `content` is like `element` but assumes the element has already been opened. It may be used in a call-back from `call(`on_begin`, Pred)` to parse individual elements after validating their headers.
    declaration
      This may be used to stop the parser after reading the first declaration. This is especially useful to parse only the `doctype` declaration.
    input
      This option is intended to be used in conjunction with the `allowed(Elements)` option of get_sgml_parser/2. It disables the parser's default to complete the parse-tree by closing all open elements.
  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 50. Using `max_errors(-1)` makes the parser continue, no matter how many errors it encounters.

    > `error(limit_exceeded(max_errors, Max), _)`

  syntax_errors(+ErrorMode)
    Defines how syntax errors are handled.
    quiet
      Suppress all messages.
    print
      Default. Pass messages to print_message/2.
    style
      Print dubious input such as attempts for redefinitions in the DTD using print_message/2 with severity `informational`.
  xml_no_ns(+Mode)
    Error handling if an XML namespace is not defined. Default generates an error. If `quiet`, the error is suppressed. Can be used together with `call(urlns, Closure)` to provide external expansion of namespaces. See also section 3.3.1.
  call(+Event, :PredicateName)
    Issue call-backs on the specified events. 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
      An open-tag has been parsed. The named handler is called with three arguments: `Handler(+Tag, +Attributes, +Parser)`.
    end
      A close-tag has been parsed. The named handler is called with two arguments: `Handler(+Tag, +Parser)`.
    cdata
      CDATA has been parsed. The named handler is called with two arguments: `Handler(+CDATA, +Parser)`, where CDATA is an atom representing the data.
    pi
      A processing instruction has been parsed. The named handler is called with two arguments: `Handler(+Text, +Parser)`, where Text is the text of the processing instruction.
    decl
      A declaration (`<!...>`) has been read. The named handler is called with two arguments: `Handler(+Text, +Parser)`, where 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.

    error
      An error has been encountered. the named handler is called with three arguments: `Handler(+Severity, +Message, +Parser)`, where Severity is one of `warning` or `error` and 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

    xmlns
      When parsing an in `xmlns` mode, a new namespace declaraction is pushed on the environment. The named handler is called with three arguments: `Handler(+NameSpace, +URL, +Parser)`. See section 3.3.1 for details.
    urlns
      When parsing an in `xmlns` mode, this predicate can be used to map a url into either a canonical URL for this namespace or another internal identifier. See section 3.3.1 for details.

#### 3.6.1 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 `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 `process_rdf_description(Element)` on each element that is directly inside an RDF element.

    :- 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)).

### 3.7 Type checking

xml_is_dom(@Term)
  True if Term is an SGML/XML term as produced by one of the above predciates and acceptable by xml_write/3 and friends.

## 4 Stream encoding issues

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 `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 `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.

## 5 library(xpath): Select nodes in an XML DOM

See also
  http://www.w3.org/TR/xpath

The library `xpath.pl` provides predicates to select nodes from an XML DOM tree as produced by `library(sgml)` based on descriptions inspired by the XPath language.

The predicate xpath/3 selects a sub-structure of the DOM non-deterministically based on an XPath-like specification. Not all selectors of XPath are implemented, but the ability to mix xpath/3 calls with arbitrary Prolog code provides a powerful tool for extracting information from XML parse-trees.

[semidet]xpath_chk(+DOM, +Spec, ?Content)
  Semi-deterministic version of xpath/3.
[nondet]xpath(+DOM, +Spec, ?Content)
  Match an element in a DOM structure. The syntax is inspired by XPath, using () rather than `[]` to select inside an element. First we can construct paths using / and //:
  ``//``Term
    Select any node in the DOM matching term.
  ``/``Term
    Match the root against Term.
  Term
    Select the immediate children of the root matching Term.

