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Re: What's wrong with namespaces? Some observations and sugges

  • From: Dimitre Novatchev <dnovatchev@gmail.com>
  • To: Michael Kay <mike@saxonica.com>
  • Date: Sun, 5 Dec 2010 16:55:35 -0800

Re:  What's wrong with namespaces? Some observations and sugges
On Sun, Dec 5, 2010 at 3:41 PM, Michael Kay <mike@saxonica.com> wrote:
>
>> An interesting question is how we would write compact XPath
>> expressions for XML2:
>>
>>      /*/head/body
>>
>> should select the :org.w3c.html.body element.
>>
>> Should we have to write:
>>
>>     /*/head/body/:org.w3c.svg.svg
>>
>> or
>>
>> could it be:
>>
>>     /*/head/body/:~svg.svg
>>
>
> My suggestion would be:
>
> select=":org.w3c.html.table"
>
> selects an element with this precise name
>
> select="table"
>
> selects a table element in any namespace - that is, an element that has
> "table" as the final part of its name.
>
> Generally I think it's better if XPath is designed to over-retrieve rather
> than under-retrieve - it's easier for the user to see what they did wrong if
> they get too much data back than if they get nothing back.

Yes,  this principle  is absolutely necessary for a single location step.

When evaluating an expression containing more than one location step a
useful convention can be used:

... /Name1/Name2

Select all "Name2" elements that are children of "Name1" elements and
are in the same namespace as their parents.

That is,  it is necessary to specify the full namespace or a part of
it on a child element, only if the child element is not in the same
namespace as the parent element.

In this way,

   /*/head/body

selects the

  org.w3c.html.body

element. Here the top element, its child "head" and its child "body"
all are in the "org.w3c.html" namespace.

On the other side:

   /*/head/body/svg

selects nothing, because the "svg" element isn't in the namespace
("org.w3c.html" ) of its parent.

To select the "svg" element, either its full namespace or a part of
the full namespace that is unique among all namespaces needs to be
specified:

  /*/head/body/:org.w3c.svg.svg

fully specified namespace,

or:

  /*/head/body/:~svg.svg

partially specified namespace.

If we need to select all children of "Name1" regardless of their
namespace, we have this natural notation:

.../Name1/:*.Name2

Using these rules results in a compact and convenient to use
expression language while it still permits any more verbose form to be
used.

-- 
Cheers,
Dimitre Novatchev
---------------------------------------
Truly great madness cannot be achieved without significant intelligence.
---------------------------------------
To invent, you need a good imagination and a pile of junk
-------------------------------------
Never fight an inanimate object
-------------------------------------
You've achieved success in your field when you don't know whether what
you're doing is work or play


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