- From: Don Chamberlin <chamberl@almaden.ibm.com>
- Date: Wed, 11 Feb 2004 15:50:38 -0800
- To: public-qt-comments@w3.org
- Message-ID: <OF72758CC2.BEF51B88-ON88256E37.008112A3-88256E37.0082FB7C@us.ibm.com>
(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 Wednesday, 11 February 2004 18:50:54 UTC