Re: What would you like to see in XML::LibXML ?

Michael Ludwig <[email protected]> wrote:
> Aaron Crane schrieb am 29.01.2012 um 14:49 (+0000):
>> I'd particularly value the ability to annotate a node with a Perl
>> scalar, and retrieve that scalar later
>
> Interesting idea. Could you briefly explain what such annotations are
> good for? Or share a pointer?

A somewhat glib answer is that you could use them in exactly the same
ways that libxml2 allows C-language callers to use the _private field
in the node structure.  (For that matter, I assume that's precisely
why libxml2 provides it.)

More specifically, one use I've had is determining whether a given
node in a document-like XML instance represents "phrasing" content.
The sense in which I'm using that term is borrowed with light
modifications from HTML5: certain elements are candidates for phrasing
content (like <em> and <strong> in HTML, and unlike <p> or <li>), but
an element node with non-phrasing descendants should never be
considered phrasing.  I'm glossing over some subtleties here, of
course; for example, text nodes are phrasing, unless they're ignorable
whitespace.

Now, it's easy enough to write a function that takes a node, and tells
you whether it's phrasing in this sense.  But that can require
non-trivial amounts of computation — more than would be desirable in
an inner loop, in some applications.  However, since this function
would be pure (in that only changing the document can change the
answer), it can be straightforwardly cached — as long as you've got
somewhere sane to store the answer.  Currently, the only way to cache
data like that with XML::LibXML is to use attributes in an element
node, and find an attribute name that wouldn't otherwise have any
effect.  Being able to annotate non-element nodes would help in some
circumstances, and being able to use extra-DOM data would help in
others.

-- 
Aaron Crane ** http://aaroncrane.co.uk/
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