Even without schema awareness/context, surely the processor can
tell when an xpath expression will result in zero match?
No, it's very rarely possible.
For example, consider
<xsl:template match="books">
<xsl:value-of select="book[1]/authour"/>
</xsl:template>
Even if you have a schema telling you that the children of books can
include book but the children of book can't include authour, this
doesn't tell you that this path will necessarily return nothing -
because there is no way you can know that every books element that the
stylesheet processes will be valid against this schema.
In fact it's quite common for a stylesheet to create intermediate
results that (deliberately) aren't valid against the schema for the
input or output document.
It would however be possible to make a guess that this is wrong, and
issue a warning. Incorrect warnings are always very irritating, so this
is not something to be done lightly, but it's something I've wanted to
attempt for a long time.
Michael Kay
Saxonica
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