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Re: What is declarative XML? (And what's not)

• From: Rick Jelliffe <rjelliffe@allette.com.au>
• To: "Costello, Roger L." <costello@mitre.org>
• Date: Mon, 01 Jun 2009 17:03:22 +1000

Costello, Roger L. wrote:
> Do you agree that this stylesheet document is not a declarative XML document?
>   
I don't think the opposite of "declarative" is really "procedural".  The 
opposite of "procedural" is "functional".  XSLT considered functional, 
not procedural, because functions do not have side effects (variables' 
values cannot be updated.)

The opposite of "declarative" is "algorithmic": declarative says what 
you want, algorithmic says how you do it.

So I think XSLT is declarative when using the template and XPaths, but 
algorithmic when using recursion. You can see that the use of  
<xsl:when> rather than <xsl:if> betrays the concern to try to be as 
declarative as possible.

There are implementation distinctions. The more declarative some program 
is, the more that it can be parallelized and optimized and have 
evaluation re-ordered. This is the same benefit that pure "functional" 
languages have, but "declarative" is not the same as "functional" and 
"algorithmic" is not the same as "procedural".

In your example, I think it is declarative, because there is just an 
identity established, and there is nothing given about how it should be 
implemented. The following two are algorithmic however:
   (map 'sum xpath(  "/purchases/merchandise/cost" ))
or
   int cost[] = get-xpath( current-doc, "/purchases/merchandise/cost" );
   int value = 0;
   for-each (cost) {
         value += cost;
   }

In XSLT you cannot tell how a built-in function is implemented: just 
invoking a function is declarative IMHO.
 
Cheers
Rick Jelliffe