Section 6.2 Validation Plus
The RELAX-NG schema is very good at specifying “parent-child” relationships, in other words, which elements can nest directly under/within other elements. But we have situations where the possible elements depend on grandparents, great-grandparents, or older ancestors. An example is the
<var> element, which is only useful if it is contained somewhere within a
<webwork> element. You can describe these situations with RELAX-NG, but it becomes cumbersome and redundant. So our stategy is to allow some prohibited situations in the RELAX-NG schema, and use an additional stylesheet to identify the prohibited situations. Continuing our WeBWorK example, the RELAX-NG schema makes it appear that
<var> can be used many places, but the “validation-plus” stylesheet provides a helpful message indicating you have used it outside the context of a WeBWorK problem.
To use this stylesheet, simply apply it at the command-line with
xsltproc like any other stylesheet.
xsltproc -xinclude -o report.txt schema/pretext-validation-plus.xsl aota/animals.xml
The output will be a text file that indicates the suspect element by its location in the document tree.
We once used Schematron for this purpose. Its author, Rick Jelliffe, says “Schematron is a feather duster to reach the corners that other schema languages cannot reach.” Our additional stylesheet is similar.
Why do we have two tools for validation? We have explained the necessity of an extra stylesheet. Why not describe the entire grammar in this stylesheet? The reason is that RELAX-NG is a recognized standard, and so can be converted to other formats, and may also be utilized by XML editors or integrated development environments (IDE) to provide features like code completion. Besides, it would be very tedious to provide all the code for checking everything that is possible and everything that is not.