  The Terms above are of type callable. The functor specifies the element name. The element name '*' refers to any element. The name `self` refers to the top-element itself and is often used for processing matches of an earlier xpath/3 query. A term NS:Term refers to an XML name in the namespace NS. Optional arguments specify additional constraints and functions. The arguments are processed from left to right. Defined conditional argument values are:

  index(?Index)
    True if the element is the Index-th child of its parent, where 1 denotes the first child. Index can be one of:
    Var
      Var is unified with the index of the matched element.
    `last`
      True for the last element.
    `last` - IntExpr
      True for the last-minus-nth element. For example, `last-1` is the element directly preceding the last one.
    IntExpr
      True for the element whose index equals IntExpr.
  Integer
    The N-th element with the given name, with 1 denoting the first element. Same as `index(Integer)`.
  `last`
    The last element with the given name. Same as `index(last)`.
  `last` - IntExpr
    The IntExpr-th element before the last. Same as `index(last-IntExpr)`.

  Defined function argument values are:

  `self`
    Evaluate to the entire element
  `content`
    Evaluate to the content of the element (a list)
  `text`
    Evaluates to all text from the sub-tree as an atom
  `normalize_space`
    As `text`, but uses normalize_space/2 to normalise white-space in the output
  `number`
    Extract an integer or float from the value. Ignores leading and trailing white-space
  `@`Attribute
    Evaluates to the value of the given attribute. Attribute can be a compound term. In this case the functor name denotes the element and arguments perform transformations on the attribute value. Defined transformations are:
    number
      Translate the value into a number using xsd_number_string/2 from `library(sgml)`.
    integer
      As `number`, but subsequently transform the value into an integer using the round/1 function.
    float
      As `number`, but subsequently transform the value into a float using the float/1 function.
    string
      Translate the value into a Prolog string.
    lower
      Translate the value to lower case, preserving the type.
    upper
      Translate the value to upper case, preserving the type.

  In addition, the argument-list can be conditions:

  Left = Right
    Succeeds if the left-hand unifies with the right-hand. If the left-hand side is a function, this is evaluated. The right-hand side is never evaluated, and thus the condition `content = content` defines that the content of the element is the atom `content`. The functions `lower_case` and `upper_case` can be applied to Right (see example below).
  `contains(Haystack, Needle)`
    Succeeds if Needle is a sub-string of Haystack.
  XPath
    Succeeds if XPath matches in the currently selected sub-DOM. For example, the following expression finds an `h3` element inside a `div` element, where the `div` element itself contains an `h2` child with a `strong` child.

        //div(h2/strong)/h3

    This is equivalent to the conjunction of XPath goals below.

           ...,
           xpath(DOM, //(div), Div),
           xpath(Div, h2/strong, _),
           xpath(Div, h3, Result)

  Examples:

  Match each table-row in DOM:

      xpath(DOM, //tr, TR)

  Match the last cell of each tablerow in DOM. This example illustrates that a result can be the input of subsequent xpath/3 queries. Using multiple queries on the intermediate TR term guarantee that all results come from the same table-row:

      xpath(DOM, //tr, TR),
      xpath(TR,  /td(last), TD)

  Match each `href` attribute in an <a> element

      xpath(DOM, //a(@href), HREF)

  Suppose we have a table containing rows where each first column is the name of a product with a link to details and the second is the price (a number). The following predicate matches the name, URL and price:

      product(DOM, Name, URL, Price) :-
          xpath(DOM, //tr, TR),
          xpath(TR, td(1), C1),
          xpath(C1, /self(normalize_space), Name),
          xpath(C1, a(@href), URL),
          xpath(TR, td(2, number), Price).

  Suppose we want to select books with genre="thriller" from a tree containing elements `<book genre=...>`

      thriller(DOM, Book) :-
          xpath(DOM, //book(@genre=thiller), Book).

  Match the elements `<table align="center">` and `<table align="CENTER">`:

          //table(@align(lower) = center)

  Get the `width` and `height` of a `div` element as a number, and the `div` node itself:

          xpath(DOM, //div(@width(number)=W, @height(number)=H), Div)

  Note that `div` is an infix operator, so parentheses must be used in cases like the following:

          xpath(DOM, //(div), Div)

## 6 Processing Indexed Files

In some cases applications wish to process small portions of large SGML, XML or RDF files. For example, the 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 `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 `structure.rdf` file of the OpenDirectory project:

    :- 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)).

