My XPath mistakenly referenced an element that doesn't

Hi Folks,

Here is my (very simple) XML document:

	<Document>Hello, world</Document>

My XSLT program contains a xsl:value-of with a simple XPath expression:

	<xsl:template match="/">
	    <xsl:value-of select="Document/foo eq 'abc'"/>
	</xsl:template>

In the XPath expression I mistakenly referenced an element -- foo -- that does
not exist.

I ran the XSLT program on the XML document. No error was generated.

My colleague argues that such behavior is bad language design:
---------------------------------------------------
Languages which define such mistakes to just return "empty" node lists or
false, or such are not helping anybody. They just turn author mistakes into
silent, hard-to-detect behaviors.  In my view this is a major mistake in the
XPath language.

All path expressions should be strongly, statically type-correct, so
Document/foo has to be a possible path. But if element foo is optional, then
any given instance may not have element foo and so a path like Document/foo
can be type correct, but meaningless for a particular data document. One can
explicitly test, e.g.,

if ( exists(Document/foo) ) then (Document/foo eq 'abc') else....

If you just use the expression without this test, and node foo doesn't exist,
then it should cause a failure.
---------------------------------------------------

Do you agree with my colleague's assessment? Is this behavior in XPath an
indication of bad language design?

/Roger