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Re: A more fundamental concept than functions: units of code t

• From: Hermann Stamm-Wilbrandt <STAMMW@de.ibm.com>
• To: xml-dev@lists.xml.org
• Date: Tue, 18 May 2010 10:05:44 +0200

> Are there other programming languages that support self-selecting units
of code?
David already pointed to C++ and Java.

Add perl, awk and even sed to that list (/regexp/ selectors).


Mit besten Gruessen / Best wishes,

Hermann Stamm-Wilbrandt
Developer, XML Compiler, L3
WebSphere DataPower SOA Appliances
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             "Costello, Roger                                              
             L."                                                           
             <costello@m...                                          To 
             rg>                       "xml-dev@lists.xml.org"             
                                       <xml-dev@lists.xml.org>             
             05/18/2010 12:27                                           cc 
             AM                                                            
                                                                   Subject 
                                        A more fundamental        
                                       concept than functions: units of    
                                       code that self-select               
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           




Hi Folks,

XSLT has xsl:template and xsl:function. On the surface they appear to be
the same: you invoke a template rule using xsl:apply-templates and you
invoke a function by dereferencing its name; you can pass parameters to
both; both return values.

Clearly the idea of "function" is a fundamental concept in Computer
Science.

It occurs to me that the functionality provided by template rules represent
an even more fundamental concept. In fact, functions are merely a subset of
the functionality provided by template rules.

Allow me to explain.

Consider these three template rules:

    <xsl:template match="Book[Publisher lt 1970]">

    <xsl:template match="Book[Publisher eq 1970]">

    <xsl:template match="Book[Publisher gt 1970]">

Suppose that Book is the context node and it has a Publisher child element
with value 1970. With this apply-templates:

    <xsl:apply-templates select="." />

the second xsl:template rule will fire. Of the three template rules, the
second one self-selected ("I meet the requisite conditions; I will now
activate myself").

Template rules have the ability to self-select. Functions never
self-select. They must always be explicitly called out.

Template rules can also be explicitly called out. Consider these revised
templates:

    <xsl:template match="*" mode="pre-1970">
        <param name="Book" />

    <xsl:template match="*" mode="eq-1970>
        <param name="Book" />

    <xsl:template match="*" mode="post-1970">
        <param name="Book" />

The second template can be explicitly called out:

    <xsl:template select="." mode="eq="1970">
        <xsl:with-param name="Book" select="." />

Each template rule is "named" using mode. Effectively the mode is used as
the name of the template rule.

So template rules are units of code that can be either explicitly called
out, or can self-select.

Functions are units of code that can only be explicitly called out.

The ability to create self-selecting units of code is very powerful. It
resounds of decentralization, local-control, Complex Systems. Wow!

Are there other programming languages that support self-selecting units of
code?

Do you agree that self-selecting units of code represents a more
fundamental concept than functions?

/Roger



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• Follow-Ups:
  • Re: A more fundamental concept than functions: units of code that self-select
    • From: Olivier Rossel <olivier.rossel@gmail.com>

• References:
  • A more fundamental concept than functions: units of code thatself-select
    • From: "Costello, Roger L." <costello@mitre.org>