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Re: Confirmation needed: block attribute

From: Jeni Tennison <jeni@jenitennison.com>
Date: Thu, 14 Feb 2002 17:49:31 +0000
Message-ID: <160443923738.20020214174931@jenitennison.com>
To: Priscilla Walmsley <pwalmsley@vitria.com>
CC: Eric van der Vlist <vdv@dyomedea.com>, xmlschema-dev@w3.org
Hi Priscilla,

>> If I specify an empty string in the element definition and nothing in
>> the type definition, the actual value for the element will be "", the 
>> actual value for the type definition will be the default one (thus 
>> "restriction") and the result will still be "restriction". Can you 
>> confirm this (since I find if rather confusing) ?
>
> No, the empty set is different from the attribute being absent. In
> this case, there would be no disallowed substitutions.

I believe that Eric was talking about how this combines with the
prohibited substitutions from the element's complex type. I think that
he was highlighting the situation where you have:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           block="restriction">

<xs:element name="foo" type="fooType" block="" />
<xs:complexType name="fooType" />

<xs:element name="bar" substitutionGroup="foo">
  <xs:complexType>
    <xs:restriction base="fooType" />
  </xs:complexType>
</xs:element>

</xs:schema>

Here, the prohibited substitutions for the fooType is the set
containing the keyword 'restriction'.

This means that, while the element makes no restrictions on what it
can be substituted with, the element's complex type *does* place a
constraint on the allowed substitutions. Specifically, it means that
the foo element cannot be substituted by the bar element, since the
bar element's type is a restriction of the foo element's type.

At least this is my interpretation of Schema Component Constraint:
Substitution Group OK (Transitive):

  For an element declaration (call it D) together with a blocking
  constraint (a subset of {substitution, extension, restriction}, the
  value of a {disallowed substitutions}) to be validly substitutable
  for another element declaration (call it C) all of the following
  must be true:

  1 The blocking constraint does not contain substitution.

  2 There is a chain of {substitution group affiliation}s from D to
    C, that is, either D's {substitution group affiliation} is C, or
    D's {substitution group affiliation}'s {substitution group
    affiliation} is C, or . . .

  3 The set of all {derivation method}s involved in the derivation
    of D's {type definition} from C's {type definition} does not
    intersect with the union of the blocking constraint, C's
    {prohibited substitutions} (if C is complex, otherwise the empty
    set) and the {prohibited substitutions} (respectively the empty
    set) of any intermediate {type definition}s in the derivation of
    D's {type definition} from C's {type definition}.

            http://www.w3.org/TR/xmlschema-1/#cos-equiv-derived-ok-rec

Cheers,

Jeni

---
Jeni Tennison
http://www.jenitennison.com/
Received on Thursday, 14 February 2002 12:49:35 GMT

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