Andrew Welch - Thoughts on XSLT

Friday, August 22, 2008

Some sample templates for use with LexEv

If your XML has been parsed using LexEv, here are some sample templates for handling the LexEv markup.

To output an entity reference:


<xsl:template match="lexev:entity">
  <xsl:value-of disable-output-escaping="yes" select="concat('&amp;', @name, ';')"/>    
</xsl:template>

To process a CDATA section as markup:


<xsl:template match="lexev:cdata">
  <xsl:apply-templates/>    
</xsl:template>

To output a DOCTYPE from the processing instructions:

In XSLT 1.0 the doctype-public and doctype-system attributes on xsl:output are static and need to be known at compile time, which means I'm afraid you have to do this:


<xsl:template match="/">
 <xsl:value-of disable-output-escaping="yes"
  select="concat('&lt;!DOCTYPE ', name(/*), '&#xa;  PUBLIC &quot;', 
   processing-instruction('doctype-public'), '&quot; &quot;',
   processing-instruction('doctype-system'), '&quot;&gt;')"/>
 <xsl:apply-templates/>
</xsl:template>

In XSLT 2.0 you can use xsl:result-document where the doctype-public and doctype-system are AVTs which mean their values can be determined at runtime:


<xsl:template match="/">
 <xsl:result-document
  doctype-public="{processing-instruction('doctype-public')}"
  doctype-system="{processing-instruction('doctype-system')}">
  <xsl:apply-templates/>
 </xsl:result-document>
</xsl:template>

Thursday, August 21, 2008

LexEv XMLReader - converts lexical events into markup

It's often a requirement to preserve entity references through to the output (which are usually lost during parsing) or to process the contents of CDATA sections as markup. The Lexical Event XMLReader wraps the standard XMLReader to convert lexical events into markup so that they can be processed. Typical uses are:

Converting cdata sections into markup:

<![CDATA[ <p> a para <p> ]]>
to:
<lexev:cdata> <p> a para </p> </lexev:cdata>

Preserving entity references:

hello—world
is converted to:
hello<lexev:entity name="mdash">—</lexev:entity>world

Preserving the doctype declaration:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
is converted to processing instructions:
<?doctype-public -//W3C//DTD XHTML 1.0 Transitional//EN?> <?doctype-system http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd?>

Marking up comments:


is converted to:
<lexev:comment> a comment </lexev:comment>

To use LexEvXMLReader with Saxon:

java -cp saxon9.jar;LexEvXMLReader.jar net.sf.saxon.Transform -x:com.andrewjwelch.lexev.LexEvXMLReader input.xml
stylesheet.xslt

Make sure LexEvXMLReader.jar is on the classpath, and then tell Saxon to use it with the -x switch (copy and paste this line -x:com.andrewjwelch.lexev.LexEvXMLReader)

To use LexEvXMLReader from Java:

XMLReader xmlReader = new LexEvXMLReader();

You can control the following features of LexEv:

enable/disable the marking up of entity references

enable/disable the marking up of CDATA sections

set the default namespace for the CDATA section markup

enable/disable the reporting of the DOCTYPE

enable/disable the marking up of comments

You can set these through the API (if you are including LexEv in an application), or from the command line using the following system properties:

com.andrewjwelch.lexev.inline-entities

com.andrewjwelch.lexev.cdata

com.andrewjwelch.doctype.cdataNamespace

com.andrewjwelch.lexev.doctype

com.andrewjwelch.lexev.comments

For example to set a system property from the command line you would use: -Dcom.andrewjwelch.lexev.comments=false

For support, suggestions and licensing, email lexev@andrewjwelch.com

Friday, July 18, 2008

Kernow 1.6.1

Kernow 1.6.1 (beta) is now availble both as a download and via web start.

Notable things in this release:

- Line numbers on the editor panes in the sandboxes (thanks to a new version of Bounce). You might not think so, but getting line numbers down the side of the editor pane is really involved. It's like block indenting (pressing tab or shift-tab when a block of text is selected) in that it's very low level and requires a lot of coding. Why it's not an intergral part of the editor pane I don't know...

- Improved the syntax-checking-as-you-type and highlighting, and added the ability to disable it.

- The output area is now also a JEditorPane using Bounce so it supports tag highlighting. This might slow things down because now it's an HTML document where every addition is inserted at the end of the document, instead of just appending to a JTextArea... if this proves to be A Bad Thing I'll revert it back to a plain old text area with plain text.

- You can now select which tabs are visible (in options -> tabs) so if you never use certain tabs (like Batch or Schematron) you can remove them.

- If you have Saxon SA you can use XML Schema 1.1 (options -> validation)

- Improved the parameters dialog to make it less fiddly to enter params

- Slight graphical tweaks and likely other things that I've forgotten...

Thursday, July 17, 2008

The Nimbus Look and Feel

This is "Nimbus" - the new look and feel that comes with Java 6 Update 10. This is a cross platform l&f which means it should look the same on all platforms. Kernow currently uses the "platform default" look and feel so it should look like a native app on the platform it's run on, but it's hard to make sure it looks right - often what looks ok on Windows will have obscured buttons on Linux... something I should've fixed but never did.

