The reference to XPATH 2.0 isn't relevant, XPATH 1.0 != XPATH 2.0.
And at the end of the letter you point out yourself that you wouldn't
want to use empty node-sets as the context but that is exactly what your
proposal requires.
Yaron
Assaf Arkin wrote:
> Yaron Y. Goland wrote:
>
> > I realize I'm being thick but I don't believe the sections you quote
> > solve the problem. The key issue is - is there such a thing as an
> > empty node in XPATH? I believe the answer to be no.
>
> I believe the answer is found in the XPath data model specification:
>
> "Document Nodes *must* satisfy the following constraints.
>
> 1.
>
> The *children* *must* consist exclusively of Element, Processing
> Instruction, Comment, and Text Nodes if it is not empty.
> Attribute, namespace, and Document Nodes can never appear as children
>
> 2.
>
> If a node /N/ is among the *children* of a Document Node /D/, then
> the *parent* of /N/ *must* be /D/.
>
> 3.
>
> If a node /N/ has a *parent* Document Node /D/, then /N/ *must* be
> among the *children* of /D/."
>
> http://www.w3.org/TR/xpath-datamodel/#DocumentNode
>
> You may also want to look at XPath implementations to see what they
> accept as context node as part of their formal API. I will be interested
> in hearing of an implementation that doesn't accept an empty context
> node. For example, the Java 1.5 XPath library:
>
> http://java.sun.com/j2se/1.5.0/docs/api/javax/xml/xpath/package-summary.html
>
> (you can see the example at the bottom)
>
> > An alternative argument is - is there such a thing as an empty
> > node-list in XPATH? There I think the answer is potentially yes. But
> > it isn't clear to me if an empty node-list can be legally used as a
> > context for an expression because all de-references would be illegal.
> > E.g. /foo would be an automatic fault but "/" would NOT. That worries
> > me alot. I think it's misleading to allow "/" to work but "/foo" not
> > to. I think it's better for users if we just ban "/".
>
> I don't know of many implementations that support a node-set as an
> evaluation context, and I wouldn't want to rely on anything that's
> implementation specific.
>
> Assaf
>
> >
> > Yaron
> >
> > Assaf Arkin wrote:
> >
> >> Yaron Y. Goland wrote:
> >>
> >> > This proposal is only relevant if we don't decide to adopt a
> >> mechanism
> >> > to map properties to XPATH variables.
> >> >
> >> > As for the desire to use XPATH 1.0 in a standard manner,
> >> unfortunately
> >> > that is quite impossible given our semantics. It is explicitly
> >> illegal
> >> > to have a null context node in XPATH 1.0. In section 1, the
> >> > introduction to XPATH 1.0, a series of requirements are listed for
> >> > defining the context in which a XPATH executes. List in there is a
> >> > node (explicitly referred to as the context node). This is then
> >> > followed by a requirement for a pair of non-zero positive integers
> >> > identifying the context position and context size. If the context is
> >> > empty or null then these two numbers would have to be 0 and that's
> >> > explicitly prohibited. Hence why we already are forced into a
> >> > situation where we have to require a BPEL specific XPATH processor.
> >>
> >> It is illegal to have a null context node, it is permissible to have an
> >> empty context node. My suggestion takes XPath into account so it doesn't
> >> violate any XPath rules. So my suggestion still stands, now let me
> >> explain why it works.
> >>
> >> The context size refers to the size of the node-set which contains the
> >> current context node, while the position refers to the position of the
> >> current context node in that node-set. When you begin evaluating an
> >> XPath expression - the initial values are 1 and 1, meaning one element
> >> in the node-set (the context node) in the first ordinal position. It
> >> matters not what the contents of the context node is, i.e. whether it's
> >> a document, an element, a text node, or an empty node.
> >>
> >> The XPath 1.0 specification is easy to misread. If you try to implement
> >> it, you will soon catch this subtelty when you have to implement
> >> functions like count() and position(), but without going deep it's easy
> >> to misread it. I suggest referring to the XPath 2.0 specification, which
> >> is not fundamentally different on this issue, just more expressive and
> >> precise:
> >>
> >> *
> >>
> >> Definition: The *context item* is the item currently being
> >> processed. An item is either an atomic value or a
> >> node.][Definition: When the context item is a node, it can also be
> >> referred to as the *context node*.] The context item is returned
> >> by an expression consisting of a single dot (|.|). When an
> >> expression |E1/E2| or |E1[E2]| is evaluated, each item in the
> >> sequence obtained by evaluating |E1| becomes the context item in
> >> the inner focus for an evaluation of |E2|.
> >>
> >> *
> >>
> >> [Definition: The *context position* is the position of the context
> >> item within the sequence of items currently being processed.] It
> >> changes whenever the context item changes. Its value is always an
> >> integer greater than zero. The context position is returned by the
> >> expression |fn:position()|. When an expression |E1/E2| or |E1[E2]|
> >> is evaluated, the context position in the inner focus for an
> >> evaluation of |E2| is the position of the context item in the
> >> sequence obtained by evaluating |E1|. The position of the first
> >> item in a sequence is always 1 (one). The context position is
> >> always less than or equal to the context size.