The following code extracts the RDF element with required id:

    rdf_element(Id, Term) :-
            location(Id, File, Offset),
            load_structure(File, Term,
                           [ dialect(xml),
                             offset(Offset),
                             parse(element)
                           ]).

## 7 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 `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 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 `SGML_CATALOG_FILES` (compatible with the SP package).

sgml_register_catalog_file(+File, +Location)
  Register the indicated File as a catalog file. Location is either `start` or `end` and defines whether the catalog is considered first or last. This predicate has no effect if File is already part of the catalog.

  If no files are registered using this predicate, the first query on the catalog examines `SGML_CATALOG_FILES` and fills the catalog with all files in this path.

Two types of lines are used by this package.

> `DOCTYPE` doctype file
> `PUBLIC` `"`Id`"` file

The specified file path is taken relative to the location of the catolog file. For the `DOCTYPE` declaraction, `library(sgml)` first makes an attempt to resolve the `SYSTEM` or `PUBLIC` identifier. If this fails it tries to resolve the doctype using the provided catalog files.

Strictly speaking, `library(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.

## 8 library(pwp): Prolog Well-formed Pages

author
  Richard O'Keefe
To be done
  Support compilation of PWP input files

PWP is an approach to server-side scripting using Prolog which is based on a simple key principle:

- The source form of a PWP should be WELL-FORMED XML

Especially when generating XML rather than HTML, this is such an obvious thing to do. We have many kinds of XML checking tools.

- We can tell whether an XML document is WELL FORMED (all the punctuation is right, all tags balance) using practically any decent parser, including SWI Prolog's 'sgml'.
- If we can write a Document Type Description then we can check that a document is VALID using tools like Open SP (formerly James Clark's SP) or SWI Prolog's 'sgml'. This does not guarantee that the output will be valid, but it does catch a lot of errors very early.
- If we can write an XML Schema then we can check that a document is schema-valid. (SWI Prolog's 'sgml' does not yet come with a schema validator, but who knows what the future holds?).
- Since an XML document is just a data structure, we can use any checking tool that we can write in Prolog, IF the input is well-formed so that we can load a template as a Prolog data structure.

Having decided that the input should be well formed, that means NO NEW SYNTAX

None of the weird and horrible <% ... %> or whatever not-quite-XML stuff you see in other template systems, making checking so very hard (and therefore, making errors so distressingly common).

That in turns means that PWP "markup" must be based on special elements or special attributes. The fact that an XML parser must allow undeclared attributes on any element even when validating, but must not allow undeclared elements, suggests doing this through attributes. In particular, one should be able to take an existing DTD, such as an XHTML DTD, and just use that without modification. So the design reduces to

- Allow dynamic data selection, insertion, and transformation just using a small number of extra attributes.

This description uses the following name space:

    xmlns:pwp='http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl'

The attributes are

- pwp:ask = Query
- pwp:use = Term
- pwp:how = text `|` xml
- pwp:tag = QName or '-'
- pwp:att = '' `|` 'one non-alphanumeric character'

Here's what they mean. Each element is expanded in the context of a set of variable bindings. After expansion, if the tag is not mapped to '-', all attributes in the pwp: namespace are removed and the children elements are recursively expanded.

- pwp:ask = Query

  - Query is a Prolog goal. For each solution of Query, the element is further processed with the new variables of Query added to the context.
  - If Query is not a well formed Prolog goal, or if execution of Query throws an exception, page transformation comes to a complete halt and no page is generated.

- pwp:use = Term
- pwp:how = text `|` xml `|` text-file `|` xml-file `|` pwp-file

      Term is a Prolog term; variables in Term are bound by the context.
      An empty Term is regarded as a missing value for this attribute.
      The Prolog variable CONTEXT refers to the entire context, a list
      of Name = Value, where Name is a Prolog atom holding the name of        the context variable and Value is an arbitrary Prolog term.

  - If pwp:how is text, The value of Term is used to define a sequence of characters.

