The beauty and power of declarative syntaxes and declarativeprogramming

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

Hi Folks,

While reading a paper [1] by Dimitre Novatchev I gained a deeper insight and appreciation of the beauty and power of declarative syntaxes and declarative programming languages.

Consider this XML:

    <?xml version="1.0"?>
    <aircraft>
        <altitude units="feet">
            12000
        </altitude>
    </aircraft>

It is a declaration of an aircraft's altitude in feet.

It is just a declaration. There are no moving parts, no "state" changes. It just declares what is.

This is really nice. It's easy to understand and verify at a glance.

XML is a declarative syntax.


Imagine a parallel universe where altitudes are expressed in meters, not feet.

We can declaratively express the relationship between the two universes:

    $alt = the value of altitude in feet
    <aircraft>
        <altitude units="meters">
            $alt * 0.3048
        </altitude>
    </aircraft>

This is a declaration of how an aircraft's altitude in feet relates to an altitude in meters.

Observe that there are no moving parts. It just declares the relationship.

At a glance you can determine whether the relationship is correct.

This is declarative programming. You simply declare the relationships.

Contrast with non-declarative programming, where you write code to convert feet to meters. "Convert" implies moving parts and changes. Whenever there's movement, things become much more complex and difficult to understand and verify. 


XSLT is a declarative programming language.

Rather than using my fictitious syntax, we can express the relationship using a standard XSLT syntax:

    <xsl:variable name="alt" select="/aircraft/altitude" />
    <aircraft>
        <altitude units="meters">
            <xsl:value-of select="$alt * 0.3048" />
        </altitude>
    </aircraft>


Breaking away from the non-declarative style of programming is difficult, as that's the way we're taught in school. I can hear my professor: "A program is like a recipe, first do step 1, then step 2, and so forth." Declarative programming isn't like that at all. Declarative programming simply involves declaring relationships. This is beautiful and powerful!

/Roger

[1] Higher-Order Functional Programming with XSLT 2.0 and FXSL by Dimitre Novatchev
http://www.idealliance.org/papers/extreme/proceedings/xslfo-pdf/2006/Novatchev01/EML2006Novatchev01.pdf