Re: [xsl] String conversion problem when string is large

[Wendell Piez <wapiez@xxxxxxxxxxxxxxxx>]

Dear Mike,

On 3/20/2012 7:50 PM, Michael Kay wrote:
> > I have much to learn. Those two things look identical to me.
> Sadly, it's a very common mistake. Any variable-like element with
> content (xsl:variable, xsl:param, xsl:with-param) creates a temporary
> tree; in this case a tree containing a document node and a single text
> node child. That's a much more heavyweight data structure than a simple
> string, as your memory problems demonstrate. It behaves like a string in
> most circumstances, but not all - for example, conversion to a boolean
> behaves differently. That makes it quite hard to optimize. Saxon does
> its best by using a special tree model that can only hold a document
> node with a single text node child. I happened to notice in your stack
> trace that this wasn't being used here, it was using a standard TinyTree
> - I'm not sure why the optimization isn't kicking in, there can be any
> number of reasons. But using a string is so much better, it reduces the
> size of your code and makes all this optimization effort (which is
> really just amelioration of bad code) completely unnecessary.
>
> Even better, declare it as a string so the optimizer knows what to expect!

Just to confirm, do I read correctly that

<xsl:param name="string" as="xs:string">string</xsl:param>

will be passed and processed as a string rather than a tree?

I have been working in the belief (also in analogous cases) that it 
does, but as long as we're on the topic I wonder if you can speak to it.

Plus, I am wondering whether I can know that this will happen in any 
XSLT 2.0 processor, not only Saxon.

The main reason I need this settled in my mind is that, as you say, this 
comes up very commonly; and one of the most mystifying pieces of advice 
we give to beginners is to prefer using @select, when possible, to a 
literal value in a parameter or variable assignment, even though the 
latter is legal and it works, under one definition of "work". If the 
workaround is really this easy (and if it even suggests the nature of 
the underlying problem, as this appears to), that might help.

In other words, if

<xsl:param name="string" as="xs:string">string</xsl:param>

is long for

<xsl:param name="string" select="'string'"/>

we should see similarly that

<xsl:param name="string">string</xsl:param>

is another way of saying

<xsl:param name="string" as="document-node()">
  <xsl:document>string</xsl:document>
</xsl:param>

Is this correct?

Thanks as always,
Wendell

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Wendell Piez                            mailto:wapiez@xxxxxxxxxxxxxxxx
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