    - A number produces the same characters that write/1 would.
    - An atom produces the same characters that write/1 would.
    - A string produces the same characters that write/1 would.
    - A list of character codes produces those characters.
    - The following terms produce the same sequence of characters that the corresponding goal would have sent to the current output stream:

      - `write(Datum)`
      - `writeq(Datum)`
      - `write_canonical(Datum)`
      - `print(Datum)`
      - `print(Datum)`
      - `format(Format)`
      - `format(Format, Arguments)`

    - A singleton list [X] defines the characters that X defines.
    - Any other term F(T1,...,Tn) defines the characters that T1 defines, followed by the characters that T2 defines, ..., followed by the characters that Tn defines.

  - If pwp:how is xml,

    > The value of Term must be an XML term as defined in the SGML2PL documentation or a list of such terms. A single term is taken as if it had been [Term]. The resulting list of terms replaces the children of the current element and will not be further processed.

  - If pwp:how is text-file,

    > The value of Term is used to define a sequence of characters. That sequence of characters is used as a file name. The file is read as a sequence of characters, and that sequence used as character data.

  - If pwp:how is xml-file,

    > The value of Term is used to define a sequence of characters. That sequence of characters is used as a file name. The file is loaded as XML, and the sequence of XML items thus obtained used. This means that PWP provides XML inclusion without depending on the parser to support XInclude.

  - If pwp:how is pwp-file,

    > Like xml-file, but PWP attributes are evaluated and processed. The current context variables are passed to the PWP processor.

    The default value for pwp:how is text.

- pwp:tag = QName or '-'

      If pwp:tag is missing or the value is empty, the current element
      appears in the output (after further processing) with its present
      tag.  If pwp:tag is a QName, the current element appears (...)
      with that as its tag.  That option is most useful in DTDs, where
      an "authoring" DTD may use one tag and have it automatically mapped
      to another tag in the output, e.g., <item> -> <li>.  Finally, if
      pwp:tag is '-', the children of the current element (either the
      result of pwp:use or the transformed original children, whichever
      applies) appear in the output but there is no element around them.

      A missing or empty pwp:ask is just like pwp:ask = 'true'.

- pwp:att = '' `|` 'one non-alphanumeric character'.

      Attributes in the pwp namespace are not affected by this attribute.
      Such attributes are always stripped out and never substituted into.

      If pwp:att is missing or empty, attributes of the current
      element are copied over to the output unchanged.

      If pwp:att = 'c' for some non-alphanumeric character c,
      each attribute is examined for occurrences of c(...)c and c[...]c
      which are as short as possible.
      There is no one character which could be used every time, so you
      have to explicitly choose a substitution marker which is safe
      for the data you do not want to be altered.  None of the pwp
      attributes are inherited, least of all this one.

      Text outside c(...)c groups is copied unchanged; text inside
      a c(...)c group is parsed as a Prolog term and treated as if by
      pwp:how = text. Text inside a c[...]c group is evaluated (in the
      current context), and if it fails, the entire attribute will be
      removed from the element.

Examples:

1. A "Hello World" like example

       <html
         xmlns:pwp="http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl"
         pwp:ask = "ensure_loaded(msg), once(msg(Greeting))">
         <head>
           <title pwp:use="Greeting"/>
         </head>
         <body>
           <p><span pwp:use="Greeting" pwp:tag='-'/></p>
         </body>
       </html>

   where `msg.pl` contains

       msg('Hello, World!').

   This example illustrates an important point. Prolog Well-Formed Pages provide NO way to physically incorporate Prolog clauses into a page template. Prolog clauses must be put in separate files which can be checked by a Prolog syntax checker, compiler, cross-referencer, &c WITHOUT the Prolog tool in question needing to know anything whatsoever about PWP. You load the files using pwp:ask on the root element.

2. Binding some variables and using them

       <html
         xmlns:pwp="http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl">
         <head><title>Example 2</title></head>
         <body pwp:ask="Hello = 'Hello world', A = 20, B = 22">
           <h1 pwp:use="Hello"/>
           <p>The answer is <span pwp:use="C" pwp:ask="C is A+B"/>.</p>
         </body>
       </html>

3. Making a table We are given a Prolog database `staff.pl` defining `staff(NickName, FullName, Office, Phone, E_Mail_Address)`. `status(NickName, full_time | part_time)`. We want to make a phone list of full time staff.

