Re: compiling XPath to Scheme source code

Hello!

>   I'm working on translating XPath expressions to Scheme
> source code which evaluates the XPath.

Isn't it like the specialization of the general sxpath function of the
type
	sxpath:: SXPath -> SXML -> Nodelist
to the particular value of SXPath? Then we are talking about Partial
Evaluation (PE for short) -- which is a vast and developed area. I
wonder why Michael Sperber hasn't said anything: he wrote several
well-cited papers on this topic (as well as a partial evaluator for
the full Scheme, right Mike?). You can see the extensive bibliography
at

	http://library.readscheme.org/page10.html


> ; Translate the xpath to a Scheme source code
> (define xpath-proc-src2 (xpath->scheme xpath-str2))

> ; A value of the variable "xpath-proc-src2" is:

> ; (node-join
> ;   *root*
> ;   (sxml:child (ntype?? 'AAA))
> ;   (sxml:child (ntype?? 'CCC)))
> ;
> ; "eval" will fail on this expression because the variable
> ; "*root*" is not bound. So the new step appears.

> ; Bind variables.
> (define xpath-proc-src2b
>   (sxlet ((*root* tree2)) xpath-proc-src2))

> ; A value of the variable "xpath-proc-src2b" is:
> ;
> ; (let ((*root* (lambda (dummy) tree2)))
> ;      (node-join
> ;         *root*
> ;         (sxml:child (ntype?? 'AAA))
> ;         (sxml:child (ntype?? 'CCC))))

So, xpath->scheme can generate open code (which has free variables),
and sxlet will generate code that captures those variables with
bindings. Hmm, sounds a bit risky. Dealing with open code is quite
error-prone and difficult. Perhaps we should consider the type of
sxpath to be
	sxpath:: SXPath -> Root -> OtherVar -> SXML -> Nodelist
or
	sxpath:: SXPath -> OtherVarDict -> SXML -> Nodelist
and specialize with respect to SXPath and OtherVarDict. So,

	(xpath->scheme-no-dict xpath-str2)
will generate something like
	(lambda (dict)
	  (node-join
	   (lookup-def '*root* dict)
	   (sxml:child (ntype?? 'AAA))
	   (sxml:child (ntype?? 'CCC))))
whereas
	(xpath->scheme xpath-str2 
	   '((*root* tree2))) ; statically known dict
will generate
       (node-join
	   tree2
	   (sxml:child (ntype?? 'AAA))
	   (sxml:child (ntype?? 'CCC)))


	Cheers,
	Oleg


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