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Indirection in XML

  • From: "Costello, Roger L." <costello@mitre.org>
  • To: "xml-dev@lists.xml.org" <xml-dev@lists.xml.org>
  • Date: Sat, 6 May 2017 20:41:46 +0000

Indirection in XML

Hi Folks,

In the following XML document, an entity is defined and then it is used in the XML:

<?xml version="1.0"?>
<!DOCTYPE Commentary [
<!ENTITY adjective "insightful">
    His presentation was


Instead is using the word ‘insightful’ directly in the <Commentary> element, it is used indirectly, via the entity. The entity introduced a level of indirection.


In the following XML Schema, a simpleType is defined and then it is referenced in the declaration of the ‘title’ element:

<xs:element name="title" type="person-title" />

<xs:simpleType name="person-title">
<xs:restriction base="xs:string">
<xs:enumeration value="Mr." />
<xs:enumeration value="Mrs." />
<xs:enumeration value="Miss" />

Instead of providing the simpleType directly in the ‘title’ element declaration, it is provided indirectly, via a reference. The reference introduced a level of indirection.


In the following XSLT document, a variable is defined and then later it is dereferenced in an output statement:

<xsl:variable name="author" select="/Book/Author[1]/text()"/>

. . .

<xsl:sequence select="('The author is: ', $author)" />


Instead of showing the author directly in the output statement, the author is shown indirectly, via a variable. The variable introduced a level of indirection.


All problems in computer science can be solved by another level of indirection. – David Wheeler



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