Steve Ball
2000 Zveno Pty Ltd

Zveno makes this software and all associated data and documentation ('Software') available free of charge for any purpose. Copies may be made of this Software but all of this notice must be included on any copy.

The Software was developed for research purposes and Zveno does not warrant that it is error free or fit for any purpose. Zveno disclaims any liability for all claims, expenses, losses, damages and costs any user may incur as a result of using, copying or modifying the Software.

TclXML n
::xml::parser ::xml::parserclass XML parser support for Tcl
`package require xml` `package require parserclass` xml 2.6 ::xml ::sgml ::xml::tclparser `::xml::parserclass` `option` arg arg `::xml::parser` name `-option` value `parser` option arg

## Description

TclXML provides event-based parsing of XML documents. The application may register callback scripts for certain document features, and when the parser encounters those features while parsing the document the callback is evaluated.

The parser may also perform other functions, such as normalisation, validation and/or entity expansion. Generally, these functions are under the control of configuration options. Whether these functions can be performed at all depends on the parser implementation.

The TclXML package provides a generic interface for use by a Tcl application, along with a low-level interface for use by a parser implementation. Each implementation provides a class of XML parser, and these register themselves using the ::xml::parserclass create command. One of the registered parser classes will be the default parser class.

Loading the package with the generic package require xml command allows the package to automatically determine the default parser class. In order to select a particular parser class as the default, that class' package may be loaded directly, eg. package require expat. In all cases, all available parser classes are registered with the TclXML package, the difference is simply in which one becomes the default.

## Commands

### ::xml::parserclass

The ::xml::parserclass command is used to manage XML parser classes.

#### Command Options

The following command options may be used:

create
  create name `-createcommand` script `-createentityparsercommand` script `-parsecommand` script `-configurecommand` script `-getcommand` script `-deletecommand` script

  Creates an XML parser class with the given name.

destroy
  destroy name

  Destroys an XML parser class.

info
  `info` names

  Returns information about registered XML parser classes.

### ::xml::parser

The ::xml::parser command creates an XML parser object. The return value of the command is the name of the newly created parser.

The parser scans an XML document's syntactical structure, evaluating callback scripts for each feature found. At the very least the parser will normalise the document and check the document for well-formedness. If the document is not well-formed then the `-errorcommand` option will be evaluated. Some parser classes may perform additional functions, such as validation. Additional features provided by the various parser classes are described in the section Parser Classes

Parsing is performed synchronously. The command blocks until the entire document has been parsed. Parsing may be terminated by an application callback, see the section Callback Return Codes. Incremental parsing is also supported by using the `-final` configuration option.

#### Configuration Options

The ::xml::parser command accepts the following configuration options:

`-attlistdeclcommand`
  `-attlistdeclcommand` script

  Specifies the prefix of a Tcl command to be evaluated whenever an attribute list declaration is encountered in the DTD subset of an XML document. The command evaluated is:

  `script` name attrname type default value

  where:

  - name Element type name
  - attrname Attribute name being declared
  - type Attribute type
  - default Attribute default, such as #IMPLIED
  - value Default attribute value. Empty string if none given.
`-baseurl`
  `-baseurl` URI

  Specifies the base URI for resolving relative URIs that may be used in the XML document to refer to external entities.

`-characterdatacommand`
  `-characterdatacommand` script

  Specifies the prefix of a Tcl command to be evaluated whenever character data is encountered in the XML document being parsed. The command evaluated is:

  `script` data

  where:

  - data Character data in the document
`-commentcommand`
  `-commentcommand` script

  Specifies the prefix of a Tcl command to be evaluated whenever a comment is encountered in the XML document being parsed. The command evaluated is:

  `script` data

  where:

  - data Comment data
`-defaultcommand`
  `-defaultcommand` script

  Specifies the prefix of a Tcl command to be evaluated when no other callback has been defined for a document feature which has been encountered. The command evaluated is:

  `script` data

  where:

  - data Document data
`-defaultexpandinternalentities`
  `-defaultexpandinternalentities` boolean

  Specifies whether entities declared in the internal DTD subset are expanded with their replacement text. If entities are not expanded then the entity references will be reported with no expansion.

