Hi Folks,
I am learning the Alloy modelling language. It is so cool. One interesting aspect of Alloy is this rule:
An expression is erroneous if it can be shown
to be redundant, using types alone.
A common and simple case of redundancy is an expression that is equal to the empty relation (set). For example, if sets A and B are disjoint, then (A intersect B) is empty and therefore redundant
(and therefore erroneous).
I am trying to see if any of the XML technologies have an equivalent rule.
It seems that XSD does not. Consider: the following schema says that an XML instance document can never have a <Foo> element:
<xs:element
name="Book">
<xs:complexType>
<xs:sequence>
<xs:element
name="Title"
type="xs:string"/>
<xs:element
name="Author"
type="xs:string"/>
<xs:element
name="Date"
type="xs:string"/>
<xs:element
name="Foo"
minOccurs="0"
maxOccurs="0"
type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
The Foo element declaration is redundant – it declares an element that can never manifest in XML instances. Redundancy is not an error in XSD.
It also seems that redundancy is not an error in XSLT/XPath. Consider: the following XSLT has an XPath expression that can never be satisfied:
<xsl:template
match="/">
<xsl:choose>
<xsl:when
test="name(/*[position()
eq 0]) eq 'foo'">foo</xsl:when>
<xsl:when
test="name(/*[position()
eq 1]) eq 'bar'">bar</xsl:when>
<xsl:when
test="name(/*[position()
eq 1]) eq 'blah'">blah</xsl:when>
</xsl:choose>
</xsl:template>
In the vast universe of XML technologies, is there anywhere a rule that requires redundancy be treated as an error?
/Roger
Cart
