- From: Michael Kay <mhk@mhk.me.uk>
- Date: Fri, 13 Feb 2004 10:06:43 -0000
- To: "'Don Chamberlin'" <chamberl@almaden.ibm.com>, "'Daniela Florescu'" <danielaf@bea.com>
- Cc: <public-qt-comments@w3.org>
- Message-ID: <000a01c3f219$156fab90$6401a8c0@pcukmka>
Further candidates for defaulting the argument to the context item are node-name() and document-uri(), if we want consistency (which we do, despite appearances!). Michael Kay -----Original Message----- From: public-qt-comments-request@w3.org [mailto:public-qt-comments-request@w3.org] On Behalf Of Don Chamberlin Sent: 12 February 2004 22:26 To: Daniela Florescu Cc: public-qt-comments@w3.org Subject: Re: [XQuery] IBM-XQ-007: Last step in a path expression Hi Dana, Thanks! Yes, I would support your proposal to make the context item the default operand for data(), and I agree that this enhances the value of the proposal to allow path expressions to return atomic values. --Don Chamberlin Daniela Florescu <danielaf@bea.com> 02/12/2004 12:00 PM To Don Chamberlin <chamberl@almaden.ibm.com> cc public-qt-comments@w3.org Subject Re: [XQuery] IBM-XQ-007: Last step in a path expression I understand Don, thank you. I totally agree with this version of the proposal. However, please remark that the following expression that most of my customers would like to write: $x/a/b/data() would still not be valid because data() needs an explicit argument. Can we also make the current node the implicit argument for data() ( in case there is no argument) ? Best regards, Dana On Feb 12, 2004, at 10:30 AM, Don Chamberlin wrote: > > Dana, > All of your questions are answered in the proposal I cited, originally > posted by Sarah Wilkin, which is very brief, clear and complete. > Please take the time to read it: > http://lists.w3.org/Archives/Public/public-qt-comments/2004Feb/ > 0100.html > > To recapitulate, duplicate elimination and doc-order-sorting is > applied if the result is a sequence of nodes. If the result contains a > mixture of nodes and atomic values, a type error is raised. If the > statically inferred type contains both nodes and values, and static > type checking is in effect, the type error is a static error. This is > consistent with our handling of type errors for all other operators. > > Note that this proposal does not "overload" the "/" operator--it > simply eliminates the unnecessary and unmotivated restriction that > "each evaluation of E2 must result in a sequence of nodes, otherwise a > type error is raised." Eliminating this restriction makes the "/" > operator more useful and does not conflict with any other part of the > language. > > --Don Chamberlin > > > > > > Daniela Florescu <danielaf@bea.com> > > 02/11/2004 05:31 PM > > To > Don Chamberlin <chamberl@almaden.ibm.com> > > cc > public-qt-comments@w3.org > > Subject > Re: [XQuery] IBM-XQ-007: Last step in a path expression > > > > > > > > Don, > > but in this new proposal, under which conditions would you apply > sorting > by doc order and duplicate elimination ? > > What if the dynamic answer contains a mixture of nodes and values? > > And what if the statically inferred type contains both nodes and > values > ? > Don't you want to know at compile time if you have to do a sort or not > ? > > I am not sure I understand the proposal as written. > > Best regards > Dana > > > On Feb 11, 2004, at 3:50 PM, Don Chamberlin wrote: > > > > > (IBM-XQ-007) Section 3.2 (Path Expressions): The definition of a > path > > expression should be revised to remove the restriction that the > > expression on the right side of "/" must return a sequence of nodes. > > > The restriction should be retained for the expression on the left > side > > of "/". In effect, this would permit the last step in a path to > return > > one or more atomic values. This feature has recently been requested > by > > Sarah Wilkin > > (http://lists.w3.org/Archives/Public/public-qt-comments/2004Feb/ > > 0100.html) who proposes the following rule: When evaluating E1/E2, > if > > each evaluation of E2 returns a sequence of nodes, they are combined > > > in document order, removing duplicates; if each evaluation of E2 > > returns a sequence of atomic values, the sequences are concatenated > in > > the order generated; otherwise a type error is raised. Like all type > > > errors, this error can be raised either statically or dynamically, > > depending on the implementation. This rule provides well-defined > > static and dynamic semantics for path expressions. > > > > To illustrate the usability advantages of this proposal, consider a > > document containing "employee" elements, each of which has child > > elements "dept", "salary", and "bonus". To find the largest total > pay > > (salary + bonus) of all the employees in the Toy department, here is > > > what I think many users will write: > > > > max( //employee[dept = "Toy"]/(salary + bonus) ) > > > > Unfortunately in our current language this is an error because the > > final step in the path does not return a sequence of nodes. The user > > > is forced to write the following: > > > > max( for $e in //employee[dept = "Toy"] return ($e/salary + > $e/bonus) ) > > > > This expression is complex and error-prone (users will forget the > > parentheses or will forget to use the bound variables inside the > > return clause). There is no reason why this query cannot be > expressed > > in a more straightforward way. Users will try to write it as a path > > expression and will not understand why it fails. > > > > Another very common example is the use of data() to extract the > typed > > value from the last step in a path, as in this case: > > //book[isbn="1234567"]/price/data(). This very reasonable > expression > > is also an error and the user is forced to write > > data(//book[isbn="1234567"]/price). > > > > Note that I am NOT asking for a general-purpose mapping operator, > > which I think is not in general needed since we already have a > > for-expression. Instead, I think we should simply relax the > unnatural > > and unnecessary restriction that is currently placed on path > > expressions. This will remove a frequent source of errors and will > > improve the usefulness of path expressions, without precluding us > from > > introducing a general-purpose mapping operator later if a consensus > > emerges to do so. > > > > --Don Chamberlin >
Received on Friday, 13 February 2004 05:06:04 UTC