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

From: Michael Kay <mike@saxonica.com>
Date: Wed, 13 May 2009 00:02:30 +0100
To: "'Axel Polleres'" <axel.polleres@deri.org>, <public-qt-comments@w3.org>
Cc: "'RIF WG Chairs'" <team-rif-chairs@w3.org>
Message-ID: <593A988C946045409B511B8991A8B57E@Sealion>
> 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
Received on Tuesday, 12 May 2009 23:03:14 GMT

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