Re: [xsl] XML tags as map keys and impact on XSLT/XPath

[Michael Kay <mike@xxxxxxxxxxxx>]

On 18/06/2010 07:28, Wolfgang Laun wrote:
> Every now and then, people (not me) want to represent a Map<K,V>  in XML by using
> s.th. like
>     <map>
>       <k1>v1</k1>
>       <k2>v2</k2>
>       ...
>     </map>
> with ki from K and vi from V. Apart from the obvious limitation for K's values,
> I feel that this is somehow violating the spirit of XML. But this is not a list
> for XML, and I don't want to risk a red or yellow card.
>
> So, more specifically: Doesn't such a "structure" complicate the writing
> of XSLT constructs? Aren't there any statements or expressions that
> won't be usable at all? (I don't need an exhaustive list of what isn't
> possible - I'm more interested in a general judgment.)
>
>    

I agree: in general it's a poor way of using XML, and it makes it more 
difficult to process using XSLT.

The exception is when the set (k1, k2, k3....) is very predictable and 
unlikely to change. There's room for debate about this. A schema used in 
the XMark benchmark uses the names of continents as element names 
(<Asia>, <Europe>, etc). That feels wrong to me, just as it would feel 
wrong to use the names of continents as Java variable names. There are 
many borderline cases: should one use <home-phone>, <work-phone>, and 
<mobile-phone>, or should one use <phone role="home"> etc? Probably the 
latter, because it makes it easier to change the set of roles and easier 
to process all phone numbers in a generic way. But in the end, drawing 
the line between data and metadata is subjective.

The objection about XSLT processing can in principle be overcome if the 
elements in the set are declared in a schema as having either a common 
type or as members of a substitution group; doing this gives you a 
handle in a schema-aware stylesheet to define match patterns that match 
all elements in the set, or path expressions that select them all. But 
fixing the set of values in a schema in some ways compounds the error, 
because it makes it even more difficult to change the value set over 
time as requirements evolve.

SGML old-timers at this point will start reminiscing about architectural 
forms. I mention this only to point out that it's an issue that has been 
around for a long time.

Michael Kay
Saxonica