`-doctypecommand`
  `-doctypecommand` script

  Specifies the prefix of a Tcl command to be evaluated when the document type declaration is encountered. The command evaluated is:

  `script` name public system dtd

  where:

  - name The name of the document element
  - public Public identifier for the external DTD subset
  - system System identifier for the external DTD subset. Usually a URI.
  - dtd The internal DTD subset

  See also `-startdoctypedeclcommand` and `-enddoctypedeclcommand`.

`-elementdeclcommand`
  `-elementdeclcommand` script

  Specifies the prefix of a Tcl command to be evaluated when an element markup declaration is encountered. The command evaluated is:

  `script` name model

  where:

  - name The element type name
  - model Content model specification
`-elementendcommand`
  `-elementendcommand` script

  Specifies the prefix of a Tcl command to be evaluated when an element end tag is encountered. The command evaluated is:

  `script` name args

  where:

  - name The element type name that has ended
  - args Additional information about this element

  Additional information about the element takes the form of configuration options. Possible options are:

  - `-empty` boolean The empty element syntax was used for this element
  - `-namespace` uri The element is in the XML namespace associated with the given URI
`-elementstartcommand`
  `-elementstartcommand` script

  Specifies the prefix of a Tcl command to be evaluated when an element start tag is encountered. The command evaluated is:

  `script` name attlist args

  where:

  - name The element type name that has started
  - attlist A Tcl list containing the attributes for this element. The list of attributes is formatted as pairs of attribute names and their values.
  - args Additional information about this element

  Additional information about the element takes the form of configuration options. Possible options are:

  - `-empty` boolean The empty element syntax was used for this element
  - `-namespace` uri The element is in the XML namespace associated with the given URI
  - `-namespacedecls` list The start tag included one or more XML Namespace declarations. list is a Tcl list giving the namespaces declared. The list is formatted as pairs of values, the first value is the namespace URI and the second value is the prefix used for the namespace in this document. A default XML namespace declaration will have an empty string for the prefix.
`-endcdatasectioncommand`
  `-endcdatasectioncommand` script

  Specifies the prefix of a Tcl command to be evaluated when end of a CDATA section is encountered. The command is evaluated with no further arguments.

`-enddoctypedeclcommand`
  `-enddoctypedeclcommand` script

  Specifies the prefix of a Tcl command to be evaluated when end of the document type declaration is encountered. The command is evaluated with no further arguments.

`-entitydeclcommand`
  `-entitydeclcommand` script

  Specifies the prefix of a Tcl command to be evaluated when an entity declaration is encountered. The command evaluated is:

  `script` name args

  where:

  - name The name of the entity being declared
  - args Additional information about the entity declaration. An internal entity shall have a single argument, the replacement text. An external parsed entity shall have two additional arguments, the public and system indentifiers of the external resource. An external unparsed entity shall have three additional arguments, the public and system identifiers followed by the notation name.
`-entityreferencecommand`
  `-entityreferencecommand` script

  Specifies the prefix of a Tcl command to be evaluated when an entity reference is encountered. The command evaluated is:

  `script` name

  where:

  - name The name of the entity being referenced
`-errrocommand`
  `-errorcommand` script

  Specifies the prefix of a Tcl command to be evaluated when a fatal error is detected. The error may be due to the XML document not being well-formed. In the case of a validating parser class, the error may also be due to the XML document not obeying validity constraints. By default, a callback script is provided which causes an error return code, but an application may supply a script which attempts to continue parsing. The command evaluated is:

  `script` errorcode errormsg

  where:

  - errorcode A single word description of the error, intended for use by an application
  - errormsg A human-readable description of the error
`-externalentitycommand`
  `-externalentitycommand` script

