RE: Possibility of lazy evaluation of static parameters / variables / use-when / shadow attributes

It is interesting that you mention that (the deferred JIT compilation). We are currently failing some tests because we don't raise a static error, simply because a construct is never used and for most constructs (notably declarations), we defer compilation until it is needed. For instance, if a static variable is never used, and it uses itself a variable that is not declared, it may never raise an error (conversely, we *do* raise XPath syntax errors).

Considering that you are on the same path, we may think of something spec-wise? I have not been able to find where we say that static expressions *must* raise an error in the static phase, but it may be inferred from the fact that static errors themselves *must* be raised.

While in my original mail I expressed my preference for requiring such errors to be raised, I think there's something to be said for lazy compilation. XPath (and by extension, XSLT) has a class of errors that are considered static, but these aren't necessarily syntax errors (i.e., a function name that is not found is not a syntax error, but still raises a static error).

The example with an unknown function is interesting: a function may be available from a separate library. Both Java and .NET, iirc, JIT-compile a library on first use. It can be beneficial for performance if a named function call only requires resolution on moment of its use (if never used, the containing library is never called, and is never JITted).

Cheers,
Abel

> -----Original Message-----
> From: Michael Kay [mailto:mike@saxonica.com]
> Sent: Friday, October 07, 2016 9:34 AM
> To: Abel Braaksma
> Cc: Public XSLWG
> Subject: Re: Possibility of lazy evaluation of static parameters / variables /
> use-when / shadow attributes
> 
> The status quo in the spec is that you're not allowed to do any dynamic
> evaluation if the stylesheet contains any static errors, and that would include
> (I think) a dynamic error in the evaluation of a static expression.
> 
> Having said that, we're probably going to start introducing user-selectable JIT
> compilation options that might defer static analysis of some constructs until
> they are first used, whether the spec allows it or not. We're spending too
> much time repeatedly compiling DocBook stylesheets that contain 500
> template rules designed to process elements that don't actually appear in
> your source document.
> 
> Michael Kay
> Saxonica
> 
> 
> > On 7 Oct 2016, at 00:04, Abel Braaksma <abel.braaksma@xs4all.nl> wrote:
> >
> > In many places in the spec we mention or hint that a processor is not
> required to evaluate an unused variable. How does this work for static
> parameters and variables? Do we require them to be processed? I browsed
> through the use-when and shadow attribute sections but couldn't readily
> find this.
> >
> > This may be important, for instance to detect static errors resulting from
> the dynamic evaluation of static expressions.
> >
> > Example:
> >
> > <xsl:variable name="x" use-when="1 div 0 eq 0" />
> >
> > A processor may statically know that this variable is never used. Is it
> allowed to forego the error and process the stylesheet regardless? I'm
> inclined to say no, and that static expressions (use-when, static
> variables/parameters, shadow attributes) must *always* be evaluated. This
> follows from the notion of compile-time errors in other languages, of which
> static expressions are an extension.
> >
> > Other example:
> >
> > <xsl:mode name="m" _streamable="doc('settings.xml')/*/@streamable"
> />
> >
> > It is possible that the document settings.xml contains an invalid setting for
> the streamable attribute. However, if a processor can statically infer that
> mode "m" is never used (it should be able to do so), then does it have to load
> settings.xml and process it?
> >
> > Again, I'm inclined to say yes: static errors must always be reported.
> >
> > It gets fuzzier with static variables/parameters, since we already say about
> variables that they only need to be evaluated when they are actually used:
> >
> > <xsl:variable name="foo" static="yes" select="xs:error('oh oh')"/>
> >
> > If no other static, or dynamic expression uses $foo, can it be ignored? Even
> here, I'd say: no, static expressions can never be ignored and should
> therefore be crafted more carefully than dynamic expressions.
> >
> > Note: no bug raised, I am not sure there's something wrong here.
> >
> > Cheers,
> > Abel
> >
> >
>

Received on Friday, 7 October 2016 15:16:02 UTC