- From: Michael Kay <mhk@mhk.me.uk>
- Date: Thu, 12 Feb 2004 10:24:07 -0000
- To: "'Daniela Florescu'" <danielaf@bea.com>, "'Don Chamberlin'" <chamberl@almaden.ibm.com>
- Cc: <public-qt-comments@w3.org>
- Message-ID: <000001c3f152$664e0fb0$6401a8c0@pcukmka>
I do understand the proposal as written, and am ambivalent about it. We do need a mapping operator, but I greatly prefer to use a new operator ("!"), because the semantics are sufficiently different from "/" as to cause confusion if "/" is overloaded, and a general mapping operator would also allow atomic values on the left. It seems odd to me to allow people to do a/name() but not a/name()/string-length(). When people call user-defined functions (or even system-defined functions) they aren't always very knowledgeable about whether the function is returning a set of strings or a set of text nodes, even if they wrote the function themselves. It will be confusing if the two cases behave differently. Michael Kay -----Original Message----- From: public-qt-comments-request@w3.org [mailto:public-qt-comments-request@w3.org] On Behalf Of Daniela Florescu Sent: 12 February 2004 01:32 To: Don Chamberlin 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.htm l) 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 Thursday, 12 February 2004 05:24:01 UTC