Re: [xsl] My XPath mistakenly referenced an element that doesn't exist and I got no error message ... is this bad language design?

["Wendell Piez wapiez@xxxxxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

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, 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 without 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 <
xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:

> 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
> 
>
>

-- 
...Wendell Piez... ...wendell -at- nist -dot- gov...
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