Re: My XPath mistakenly referenced an element that do

I wanted to put a plug in for the schema-aware usage.

When I was working with a team converting one complex vocabulary to another
complex vocabulary, the error reporting from the schema-aware processing
was incredibly helpful.

Obviously, this only works for cases where those vocabularies exist, but in
those cases, schema-aware error reporting is a fabulous tool.

On Thu, Oct 14, 2021 at 6:22 PM Dimitre Novatchev dnovatchev@xxxxxxxxx <
xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:

> > Even adding req() as a synonym for one-or-more() might be enough to
> encourage people to write req(Document)/req(foo) in order to get better
> diagnostics.
>
> Why not just:
>
>        req(Document/foo)
>
>
> Thanks,
> Dimitre
>
> On Thu, Oct 14, 2021 at 8:17 AM Michael Kay mike@xxxxxxxxxxxx <
> xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
>> Having said all this, of course you (and your friend) are perfectly
>> correct: the fact that "void paths" are not statically detected is a major
>> cause of debugging headaches, and a solution to the problem that prevents
>> those headaches would be highly desirable.
>>
>> I've sometimes wondered whether a "soft" checker (lint-like) that warns
>> you of potentially void paths (by reference to a schema and/or instance
>> document) might not be a valuable tool.
>>
>> Or a run-time checker might be less complicated than a compile-time
>> checker: Add an option selection="strict" at the xsl:transform level, and
>> then
>>
>> Document/foo is treated as one-or-more(Document)/one-or-more(foo)
>>
>> while if you want zero-or-more(Document)/zero-or-more(foo)
>>
>> then you have to use some new syntax like opt(Document)/opt(foo)
>>
>> Even adding req() as a synonym for one-or-more() might be enough to
>> encourage people to write req(Document)/req(foo) in order to get better
>> diagnostics.
>>
>> Or perhaps a "\" operator that behaves like "/" except the RHS is not
>> allowed to be empty.
>>
>> Michael Kay
>> Saxonica
>>
>> > On 14 Oct 2021, at 16:06, Michael Kay mike@xxxxxxxxxxxx <
>> xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>> >
>> > This was the thinking that led to the design of the "static typing"
>> feature in XQuery 1.0, which has been a notable failure. Even it its
>> diluted form as implemented in schema-aware XPath/XQuery without static
>> typing (where Saxon will potentially give you a warning here) it has not
>> been a conspicuous success.
>> >
>> > The main reason, I think, is it requires extra coding effort up-front,
>> and only gives you returns when debugging, later, if you don't get things
>> right first time. Not many people are prepared to make that investment.
>> >
>> > Looking at your example, match="/" will match the document node of any
>> XML document, whatever its type. To catch an error here, you first have to
>> make it clear that the rule is only intended to apply to a particular kind
>> of document, for example by writing
>> >
>> > match="document-node(schema-element(Document))"
>> >
>> > which is quite a bit more verbose than your original; and it also
>> requires you (a) to import the schema for this document type, and (b) to
>> validate the instance against the schema before applying the stylesheet.
>> >
>> > I would still encourage people to do that: it will detect many of your
>> coding mistakes. But realistically, we're all inclined to take the path of
>> least resistance, even if that path leads you into dragon-infested
>> territory.
>> >
>> > Michael Kay
>> > Saxonica
>> >
>> >> On 14 Oct 2021, at 14:45, 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
>> >>
>> >>
>> >
>> >
>>
>>
>>
>
> --
> Cheers,
> Dimitre Novatchev
> ---------------------------------------
> Truly great madness cannot be achieved without significant intelligence.
> ---------------------------------------
> To invent, you need a good imagination and a pile of junk
> -------------------------------------
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> biggest mistake of all
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