W3C home > Mailing lists > Public > public-qt-comments@w3.org > December 2014

[Bug 27537] New: [XP31] Sequence on lhs of arrow operator

From: <bugzilla@jessica.w3.org>
Date: Mon, 08 Dec 2014 16:27:46 +0000
To: public-qt-comments@w3.org
Message-ID: <bug-27537-523@http.www.w3.org/Bugs/Public/>
https://www.w3.org/Bugs/Public/show_bug.cgi?id=27537

            Bug ID: 27537
           Summary: [XP31] Sequence on lhs of arrow operator
           Product: XPath / XQuery / XSLT
           Version: Last Call drafts
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: XPath 3.1
          Assignee: jonathan.robie@gmail.com
          Reporter: mike@saxonica.com
        QA Contact: public-qt-comments@w3.org

Botond Biró raised this on public-qt-comments:

What is the expected behavior if the arrow postfix operator is applied to a
sequence containing more than one item?

Does the semantic equivalence "$seq=>$f(…) is the same as $f( $seq, … )" also
hold when count($seq) != 1?
ex, ( 1, 2, 3 ) => avg() or ()=>count()

In the last call working draft the wording for arrow postfix definition implies
that a [single] item is expected, but don’t see why or if this restriction
would be necessary.

“[Definition: An arrow operator is a postfix operator that applies a function
to an item, using the item as the first argument to the function.] If $i is an
item and f() is a function, then $i=>f() is equivalent to f($i), and $i=>f($j)
is equivalent to f($i, $j).”

If the single restriction doesn't apply, then I suggest modifying the
definition to reflect this. 

And a question related to operator precedence: currently it is not possible to
use a node selection on the lhs of an arrow operator without wrapping it with
extra parenthesis:
$DAYS//@From => max() - syntax error
($DAYS//@From) => max() - ok
is it planned for the arrow operator to be moved above the step expression(i.e.
get lower precedence) so that one can use a node selection => without the need
of the extra parenthesis?

MHK comment:

I don't think there was ever any intention to restrict the LHS to a single
item. The definition should be:

[Definition: An arrow operator is a postfix operator that applies a function to
an value, using the value as the first argument to the function.] If $s is a
sequence and f() is a function, then $s=>f() is equivalent to f($i), and
$s=>f($j) is equivalent to f($s, $j).

As regards the operator precedence, I can see the case for making 

/section/para => count()

mean count(/section/para)

One can also see a case for richer expressions on the right, e.g.

$input => $function-lib?cosine

For an example of a surprise coming from the current rules, consider

-1 => abs()

which returns -1.

There would seem to be a case for decoupling the arrow operator from the
Postfix construct, and having

[95]       CastExpr ::= ArrowExpr ( "cast" "as" SingleType )?
[NN]    ArrowExpr ::= UnaryExpr "==>" UnaryExpr

-- 
You are receiving this mail because:
You are the QA Contact for the bug.
Received on Monday, 8 December 2014 16:27:48 UTC

This archive was generated by hypermail 2.4.0 : Friday, 17 January 2020 16:57:51 UTC