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RE: empty complexTypes.

From: Jason Diamond <jason@injektilo.org>
Date: Tue, 12 Dec 2000 21:55:02 -0800
To: <xmlschema-dev@w3.org>
Message-ID: <LAEMKGDBDFAKFNKPFEKLEEFFCPAA.jason@injektilo.org>
Thank you! I obviously have a lot to learn about reading schemas. Your
explanation was perfect.

Jason.

> -----Original Message-----
> From: xmlschema-dev-request@w3.org
> [mailto:xmlschema-dev-request@w3.org]On Behalf Of Morris Matsa
> Sent: Tuesday, December 12, 2000 9:24 PM
> To: xmlschema-dev@w3.org
> Cc: Jason Diamond
> Subject: Re: empty complexTypes.
>
>
>
>
> An empty complexType is allowed.  And it is useful -- I've used it in a
> schema that I wrote where the existing DTD and XML format already required
> elements with no content.
>
> I'll go through each of your three definitions of complexType content.
>
> > "Content: (annotation? , (simpleContent | complexContent | ((group | all
> |
> > choice | sequence)? , ((attribute | attributeGroup)* ,
> anyAttribute?))))"
> >
> > This looks like there has to be a simpleContent, complexContent, or one
> of
> > the particles followed by some attribute declarations.
>
> An empty complexType corresponds to the third option.  "(group | all |
> choice | sequence)?" - the "?" means zero or one of these, so zero.
> "(attribute | attributeGroup)*" - the "*" means zero or more, so zero.
> "anyAttribute?" -  zero or one so zero.  Thus this entire third option
> becomes empty, and in turn the whole content is empty.
>
> > The complexTypeModel complexType in the Schema for Schemas shows this:
> > <schema segment deleted, see below>
> > The minOccurs on the choice, element refs, and sequence default to 1,
> don't
> > they?
>
> The minOccurs do default to 1.  For the choice, choose the third option,
> the sequence, and you need exactly one of these.  Which is zero or one
> typeDefParticle (because its minOccurs is zero) followed by exactly one
> attrDecls, because of the defaults of minOccurs and maxOccurs to
> 1.  Choose
> zero typeDefParticle groups followed by one attrDecl group.  This group is
> a sequence that starts with a choice (which has a minOccurs of
> zero and can
> thus be ignored) followed by an anyAttribute element which has a minOccurs
> of zero and can thus be ignored as well.  This means that the attrDecl can
> be empty and thus the whole complexTypeModel can be empty.
>
> > Lastly, the DTD for Schemas shows this:
> >
> > <!ELEMENT %complexType; ((%annotation;)?,
> >                          (%simpleContent;|%complexContent;|
> >                           %particleAndAttrs;))>
>
> Similarly, the %annotation can be empty (zero or one because of the "?").
> The choice of the next three entities can be the %particleAndAttrs, which
> is defined as "((%mgs; | %group;)?, %attrDecls;)".  The choice
> can be empty
> (again zero or one because of the "?"), meaning that the %attrDecls is the
> entire %particleAndAttrs and thus the entire %complexType.  %attrDecls is
> in turn defined as "((%attribute;| %attributeGroup;)*,(%anyAttribute;)?)",
> where the choice can be empty (zero or more, this time a "*"), and the
> %anyAttribute can be missing (again the "?"), thus the %attrDecls can be
> empty and thus the entire %complexType can be empty.
>
>
>
> "Jason Diamond" <jason@injektilo.org>@w3.org on 12/12/2000 06:20:26 PM
>
> Sent by:  xmlschema-dev-request@w3.org
>
>
> To:   <xmlschema-dev@w3.org>
> cc:
> Subject:  empty complexTypes.
>
>
>
> Hi.
>
> 4.3.3 of the Structures CR says:
>
> "Careful consideration of the above concrete syntax reveals that a type
> definition need consist of no more than a name, i.e. that <complexType
> name="anyThing"/> is allowed. The result is however not of much use, as it
> will have empty {content type} and no allowed attributes."
>
> My careful consideration of the syntax revealed something different,
> though.
>
> "Content: (annotation? , (simpleContent | complexContent | ((group | all |
> choice | sequence)? , ((attribute | attributeGroup)* , anyAttribute?))))"
>
> This looks like there has to be a simpleContent, complexContent, or one of
> the particles followed by some attribute declarations.
>
> The complexTypeModel complexType in the Schema for Schemas shows this:
>
>  <group name="complexTypeModel">
>   <choice>
>       <element ref="simpleContent"/>
>       <element ref="complexContent"/>
>       <sequence>
>        <annotation>
>         <documentation xml:lang="en">
>    This branch is short for
>    &lt;complexContent>
>    &lt;restriction base="anyType">
>    ...
>    &lt;/restriction>
>    &lt;/complexContent></documentation>
>        </annotation>
>        <group ref="typeDefParticle" minOccurs="0"/>
>        <group ref="attrDecls"/>
>       </sequence>
>   </choice>
>  </group>
>
> The minOccurs on the choice, element refs, and sequence default
> to 1, don't
> they?
>
> Lastly, the DTD for Schemas shows this:
>
> <!ELEMENT %complexType; ((%annotation;)?,
>                          (%simpleContent;|%complexContent;|
>                           %particleAndAttrs;))>
>
> Am I reading these correctly? Since an empty complexType is essentially
> worthless, I don't see any value in allowing it. Was it removed from the
> schemas but accidentally left in the text?
>
> Thanks,
> Jason.
>
>
Received on Wednesday, 13 December 2000 00:58:44 GMT

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