> >>
> >> *
> >>
> >> [Definition: The *context size* is the number of items in the
> >> sequence of items currently being processed.] Its value is always
> >> an integer greater than zero. The context size is returned by the
> >> expression |fn:last()|. When an expression |E1/E2| or |E1[E2]| is
> >> evaluated, the context size in the inner focus for an evaluation
> >> of |E2| is the number of items in the sequence obtained by
> >> evaluating |E1|.
> >>
> >> Assaf
> >>
> >> >
> >> > Yaron
> >> >
> >> > Assaf Arkin wrote:
> >> >
> >> >> -1
> >> >>
> >> >> If we decide to manifest properties as independent XPath variables,
> >> >> then we have a syntax that is consistent in how variable values are
> >> >> accessed, directly (the entire variable) or indirectly (a
> >> property of
> >> >> the variable). I happen to be on the side that likes consistency and
> >> >> abhors making changes to XPath implementations.
> >> >>
> >> >> "We have in the past altered XPATH in order to suit our needs. For
> >> >> example, in some BPEL expressions we have banned the use of the
> >> >> global context node, a node whose presence is actually mandated by
> >> >> the XPATH specification. If we are willing to change XPATH that much
> >> >> I see no reason not to change it here as well. In for a dime, in for
> >> >> a dollar."
> >> >>
> >> >> Since we have nothing to pass in the context node, wouldn't it be
> >> >> easier if we use the XPath specification to achieve that instead of
> >> >> forcing implementations to deviate from the specification, write
> >> >> custom XPath implementations (only for BPEL), and so forth. The
> >> XPath
> >> >> specification mandates that you pass a context node, but the XPath
> >> >> specification does not require the context node to have any content
> >> >> in it. You can pass an empty root node (XPath 1.0) or an empty
> >> >> document node (XPath 2.0) and be in full comformance with the XPath
> >> >> specification and the BPEL model at the same time.
> >> >>
> >> >> Let's keep it simple (that's what this issue is all about). Get the
> >> >> semantics we want for expressions, but without having to change
> >> >> existing specifications or implementations.
> >> >>
> >> >> assaf
> >> >>
> >> >>
> >> >> Yaron Y. Goland wrote:
> >> >>
> >> >>> Issue 29 - Simplification of XPath expressions
> >> >>>
> >> >>> Proposal: Require that all arguments to BPEL defined XPATH
> >> functions
> >> >>> must be quoted strings that are statically defined within the XPATH
> >> >>> expression.
> >> >>>
> >> >>> Note: This proposal won't be necessary if we decide to manifest
> >> >>> properties as independent XPATH variables since this would let us
> >> >>> get rid of getVariableProperty.
> >> >>>
> >> >>> Rationale:
> >> >>>
> >> getVariableProperty(getVariableProperty(foo,bar),getVariableProperty(ick,bick))
> >>
> >> >>> is completely legal in XPATH. This is a problem because such an
> >> >>> expression makes it impossible to statically analyze the expression
> >> >>> that contains the function call and determine what variable is
> >> being
> >> >>> referenced. This is problematic for static analysis of the process
> >> >>> because it makes it impossible to determine what variable and
> >> >>> property are being accessed so there is no way to check if that
> >> >>> variable is accessible from that point in the BPEL process
> >> >>> definition or if the variable has the referenced property
> >> defined on
> >> >>> it. Even worse (in my mind anyway) is that the previous plays havoc
> >> >>> with optimizations for compensation handling. If the previous
> >> >>> function was contained within a compensation handler then there
> >> >>> would be no way to know what variable was being accessed and so the
> >> >>> BPEL process would have no choice but to persist all variables
> >> >>> visible from the compensation handler, major yuck!
> >> >>> We have in the past altered XPATH in order to suit our needs.
> >> >>> For example, in some BPEL expressions we have banned the use of the
> >> >>> global context node, a node whose presence is actually mandated by
> >> >>> the XPATH specification. If we are willing to change XPATH that
> >> much
> >> >>> I see no reason not to change it here as well. In for a dime, in
> >> for
> >> >>> a dollar.
> >> >>>
> >> >>> Section 9.1 -
> >> >>>
> >> >>> Add a new paragraph after the following paragraph "Any qualified
> >> >>> names used within XPath expressions are resolved by using namespace
> >> >>> declarations currently in scope in the WS-BPEL document at the
> >> >>> location of the expression.":
> >> >>>
> >> >>> The arguments to all XPATH functions defined in this specification
> >> >>> MUST be given as quoted strings. The previous requirement MUST be
> >> >>> statically enforced. It is therefore illegal to pass into a BPEL
> >> >>> XPATH function any XPATH variables, the output of XPATH
> >> functions, a
> >> >>> XPATH location path or any other value that is not a quoted string.
> >> >>> This means, for example, that getVariableProperty("varA","propB")
> >> >>> meets the previous requirement while
> >> >>>
> >> getVariableProperty($varA,string(getVariableProperty("varB","propB"))
> >> >>> does not. Note that the previous requirement institutes a
> >> >>> restriction which does not exist in the XPATH standard.
> >> >>>
> >> >>>
> >> >>>
> >> ---------------------------------------------------------------------
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> >> >>> For additional commands, e-mail:
> >> >>>
> >> >>>
> >> >>
> >> >
> >> >
> >>
> >
> >
>