       <html
         xmlns:pwp="http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl"
         pwp:ask='ensure_loaded(staff)'>
         <head>
           <title>Phone list for Full-Time staff.</title>
         </head>
         <body>
           <h1>Phone list for Full-Time staff.</h1>
           <table
             pwp:ask = "setof(FullName-Phone,
                              N^O^E^(
                                status(N, full_time),
                                staff(N, FullName, O, Phone, E)
                              ),
                              Staff_List)">
             <tr><th>Name</th><th>Phone</th></tr>
             <tr pwp:ask="member(FullName-Phone, Staff_List)">
               <td pwp:use="FullName"/>
               <td pwp:use="Phone"/>
             </tr>
           </table>
         </body>
       </html>

4. Substituting into an attribute Same data base as before, but now we want to make a mailing list page.

       <html
         xmlns:pwp="http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl"
         pwp:ask='ensure_loaded(staff)'>
         <head>
           <title>Phone list for Full-Time staff.</title>
         </head>
         <body>
           <h1>Phone list for Full-Time staff.</h1>
           <table
             pwp:ask = "setof(FullName-E_Mail,
                              N^O^P^staff(N, FullName, O, P, E_Mail),
                              Staff_List)">
             <tr><th>Name</th><th>Address</th></tr>
             <tr pwp:ask="member(FullName-E_Mail, Staff_List)">
               <td pwp:use="FullName"/>
               <td><a pwp:use="E_Mail"
                      pwp:att='$' href="mailto:$(E_Mail)$"/></td>
             </tr>
           </table>
         </body>
       </html>

5. If-then-else effect A page that displays the value of the 'SHELL' environment variable if it has one, otherwise displays 'There is no default shell.'

       <html
         xmlns:pwp="http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl">
         <head><title>$SHELL</title></head>
         <body>
           <p pwp:ask="getenv('SHELL', Shell)"
           >The default shell is <span pwp:tag="-" pwp:use="Shell"/>.</p>
           <p pwp:ask="\+getenv('SHELL',_)">There is no default shell.</p>
         </body>
       </html>

   There is one other criterion for a good server-side template language:

   It should be possible to compile templates so as to eliminate most if not all interpretation overhead.

   This particular notation satisfies that criterion with the limitation that the conversion of a term to character data requires run-time traversal of terms (because the terms are not known until run time).

[det]pwp_files(:In:atom, +Out:atom)
  loads an Xml document from the file named In, transforms it using the PWP attributes, and writes the transformed version to the new file named Out.
[det]pwp_stream(:Input:input_stream, +Output:output_stream, +Context:list)
  Loads an Xml document from the given Input stream, transforms it using the PWP attributes, and writes the transformed version to the given Output stream. Context provides initial contextual variables and is a list of Name=Value.
pwp_xml(:In:list(xml), -Out:list(xml), +Context)
  maps down a list of XML items, acting specially on elements and copying everything else unchanged, including white space. The Context is a list of 'VariableName'=CurrentValue bindings.

## 9 Writing markup

### 9.1 Writing documents

The library `library(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 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.

xml_write(+Stream, +Term, +Options)
  Write the XML header with encoding information and the content of the document as represented by Term to Stream. This predicate deals with XML with or without namespaces. If namespace identifiers are not provided they are generated. This predicate defines the following Options
  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.
  doctype(Doctype)
    Document type to include in the header. When omitted it is taken from the outer element.
  header(Bool)
    If Bool is `false`, the XML header is suppressed. Useful for embedding in other XML streams.
  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 `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.
  indent(Integer)
    Set the initial element indentation. It more than zero, the indent is written before the document.
  nsmap(Map)
    Set the initial namespace map. Map is a list of Name = URI. This option, together with `header` and `ident` is added to use xml_write/3 to generate XML that is embedded in a larger XML document.
  net(Bool)
    Use/do not use Null End Tags. For XML, this applies only to empty elements, so you get `<foo/>` (default, `net(true)`) or `<foo></foo>` (`net(false)`). For SGML, this applies to empty elements, so you get `<foo>` (if foo is declared to be `EMPTY` in the DTD), `<foo></foo>` (default, `net(false)`) or `<foo//` (`net(true)`). In SGML code, short character content not containing ``/`` can be emitted as `<b>xxx</b>` (default, `net(false)` or `<b/xxx/` (`net(true)`)
sgml_write(+Stream, +Term, +Options)
  Write the SGML `DOCTYPE` header and the content of the document as represented by Term to Stream. The Options are described with xml_write/3.
html_write(+Stream, +Term, +Options)
  Same as sgml_write/3, but passes the HTML DTD as obtained from dtd/2. The Options are described with xml_write/3.

