Linking in XML

XML 'extensions' are separated from, but related to, the XML language.

Note: You can use the extensions on SGML documents, and largely on SGML-correct HTML documents for that matter, but an SGML processor will not understand it. An XML processor is deemed to understand these.

SGML:

• Links go from element to named element, and from entity to entity.

  Note: XML identification of external entities is more specific than SGML identification.

• All additional linking cannot be validated (application / architecture).

XML linking relates to:

• HTML
  • XML takes the URL-based addressing from HTML (IETF RFC 1738).
  • Absolute and relative addresses supported.

• HyTime
  • XML adapts architecture attributes for link recognition.
    • Element architecture: XML-LINK=
    • Attribute name mapping: XML-ATTRIBUTES=
    • Normally, these values are #FIXED in the DTD.

  • Link types: linking 'inspired' on <clink> and <ilink> (but much simpler).

• TEI
  • Extended pointer syntax.

Short overview:

'Links'

• Links are what you see 'on screen' as a link source.
• Links have a role, a 'title', be in- or out-of-line, and have a required behavior.
• Simple link: <SIMPLE>, refer to a single object directly. cf. <A HREF=> and HyTime <clink>
• <SIMPLE HREF> holds the link end identification ('locator').
• Extended link: <EXTENDED>, 'collect' several resources for link end. cf. <ilink>.
• Link behavior is expressed by SHOW=(embed,replace,new) and ACTUATE=(auto,user).

'Locators'

• <EXTENDED> content holds locators, <LOCATOR>. (Child nodes)
• <LOCATOR HREF> holds the link end identification ('locator').
• You may not see locators on-screen; they are the information base for an <EXTENDED>.
• Other child elements may be 'shown' or disregarded.

  Note: ...as implied by the <EXTENDED INLINE=(y,n)> attribute).

• How the link ends of locators are presented is open to the software (pragmatics, meaning).

HREF value

• HREF is URL
• URL may be extended by:
  • name (i.e. element ID, not an HTML name!)
  • a TEI extended pointer expression [sperb92], chapter 14.

  Example:

  Examples: [1] <SIMPLE HREF="HTTP://HERE.AND.THERE/SOME/FILE.XML#A27"></> <SIMPLE HREF="HTTP://HERE.AND.THERE/SOME/FILE.XML#ID(A27)"></> <LOCATOR TITLE="SECTION ON COLOR MERGER" HREF="#ID(A27)DESCENDANT(1,SECT)"/>

• Keywords such as CHILD and ID are used to walk through the element tree.
• When a span of elements is intended, separate by ".."

Extended link groups

• <GROUP>, consists of <DOCUMENT>
• Used to collect links to external sources that have to be available for correct document processing.
• <DOCUMENT> is only a link to the source.