  Specifies the prefix of a Tcl command to be evaluated to resolve an external entity reference. If the parser has been configured to validate the XML document, a default script is supplied that resolves the URI given as the system identifier of the external entity and recursively parses the entity's data. If the parser has been configured as a non-validating parser, then by default external entities are not resolved. This option can be used to override the default behaviour. The command evaluated is:

  `script` name baseuri uri id

  where:

  - name The Tcl command name of the current parser
  - baseuri An absolute URI for the current entity which is to be used to resolve relative URIs
  - uri The system identifier of the external entity, usually a URI
  - id The public identifier of the external entity. If no public identifier was given in the entity declaration then id will be an empty string.
`-final`
  `-final` boolean

  Specifies whether the XML document being parsed is complete. If the document is to be incrementally parsed then this option will be set to false, and when the last fragment of document is parsed it is set to true. For example,

      set parser [::xml::parser -final 0]
      $parser parse $data1
      .
      .
      .
      $parser parse $data2
      .
      .
      .
      $parser configure -final 1
      $parser parse $finaldata

`-ignorewhitespace`
  `-ignorewhitespace` boolean

  If this option is set to true then spans of character data in the XML document which are composed only of white-space (CR, LF, space, tab) will not be reported to the application. In other words, the data passed to every invocation of the `-characterdatacommand` script will contain at least one non-white-space character.

`-notationdeclcommand`
  `-notationdeclcommand` script

  Specifies the prefix of a Tcl command to be evaluated when a notation declaration is encountered. The command evaluated is:

  `script` name uri

  where:

  - name The name of the notation
  - uri An external identifier for the notation, usually a URI.
`-notstandalonecommand`
  `-notstandalonecommand` script

  Specifies the prefix of a Tcl command to be evaluated when the parser determines that the XML document being parsed is not a standalone document.

`-paramentityparsing`
  `-paramentityparsing` boolean

  Controls whether external parameter entities are parsed.

`-parameterentitydeclcommand`
  `-parameterentitydeclcommand` script

  Specifies the prefix of a Tcl command to be evaluated when a parameter entity declaration is encountered. The command evaluated is:

  `script` name args

  where:

  - name The name of the parameter entity
  - args For an internal parameter entity there is only one additional argument, the replacement text. For external parameter entities there are two additional arguments, the system and public identifiers respectively.
`-parser`
  `-parser` name

  The name of the parser class to instantiate for this parser object. This option may only be specified when the parser instance is created.

`-processinginstructioncommand`
  `-processinginstructioncommand` script

  Specifies the prefix of a Tcl command to be evaluated when a processing instruction is encountered. The command evaluated is:

  `script` target data

  where:

  - target The name of the processing instruction target
  - data Remaining data from the processing instruction
`-reportempty`
  `-reportempty` boolean

  If this option is enabled then when an element is encountered that uses the special empty element syntax, additional arguments are appended to the `-elementstartcommand` and `-elementendcommand` callbacks. The arguments -empty 1 are appended. For example:

  `script` `-empty` 1
`-startcdatasectioncommand`
  `-startcdatasectioncommand` script

  Specifies the prefix of a Tcl command to be evaluated when the start of a CDATA section section is encountered. No arguments are appended to the script.

`-startdoctypedeclcommand`
  `-startdoctypedeclcommand` script

  Specifies the prefix of a Tcl command to be evaluated at the start of a document type declaration. No arguments are appended to the script.

`-unknownencodingcommand`
  `-unknownencodingcommand` script

  Specifies the prefix of a Tcl command to be evaluated when a character is encountered with an unknown encoding. This option has not been implemented.

`-unparsedentitydeclcommand`
  `-unparsedentitydeclcommand` script

  Specifies the prefix of a Tcl command to be evaluated when a declaration is encountered for an unparsed entity. The command evaluated is:

  `script` system public notation

  where:

  - system The system identifier of the external entity, usually a URI
  - public The public identifier of the external entity
  - notation The name of the notation for the external entity
`-validate`
  `-validate` boolean

  Enables validation of the XML document to be parsed. Any changes to this option are ignored after an XML document has started to be parsed, but the option may be changed after a reset.