### 9.2 XML Quote primitives

In most cases, the preferred way to create an XML document is to create a Prolog tree of `element(Name, Attributes, Content)` terms and call xml_write/3 to write this to a stream. There are some exceptions where one might not want to pay the price of the intermediate representation. For these cases, this library contains 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 encoding argument. Accepted values are `ascii`, `iso_latin_1`, `utf8` and `unicode`. Versions with two arguments are provided for backward compatibility, making the safe `ascii` encoding assumption.

xml_quote_attribute(+In, -Quoted, +Encoding)
  Map the characters that may not appear in XML attributes to entities. Currently these are `<>&"`.4Older versions also mapped `'` to `&apos;`. Characters that cannot represented in Encoding are mapped to XML character entities.
xml_quote_attribute(+In, -Quoted)
  Backward compatibility version for xml_quote_attribute/3. Assumes `ascii` encoding.
xml_quote_cdata(+In, -Quoted, +Encoding)
  Very similar to xml_quote_attribute/3, but does not quote the single- and double-quotes.
xml_quote_cdata(+In, -Quoted)
  Backward compatibility version for xml_quote_cdata/3. Assumes `ascii` encoding.
xml_name(+In, +Encoding)
  Succeed if 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 Encoding cannot represent one of the characters, this function fails. Character classification is based on http://www.w3.org/TR/2006/REC-xml-20060816.
xml_name(+In)
  Backward compatibility version for xml_name/2. Assumes `ascii` encoding.

### 9.3 XSD lexical forms

The predicates in this section translate between values and their lexical forms for XML-Schema data types. They are implementated in C to achieve the best possible performance.

[det]xsd_number_string(?Number, ?String)
  This predicate is similar to number_string/2, but accepts floating point numbers according to the XML syntax rather than the Prolog syntax. In particular, XML does not require a `0' (zero) before and after the decimal dot and accepts the constants `NaN` and `INF`. If a Prolog float is converted into a string it returns the XML canonical form. This form always has one digit before the decimal dot, at least one digit after it and an exponential component using the capital `E`. This predicate behaves as number_string/2 for integers.

  Throws a `syntax_error(xsd_number)` if String is given and is not a well-formed XSD number.

[det]xsd_time_string(?DateTime, ?Type, ?String)
  Serialize and deserialize the XSD date and time formats. The converion is represented by the table below.

  Prolog term | Type | XSD string
  date(Y,M,D) | xsd:date | YYYY-MM-DD
  date_time(Y,M,D,H,Mi,S) | xsd:dateTime | YYYY-MM-DDTHH-MM-SS
  date_time(Y,M,D,H,Mi,S,0) | xsd:dateTime | YYYY-MM-DDTHH-MM-SSZ
  date_time(Y,M,D,H,Mi,S,TZ) | xsd:dateTime | YYYY-MM-DDTHH-MM-SS[+-]HH:MM
  time(H,M,S) | xsd:time | HH:MM:SS
  year_month(Y,M) | xsd:gYearMonth | YYYY-MM
  month_day(M,D) | xsd:gMonthDay | MM-DD
  D | xsd:gDay | DD
  M | xsd:gMonth | MM
  Y | xsd:gYear | YYYY

  For the Prolog term all variables denote integers except for S, which represents seconds as either an integer or float. The TZ argument is the offset from UTC in seconds. The Type is written as xsd:name, but is in fact the full URI of the XSD data type, e.g., `http://www.w3.org/2001/XMLSchema#date`. In the XSD string notation, the letters YMDHS denote digits. The notation SS is either a two-digit integer or a decimal number with two digits before the floating point, e.g. `05.3` to denote 5.3 seconds.

