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Re: Meaning of block="substitution"?

From: Curt Arnold <curta@hyprotech.com>
Date: Sat, 2 Dec 2000 11:28:27 -0600
Message-ID: <001301c05c85$4a433c60$f00ba8c0@houston.rr.com>
To: <xmlschema-dev@w3.org>
Cc: <www-xml-schema-comments@w3.org>
I started writing this as a quick response to Roger's question on the
xmlschema-dev list
(http://lists.w3.org/Archives/Public/xmlschema-dev/2000Dec/0012.html) (he
has a course to teach the next two days of course), but on a cursory check
of the schema draft, it doesn't follow what it would have expected.  I
haven't had a chance to research this fully,  maybe I haven't worked out all
the details.

---   from the hip answer to Roger's question -----

I think that you would typically (possibly exclusively) use block on a
reference (not a definition) of an element.  A substitution group target
must be an immediate child of <schema> (not a locally defined element).
You'll have to really did into the schema draft to see under what conditions
block is allowed, but in some ways it should follow similar rules to
minOccurs and maxOccurs (which make sense in the context of a complexType
definition, but doesn't make sense for a global definition).

If you are able to use a block attribute on a global definition, it is
possible that a reference in a particular complexType would change it.

<xsd:schema xmlns="http://www.example.org/train"
targetNamespace="http://www.example.org/train"
xmlns:xsd="http://www.w3.org/2000/10/XMLSchema">
    <xsd:element name="subway" type="xsd:string" block="substitution"/>
    <xsd:element name="train" substitutionGroup="subway" type="xsd:string"/>
    <xsd:element name="notrains">
        <xsd:complexType>
            <xsd:element ref="subway" block="substitution"/>
        </xsd:complexType>
    </xsd:element>
    <xsd:element name="anything">
        <xsd:complexType ref="subway" block=""/>
    </xsd:element>
</xsd:schema>

-------- End of naive answer --------------

So I went to check the schema draft to see if I got things right and found a
few things that seemed counter-intuitive.

Section 4.3.2
Schema Representation Constraint: Element Declaration Representation OK

1.3  If the item's parent is not schema, then 1.3.1  One of ref or name must
be present, but not both;
1.3.2  If ref is present, then all of complexType, simpleType, key, keyref,
unique, nullable, default, fixed, block and type must be absent, i.e. only
minOccurs, maxOccurs, id are allowed in addition to ref, along with
annotation;

In section 1.3.2, I expected block to be allowed on a ref (and possibly
prohibited if name was present).  What sense does block="substitution" make
unless it can be applied to a reference to an examplar.  If it has to appear
or an element reference, the only thing is does is dispute another element's
attempt to use it as an examplar.

Disallowing default, fixed, and nullable on references is also questionable
(but off topic)

The schema for schema does not appear to allow block="" or block="none" (to
negate a blockDefault="substitution") though the description for disallowed
substitutions implies that a value that normalized to an empty string would
be acceptible.  It would seem useful that if you could set a strict value
for blockDefault (and it probably goes for fixedDefault too) that you should
be able to override it with a more permissive value.
Received on Saturday, 2 December 2000 12:27:59 GMT

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