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Re: Clarification request on XPath/XQuery functions and operators regarding implicit casts

From: Axel Polleres <axel.polleres@deri.org>
Date: Wed, 13 May 2009 00:35:17 +0100
Message-ID: <4A0A07B5.40402@deri.org>
To: Michael Kay <mike@saxonica.com>
CC: public-qt-comments@w3.org, 'RIF WG Chairs' <team-rif-chairs@w3.org>
Thanks a lot for the quick and helpful reply!

Axel

Michael Kay wrote:
>> I would kindly ask for advice on the following questions 
>> regarding implicit casts and type promotions in XPath/XQuery functions
>>
>>
>> 1) For instance, fn:year-from-dateTime
>> (http://www.w3.org/TR/xpath-functions/#func-year-from-dateTime)
>> does not mention any implicit casts in its definition.
>> Does that imply a type error if I call
>>
>>   fn:year-from-dateTime(s)
>>
>> with an xs:string value s as parameter, even if s is in the lexical 
>> space of xs:dateTime or is a cast here implicit?
> 
> The only implicit conversions on a function call are those described at
> 
> http://www.w3.org/TR/xpath20/#id-function-calls
> 
> that is, 
> 
> * atomization of nodes
> 
> * conversion of untypedAtomic values to the required type (by casting)
> 
> * (and in other functions) promotion of numeric values, for example integer
> to double.
> 
> There is no implicit casting from xs:string. 
>> On the other hand, fn:years-from-duration
>> (http://www.w3.org/TR/xpath-functions/#func-year-from-dateTime)
>> mentions explicitly casting in its definition:
>>
>> "The result is obtained by casting $arg to an 
>> xs:yearMonthDuration (see 
>> 17.1.4 Casting to duration types) and then computing the 
>> years component 
>> as described in 10.3.1.3 Canonical representation."
>>
>> Would that imply that fn:years-from-duration(s)
>> would accept an xs:string s castable to xs:yearMonthDuration
>> as argument?
> 
> No, this casting is internal to the function, the supplied value must still
> conform to the rules given in the function signature, which require the
> value to be an instance of xs:duration (after atomization and conversion of
> untypedAtomic).
>> In this case is this different treatment of the extraction functions
>> for years from dateTime and duration intended?
>>
>> 2) Similarly,
>> op:divide-yearMonthDuration
>> (http://www.w3.org/TR/xpath-functions/#func-divide-yearMonthDuration)
>> accepts as second argument xs:double.
>>
>> Again the question is whether casting here is implicit, as e.g. the 
>> example suggests:
>>
>> op:divide-yearMonthDuration(xs:yearMonthDuration("P2Y11M"), 1.5)
>>
>> where 1.5 is a decimal value, isn't it?
> 
> The standard function calling rules allow numeric promotion: if the required
> type is xs:double, the supplied value can be xs:integer, xs:decimal, or
> xs:double.
>>
>> In general, I would like to find where in the spec I would 
>> find hints on 
>> whether conversions in functions and operators are meant to 
>> be implicit 
>> or not, and, if they are, then why in some cases, e.g.
>> fn:years-from-duration
>> casting is mentioned explicitly?
>>
> 
> Section 3.1.5 of the XPath language spec.
> 
> Best regards,
> 
> Michael Kay
> Saxonica
> 


-- 
Dr. Axel Polleres
Digital Enterprise Research Institute, National University of Ireland, 
Galway
email: axel.polleres@deri.org  url: http://www.polleres.net/
Received on Tuesday, 12 May 2009 23:36:00 GMT

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