A more fundamental concept than functions: units of code thatself-select

["Costello, Roger L." <costello@mitre.org>]

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