RE: [xml-dev] Best Practice: constrain an element's content by (1) a run

["Michael Kay" <mike@saxonica.com>]

I think the best advice is probably: if you can do it conveniently using CTA
(as you can here), then do. Otherwise use assertions.

There are a number of reasons for this.

(1) What is sometimes called the "rule of least power": don't use a chainsaw
to snap a twig.

(2) More concretely:

(2a) A schema validator is more likely to adopt a streaming implementation
for CTA than for assertions

(2b) The schema validator is likely to produce better diagnostics if you
describe the constraint using CTA

(2c) You are likely to get a more precise type annotation on the element if
you use CTA, which gives benefits when writing schema-aware stylesheets and
queries.

Regards,

Michael Kay
http://www.saxonica.com/
http://twitter.com/michaelhkay 


> -----Original Message-----
> From: Costello, Roger L. [mailto:costello@mitre.org] 
> Sent: 11 May 2009 19:53
> To: 'xml-dev@lists.xml.org'
> Subject: [xml-dev] Best Practice: constrain an element's 
> content by (1) a run-time selection of alternate types or (2) 
> a run-time selection of child elements using an XPath expression?
> 
> 
> Hi Folks,
> 
> Consider this book publication:
> 
>     <Publication kind="book">
>         <Title>Everything is Miscellaneous</Title>
>         <Author>David Weinberger</Author>
>         <Date>2007</Date>
>         <ISBN>0-8050-8811-3</ISBN>
>         <Publisher>Henry Holt and Company, LLC</Publisher>
>     </Publication>
> 
> 
> Next, consider this magazine publication:
> 
>     <Publication kind="magazine">
>         <Title>Science News</Title>
>         <Date>2005</Date>
>     </Publication>
> 
> 
> Notice the *kind* attribute in both examples.
> 
> If its value is 'book' then the content of <Publication> is:
> 
>     - Title
>     - Author
>     - Date
>     - ISBN
>     - Publisher
> 
> And if its value is 'magazine' then the content of <Publication> is:
> 
>     - Title
>     - Date
> 
> 
> 
> PROBLEM STATEMENT
> 
> What is best practice for constraining the content of Publication?
> 
> 
> 
> XML SCHEMA 1.1 PROVIDES TWO APPROACHES
> 
> XML Schema 1.1 provides two approaches to constraining the 
> content of the <Publication> element.
> 
> 
> 
> APPROACH #1: ALTERNATE TYPES
> 
> Create a BookType and a MagazineType and then select one of 
> them to be Publication's type depending on @kind:
> 
>    if @kind = 'book' then select BookType
>    else select MagazineType
> 
> 
> Here's how it is expressed in XML Schema 1.1:
> 
>    <xs:element name="Publication" type="PublicationType">
>       <xs:alternative test="@kind eq 'magazine'" 
> type="MagazineType" />
>       <xs:alternative test="@kind eq 'book'" type="BookType" />
>    </xs:element>
> 
> 
> You see the (new) <alternative> element being used to select 
> a type for Publication based on the value of @kind. 
> 
> 
> (I don't show the complexType definition for MagazineType and 
> BookType because it's the same as in XML Schema 1.0.)
> 
> 
> 
> APPROACH #2: XPATH EXPRESSION
> 
> Let the content of Publication be a collection of all the 
> elements (both book elements and magazine elements) and set 
> them optional: 
> 
>     - Title (0,1)
>     - Author (0, unbounded)
>     - Date (0,1)
>     - ISBN (0,1)
>     - Publisher (0,1)
> 
> 
> Then create an XPath expression that selects the set of 
> children for Publication depending on the value of @kind:
> 
>    if (@kind eq 'book') then
>       Title and Date and ISBN and Publisher and 
>           empty(* except 
> (Title[1],Date[1],Author,ISBN[1],Publisher[1]))
>    else
>       if (@kind eq 'magazine') then
>          Title and Date and 
>            empty(* except (Title[1],Date[1]))
>       else
>          true()
> 
> 
> Here's how it is expressed in XML Schema 1.1:
> 
> <xs:element name="Publication">
>     <xs:complexType>
>        <xs:sequence>
>             <xs:element name="Title" type="xs:string"  minOccurs="0"/>
>             <xs:element name="Author" type="xs:string" minOccurs="0"
>                                                        
> maxOccurs="unbounded"/>
>             <xs:element name="Date" type="xs:gYear"  minOccurs="0"/>
>             <xs:element name="ISBN" type="xs:string"  minOccurs="0"/>
>             <xs:element name="Publisher" type="xs:string"  
> minOccurs="0"/>
>        </xs:sequence>
>        <xs:attribute name="kind" type="xs:string" />
>        <xs:assert test="if (@kind eq 'book') then
>                           Title and Date and ISBN and Publisher and 
>                           empty(* except 
> (Title[1],Date[1],Author,ISBN[1],Publisher[1]))
>                         else
>                             if (@kind eq 'magazine') then
>                                Title and Date and empty(* 
> except (Title[1],Date[1]))
>                             else
>                                Title and Date and 
>                                   empty(* except 
> (Title[1],Date[1], Author))" />
>     </xs:complexType>
> </xs:element>
> 
> 
> You see that the content of Publication is all the book and 
> magazine elements and they are optional. 
> 
> You see an XPath expression within the (new) <assert> element 
> being used to constrain which child elements are allowed 
> within Publication based on the value of @kind.
> 
> 
> 
> TWO APPROACHES
> 
> You have seen two ways of solving the problem of constraining 
> the content of Publication:
> 
> (a) Run-time selection of alternate types
> 
> (b) Run-time selection of child elements using XPath
> 
> 
> 
> DEFINITION OF "RUN-TIME" 
> 
> By "run-time" I mean that the content of Publication is not 
> determined until an instance document is validated against a schema.
> 
> 
> 
> WHICH IS BEST PRACTICE?
> 
> Which approach is best practice? 
> 
> What are the pros and cons of each approach?
> 
> 
> /Roger
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