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Subject: [xacml-comment] 5.31 Element <AttributeSelector>


"The AttributeSelector element's RequestContextPath XML attribute SHALL 
contain a
legal XPath expression over the <xacml-context:Request> element."

The phrase 'over the' made me think for a while. This could be made 
clearer by using
the 'context node' term from the XPath specification. XPath evaluation 
with respect to a context node, the context node for this XPath 
expression is the
<xacml-context:Request> element.


"In the case where the XPath expression matches attributes in the 
request context
by AttributeId, it must also match the attribute's data-type with the 
selector's DataType."

Does the 'it' above mean the XPath expression? So, it's saying that if 
you write an
xpath expression to select an attribute from the context, and the 
expression includes
a predicate for matching with an AttributeID, then that expression MUST 
also include
a predicate that matches the selectors data type with the data type of 
the selected


"In the case of using XPath 1.0, the value of the XPath expression is 
either a node-set,
string value, numeric value, or boolean value."

This may seem a quibble, and it probably is, but even though the XPath 
says that the result of an expression can be a primitive... I do not 
believe there's any
way to form an expression that actually returns one. In my experience 
all XPath 1.0
expressions return a node-set. (I'd be very interested to be corrected 
on this point. I
just looked in the o'reilly xpath book and it has some examples that are 
plain literal
values like, 2002, or "hello", but if you follow the grammar of the 
language they're
just not valid expressions.)


"If the XPath 1.0 expression evaluates to a node-set, then each node may 
consist of
a string, numeric or boolean value, or a child node (i.e. structured 
node).  In this case,
each node is logically converted to string data by applying the "string" 
function defined
in the XPath 1.0 specification, resulting in a sequence of string data."

This is correct in spirit, but not actually correct.

In XPath 1.0 an expression evaluates to a node-set. There are seven 
kinds of node
(root, element, text, attribute, namespace, processing instruction, and 
The XPath specification describes a way of determining a <b>string-value</b>
for each type of node.


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