In addition to that, I think Saxon EE with schema-aware XSLT 2/3 and
xs:import-schema would warn you about the use of a path like
"Document/foo" if there is no match in the schema for a possible
element. I don't think it is an obligation of a schema-aware XSLT
processor but I think it is a feature of Saxon EE.

Am 14.10.2021 um 15:55 schrieb Wendell Piez wapiez@xxxxxxxxxxxxxxx:
> Roger,
>
> I think your colleague is right, but not very right.
>
> You forget that a missing 'foo' may be an error in one document and a
> feature in another document in the very same system. Indeed it is part
> of the semantics that constitute the reason why we save
> documents,B that we do not always have complete prior knowledge of
> every foo. (Otherwise what are we computing, etc.)
>
> If your schema requires foo, then use a schema-aware XPath engine, and
> your colleague has the feature he wants. That would be an appropriate
> way to layer in the requirement withoutB him having to test every case.
>
> Meanwhile, good luck making your case for why an essential feature is
> not always a problem.
>
> Cheers, Wendell
>
>
>
> On Thu, Oct 14, 2021 at 9:46 AM Roger L Costello costello@xxxxxxxxx
> <mailto:costello@xxxxxxxxx> <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx
> <mailto:xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>> wrote:
>
>     Hi Folks,
>
>     Here is my (very simple) XML document:
>
>     B  B  B  B  <Document>Hello, world</Document>
>
>     My XSLT program contains a xsl:value-of with a simple XPath
>     expression:
>
>     B  B  B  B  <xsl:template match="/">
>     B  B  B  B  B  B  <xsl:value-of select="Document/foo eq 'abc'"/>
>     B  B  B  B  </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.B  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
>
>
>
>
> --
> ...Wendell Piez... ...wendell -at- nist -dot- gov...
> ...wendellpiez.com... ...pellucidliterature.org... ...pausepress.org...
> ...github.com/wendellpiez. <http://github.com/wendellpiez.>..
> ...gitlab.coko.foundation/wendell...
> XSL-List info and archive <http://www.mulberrytech.com/xsl/xsl-list>
> EasyUnsubscribe <http://lists.mulberrytech.com/unsub/xsl-list/582271>
> (by email <>)