Anyway, what do you think?

Friday, July 11, 2008

Validating co-constrains in XML Schema 1.1 using xs:alternative

Rather than mess around with loads of assertions to check your co-constraints, XML Schema 1.1 introduces the xs:alternative instruction which allows you to change the type used to validate the element based on some condition. Instead of defining one type and then adding assertions to check the variations, just define one type per variation, then assign that type based on the condition.

To do this you first have to define a default type, then define types for each variation by restricting that type. To choose between them, use xs:alternative as a child of xs:element. Here's an example of a co-constraint - this and that are allowed based on the value of the type attribute of node - and how to validate it:

<root>
  <node type="A">
    <this/> 
  </node>
  <node type="B">
    <that/> 
  </node>
</root>

Here's the schema:

<xs:schema 
  xmlns:xs="http://www.w3.org/2001/XMLSchema" 
  elementFormDefault="qualified">

  <xs:element name="root" type="root"/>
  <xs:element name="node" type="node">
    <xs:alternative type="node-type-A" test="@type = 'A'"/>  
    <xs:alternative type="node-type-B" test="@type = 'B'"/>  
  </xs:element>
  <xs:element name="this"/>
  <xs:element name="that"/>
  
  <xs:complexType name="root">
    <xs:sequence>
      <xs:element ref="node" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>
  
  <-- Base type -->
  <xs:complexType name="node">
    <xs:sequence>
      <xs:any/>  
    </xs:sequence>
    <xs:attribute name="type" type="allowed-node-types"/>
  </xs:complexType>

  <xs:simpleType name="allowed-node-types">
    <xs:restriction base="xs:string">
      <xs:enumeration value="A"/>
      <xs:enumeration value="B"/>
    </xs:restriction>
  </xs:simpleType>      
  
  <-- Type A -->
  <xs:complexType name="node-type-A">
    <xs:complexContent>
      <xs:restriction base="node">
        <xs:sequence>
          <xs:element ref="this"/>  
        </xs:sequence>  
      </xs:restriction>  
    </xs:complexContent>
  </xs:complexType>
  
  <-- Type B -->
  <xs:complexType name="node-type-B">
    <xs:complexContent>
      <xs:restriction base="node">
        <xs:sequence>
          <xs:element ref="that"/>  
        </xs:sequence>  
      </xs:restriction>  
    </xs:complexContent>
  </xs:complexType>  
</xs:schema>

I really like this... schema 1.1 will be a joy to use.

Monday, June 16, 2008

XML Schema co-occurrence constraint workaround

Here's a potential workaround for the co-constraint problem in XML Schema.

Given some XML:


<elem type="typeA">
  <typeA/>
</elem>

<elem type="typeB">
  <typeB/>
</elem>

...the problem is you can't constrain the contents of <elem> based on the value of the type attribute.

You can do it though, if you add an xsi:type attribute to it to explicitly set its type:


<elem type="typeA" xsi:type="elem_typeA">
  <typeA/>
</elem>

<elem type="typeB" xsi:type="elem_typeB">
  <typeB/>
</elem>

with suitable type definitions in the schema:


<xs:complexType name="elem_typeA">
  <xs:sequence>
    <xs:element ref="typeA"/>
    ...

<xs:complexType name="elem_typeB">
  <xs:sequence>
    <xs:element ref="typeB"/>
    ...

...and when the XML is validated the relevant definition will be used.

This technique is far from ideal as it involves modifying the source, but only in a way which doesn't break it for anyone else. Given the various options for validating co-constraints, this could well be the most straightforward way (at least until 1.1 comes along).

Friday, April 25, 2008

The Scrabble Reference

The Scrabble Reference is an ebook I've created which allows Scrabble players to easily check if words are legal, to suggest longer words or sub-anagrams given a word and to show what words can be made given some letters (up to 15 letters).

There are two versions, TWL and SOWPODS (TWL is used in the USA, Canada, Thailand and Israel and SOWPODS in the rest of the world)

The ebook is in the Mobipocket format which deals with running the ebooks on all devices - PDAs, mobile phones, Blackberries etc so you just need to transform the input to suitable Mobipocket markup, compile the ebook and their client does the rest.

I must admit to not being interested in Scrabble, but of course I am interested in XSLT, and given the list of words allowed in Scrabble I thought I should turn them into a product - one that you can run on your phone seems perfect.

Generating the ebook was very straightforward - list of strings in, markup out... XSLT 2.0 is ideal for the task.

Relative paths and the document() function

A nice gotcha cropped up today on xsl-list...

Relative paths passed to the document() function are resolved against either the XML or the stylesheet depending on what is passed in: a node from the XML will mean the path is resolved against the XML, a string will mean it's resolved against the stylesheet.

The gotcha is this - if you modify this:

document(@path)

to this:

<xsl:variable name="path" select="@path" as="xs:string"/>
...
document($path)

and @path contains a relative path, then you could get a document not found error, or worse if your XML and XSLT are in the same directory, you won't notice...

Thoughts on XSLT