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Re: [xsl] Re: XPath as a stand-alone programming language


Subject: Re: [xsl] Re: XPath as a stand-alone programming language
From: Liam R E Quin <liam@xxxxxx>
Date: Wed, 24 Oct 2012 10:58:56 -0400

On Wed, 2012-10-24 at 14:29 +0000, Costello, Roger L. wrote:

> Is it possible to have an XPath 3.0 program that runs independent of XSLT and Query?

Yes. However, XPath still assumes a host language. For example, you
can't bind a prefix to a namespace inside XPath. More importantly, many
things people want to do with XPath are actually not within XPath's
remit - for example, I often hear "how do I use XPath to return an
element without any of its children" to which the usual answer is,
"XPath returns pointers into a document, so you can't do that. If you
want to construct a new element that's a copy of an existing one but
without children, use XQuery instead."

There are several command-line utilities for linux at least that
evaluate an XPath expression against an XML document and return the
result as serialized XML. Most of them are based on XPath 1.0, though;
the XPath 2 or 3 command-line tools I know of are all XQuery
implementations in practice. Which is fine by me as it means some of
them are massively faster - e.g. running against an index.  One could
write a stand-alone XPath implementation that ran against an index, at
least in principle, but the only ones I know of are actually embedded in
other systems such as databases, and tend to be pretty awful (e.g. not
supporting namespaces, so the element name includes the prefix and
colon, or half-supporting namespaces so there's no way to bind a prefix
and no local-name() function, or supporting an undocumented mixture of
Xpath 1 and 2 features...).

The situation seems a lot better with XQuery than with XPath, I think
largely because of developer and user motivation.

Liam

-- 
Liam Quin - XML Activity Lead, W3C, http://www.w3.org/People/Quin/
Pictures from old books: http://fromoldbooks.org/
Ankh: irc.sorcery.net irc.gnome.org freenode/#xml
Co-author, 5th edition, "Beginning XML", Wrox, Summer 2012


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