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Subject: Re: [wsbpel] Issue 29 - Proposal For Vote
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. > >> >>> > >> >>> > >> >>> > >> --------------------------------------------------------------------- > >> >>> To unsubscribe, e-mail: wsbpel-unsubscribe@lists.oasis-open.org > >> >>> For additional commands, e-mail: wsbpel-help@lists.oasis-open.org > >> >>> > >> >>> > >> >> > >> > > >> > > >> > > > > >
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