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[Bug 1523] [FO] fn:subsequence and negative start position

From: <bugzilla@wiggum.w3.org>
Date: Sun, 10 Jul 2005 22:10:31 +0000
To: public-qt-comments@w3.org
Cc:
Message-Id: <E1Drk0N-0008ML-9D@wiggum.w3.org>

http://www.w3.org/Bugs/Public/show_bug.cgi?id=1523


mike@saxonica.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|WONTFIX                     |




------- Additional Comments From mike@saxonica.com  2005-07-10 22:10 -------
I think this is a case (there are many others in F+O) where the spec is trying
to be too helpful, and where it doesn't distinguish clearly enough between rules
and helpful remarks about the consequences of the rules. I think the sentence
"If $startingLoc is zero or negative, the subsequence includes items from the
beginning of the $sourceSeq." falls into the "helpful remark" category. It's an
incomplete statement. Both the suggested answers (1,2,3,4) and (1) in fact have
the property that the result includes items from the start of the input
sequence, so this sentence doesn't help to decide which of these answers is
right. There are other cases (for example subsequence((1,2), -10, 0) where the
sentence is false - in this case the result doesn't include any items from the
start of the input sequence.

Personally, I would structure the specification of this function as:

<quote>
SUMMARY: Returns the contiguous sequence of items in the value of $sourceSeq
beginning at the position indicated by the value of $startingLoc and continuing
for the number of items indicated by the value of $length.

RULES:

In the two-argument case, returns the items in $sourceSeq whose position $p obeys:

fn:round($startingLoc) le $p 

In the three-argument case, returns the items in $sourceSeq whose position $p obeys:

fn:round($startingLoc) le $p and $p lt fn:round($startingLoc) + fn:round($length) 


NOTES

If $sourceSeq is the empty sequence, the empty sequence is returned.

If $startingLoc is zero or negative, the subsequence includes items from the
beginning of the $sourceSeq.

If $length is not specified, the subsequence includes items to the end of
$sourceSeq.

If $length is greater than the number of items in the value of $sourceSeq
following $startingLoc, the subsequence includes items to the end of $sourceSeq.

The first item of a sequence is located at position 1, not position 0.

For detailed type semantics, see Section 7.2.11 The fn:subsequence functionFS
</quote>

Specifying it this way deals unambiguously and economically with awkward corner
cases such as a position or length of infinity or NaN.

(Note also, the current text has an incorrect reference to $sourceString)
Received on Sunday, 10 July 2005 22:10:35 UTC

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