RE: [XQuery] IBM-XQ-007: Last step in a path expression

Another observation on this proposal.
 
It will allow me to get the string value of a node by writing
 
chapter/heading/string()
 
It will allow me to tokenize the value of a node by writing
 
chapter/heading/tokenize(., "\W+")
 
So I might expect that if I want to tokenize the string value of a node,
I can write:
 
chapter/heading/string()/tokenize(., "\W+")
 
But I can't! I now need to write the expression using a completely
different style, for example
 
chapter/heading/tokenize(string(), "\W+")
 
Michael Kay
 
 

-----Original Message-----
From: public-qt-comments-request@w3.org
[mailto:public-qt-comments-request@w3.org] On Behalf Of Don Chamberlin
Sent: 11 February 2004 23:51
To: public-qt-comments@w3.org
Subject: [XQuery] IBM-XQ-007: Last step in a path expression



(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