XPath Best Practice: Getting the processor to detect

Hi Folks,

Below is a summary of our discussions.  Comments welcome.  /Roger

Consider this XML document:

<?xml version="1.0"?>
<Book>
    <Title>My Life and Times</Title>
    <Author>Paul McCartney</Author>
    <Date>1998</Date>
    <ISBN>1-56592-235-2</ISBN>
    <Publisher>McMillan Publishing</Publisher> </Book>

Here is an XPath expression to count the number of <Author> elements:

    count(/Book/Authr)

Notice that Author has been accidentally misspelled in the XPath
expression.

The XML document conforms to this XML Schema:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           elementFormDefault="qualified">
    <xs:element name="Book">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="Title" />
                <xs:element ref="Author"  minOccurs="0"
maxOccurs="unbounded" />
                <xs:element ref="Date" />
                <xs:element ref="ISBN" />
                <xs:element ref="Publisher" />
            </xs:sequence>
        </xs:complexType>
    </xs:element>
    <xs:element name="Title" type="xs:string"/>
    <xs:element name="Author" type="xs:string"/>
    <xs:element name="Date" type="xs:string"/>
    <xs:element name="ISBN" type="xs:string"/>
    <xs:element name="Publisher" type="xs:string"/> </xs:schema>

Note that it is particularly important to design the XPath in such a
way that the processor catches the misspelled tag name, since the XML
Schema declares the number of occurrences of the <Author> element to be
0-to-unbounded.  The XPath count function may return a result of 0,
which is a legitimate value and so the misspelling error may go
undetected for a long time.

It should be possible for the XPath processor to detect, by consulting
the XML Schema, that Authr is not a legal child of Book and generate an
error or warning.

And it is possible.  However, it cannot be accomplished entirely within
XPath; features from the host language must be utilized.

For example, if the host language is XSLT then first create a variable
for the <Book> element and use the XSLT variable declaration capability
to specify its type, using the "as" attribute:

    <xsl:variable name="bk" select="/Book" as="schema-element(Book)" />

Then use the variable in the XPath expression:

    count($bk/Authr)

Now the processor will generate an error or warning message.  SAXON
generates this warning: "The complex type of element Book does not
allow a child element named Authr"

Using features from the host language is not an ideal situation.  The
consequence of using host-language-specific features is that the XPath
is not portable: for each host language the XPath must be redesigned
using capabilities from the host language.