  For most conversions, Type may be specified unbound and is unified with the resulting type. For ambiguous conversions, Type must be specified or an instantiation_error is raised. When converting from Prolog to XSD serialization, D, M and Y are ambiguous. When convertion from XSD serialization to Prolog, only DD and MM are ambiguous. If Type and String are both given and String is a valid XSD date/time representation but not matching Type a syntax error with the shape `syntax_error(Type)` is raised. If DateTime and Type are both given and DateTime does not satisfy Type a domain_error of the shape `domain_error(xsd_time(Type), DateTime)` is raised.

  The domain of numerical values is verified and a corresponding domain_error exception is raised if the domain is violated. There is no test for the existence of a date and thus `"2016-02-31"`, although non-existing is accepted as valid.

### 9.4 library(c14n2): C14n2 canonical XML documents

C14n2 specifies a canonical XML document. This library writes such a document from an XML DOM as returned by the XML (or SGML) parser. The process takes two steps:

- Normalise the DOM
- Call xml_write/3 with appropriate flags

[det]xml_write_canonical(+Stream, +DOM, +Options)
  Write an XML DOM using the canonical conventions as defined by C14n2. Namespace declarations in the canonical document depend on the original namespace declarations. For this reason the input document must be parsed (see load_structure/3) using the dialect `xmlns` and the option `keep_prefix(true)`.

## 10 Unsupported SGML 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

- NOTATION on entities
  Though notation is parsed, notation attributes on external entity declarations are not handed to the user.
- NOTATION attributes
  SGML notations may have attributes, declared using `<!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.

- SHORTTAG
  The SGML SHORTTAG syntax is only partially implemented. Currently, `<tag/content/` is a valid abbreviation for `<tag>content</tag>`, which can also be written as `<tag>content</>`. Empty start tags (`<>`), unclosed start tags (`<a<b</verb>`) and unclosed end tags (`<verb></a<b`) are not supported.
- SGML declaration
  The `SGML declaration' is fixed, though most of the parameters are handled through indirections in the implementation.
- The DATATAG feature
  It is regarded as superseeded by SHORTREF, which is supported. (SP does not support it either.)
- The RANK feature
  It is regarded as obsolete.
- The LINK feature
  It is regarded as too complicated.
- 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.

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.

## 11 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.

# Index

?
current_atom/1
  3.1
dtd/2
  2.1 9.1
dtd_property/2
  3.6
free_dtd/1
free_sgml_parser/1
get_sgml_parser/2
  3.6 3.6 3.6
html_write/3
iri_xml_namespace/2
iri_xml_namespace/3
  3.3.1 3.3.1
load_dtd/2
  3.4
load_dtd/3
  3.4
load_html/3
load_html_file/2
  3.1
load_sgml/3
load_sgml_file/2
  3.1
load_structure/2
  3.6 3.6 3.6.1
load_structure/3
  2.1 2.1 2.1 2.1 3.1 3.1 3.2
load_xml/3
load_xml_file/2
  3.1
new_dtd/2
new_sgml_parser/2
number_string/2
  9.3 9.3
open/4
  3.4
open_dtd/3
  3.4
print_message/2
  3.6 3.6
print_message/3
  3.6
pwp_files/2
pwp_stream/3
pwp_xml/3
seek/4
  6
set_prolog_stack/2
  3.1
set_sgml_parser/2
  3.1 3.1 3.2 3.3.1 3.6 4
sgml_parse/2
  3.1 3.6 3.6 3.6 3.6.1 6
sgml_register_catalog_file/2
  7
sgml_write/3
  9.1
xml_is_dom/1
xml_name/1
xml_name/2
  3.3.1 9.2
xml_quote_attribute/2
xml_quote_attribute/3
  9.2 9.2
xml_quote_cdata/2
xml_quote_cdata/3
  9.2
xml_write/3
  3.7 9.1 9.1 9.1 9.2
xml_write_canonical/3
xpath/3
xpath_chk/3
xsd_number_string/2
xsd_time_string/3
XML-Schema,lexical forms
  9.3