`-warningcommand`
  `-warningcommand` script

  Specifies the prefix of a Tcl command to be evaluated when a warning condition is detected. A warning condition is where the XML document has not been authored correctly, but is still well-formed and may be valid. For example, the special empty element syntax may be used for an element which has not been declared to have empty content. By default, a callback script is provided which silently ignores the warning. The command evaluated is:

  `script` warningcode warningmsg

  where:

  - warningcode A single word description of the warning, intended for use by an application
  - wanringmsg A human-readable description of the warning
`-xmldeclcommand`
  `-xmldeclcommand` script

  Specifies the prefix of a Tcl command to be evaluated when the XML declaration is encountered. The command evaluated is:

  `script` version encoding standalone

  where:

  - version The version number of the XML specification to which this document purports to conform
  - encoding The character encoding of the document
  - standalone A boolean declaring whether the document is standalone

#### Parser Command

The ::xml::parser command creates a new Tcl command with the same name as the parser. This command may be used to invoke various operations on the parser object. It has the following general form:

`name` `option` arg

`option` and the arg determine the exact behaviour of the command. The following commands are possible for parser objects:

`cget`
  `cget` -option

  Returns the current value of the configuration option given by `option`. `Option` may have any of the values accepted by the parser object.

`configure`
  `configure` -option value

  Modify the configuration options of the parser object. `Option` may have any of the values accepted by the parser object.

`entityparser`
  `entityparser` option value

  Creates a new parser object. The new object inherits the same configuration options as the parent parser object, but is able to parse XML data in a parsed entity. The option `-dtdsubset` allows markup declarations to be treated as being in the internal or external DTD subset.

`free`
  `free` name

  Frees all resources associated with the parser object. The object is not usable after this command has been invoked.

`get`
  `get` name args

  Returns information about the XML document being parsed. Each parser class provides different information, see the documentation for the parser class.

`parse`
  `parse` xml args

  Parses the XML document. The usual desired effect is for various application callbacks to be evaluated. Other functions will also be performed by the parser class, at the very least this includes checking the XML document for well-formedness.

`reset`
  `reset`

  Initialises the parser object in preparation for parsing a new XML document.

## Callback Return Codes

Every callback script evaluated by a parser may return a return code other than TCL_OK. Return codes are interpreted as follows:

- break Suppresses invocation of all further callback scripts. The `parse` method returns the TCL_OK return code.
- continue Suppresses invocation of further callback scripts until the current element has finished.
- error Suppresses invocation of all further callback scripts. The `parse` method also returns the TCL_ERROR return code.
- default Any other return code suppresses invocation of all further callback scripts. The `parse` method returns the same return code.

## Application Examples

This script outputs the character data of an XML document read from stdin.

    package require xml

    proc cdata {data args} {
        puts -nonewline $data
    }

    set parser [::xml::parser -characterdatacommand cdata]
    $parser parse [read stdin]

This script counts the number of elements in an XML document read from stdin.

    package require xml

    proc EStart {varName name attlist args} {
        upvar #0 $varName var
        incr var
    }

    set count 0
    set parser [::xml::parser -elementstartcommand [list EStart count]]
    $parser parse [read stdin]
    puts "The XML document contains $count elements"

## Parser Classes

This section will discuss how a parser class is implemented.

### Tcl Parser Class

The pure-Tcl parser class requires no compilation - it is a collection of Tcl scripts. This parser implementation is non-validating, ie. it can only check well-formedness in a document. However, by enabling the `-validate` option it will read the document's DTD and resolve external entities.

This parser implementation aims to implement XML v1.0 and supports XML Namespaces.

Generally the parser produces XML Infoset information items. That is, it gives the application a slightly higher-level view than the raw XML syntax. For example, it does not report CDATA Sections.

### Expat Parser Class

This section will discuss the Expat parser class.

## See Also

TclDOM, a Tcl interface for the W3C Document Object Model.

## Keywords

XML parse