Re: compiling XPath to Scheme source code

  Hello,

On Fri, 19 Nov 2004 08:34:29 +0300
"Oleg A. Paraschenko" <[email protected]> wrote:

...
> 
>   Your ideas are welcome.
> 

  Understanding that dealing with open code is quite error-prone and
difficult, I'm still experimenting with this approach. Here are some
more ideas why this approach is useful.

* Scheme code is more flexible than AST,
* it is well known and documented,
* it provides a sort of AOP.

  There is an advantage of Scheme source code over AST. AST is fixed
and so it can't provide additional functionality. Scheme source code
is flexible, so we can do tricks.

  For example, consider XPath "elem[position()<3]". Its representation
as AST:

(relative-location-path
  (step (axis-specifier (child))
        (node-test (local-name "elem"))
        (predicate
          (< (function-call (function-name "position"))
             (number 3)))))

  Its representation as Scheme code:

(node-join
  (node-reduce
    (sxml:child (ntype?? 'elem))
    (scxpath-filter
      (scxml:relational-cmp
        <
        (scxml:core-position)
        (xpath-value 3)))))

  In case of the Scheme code it's possible to change "scxpath-filter"
to some "take-until". In case of the AST we can't perform such change
without extending AST.

  Another issue is documentation. Scheme code is documented (R5RS)
much better than AST. (Is AST documented at all?)

--

  Use of "sxlet" allows to make local mofidications to XPath execution.
Current SCXPath is used as follows:

(define tree '(*top* (AAA (BBB) (CCC) (BBB) (BBB) (DDD (BBB)) (CCC))))
(define xpath-str "/AAA/CCC")

(define sc1 (xpath->scheme xpath-str))
(define sc2 (sxlet ((*root* tree)) sc1))
(define x (eval sc2 (interaction-environment)))
(pp (x tree))

  First, I create a Scheme code from an XPath string, then I bind
variables using the "sxlet" macro. Having done that, I use "eval"
to make a lambda and finally I apply the lambda to the tree. Result
is as expected:

((CCC) (CCC))

  But macro "sxlet" not only defines variables, it also can redefine
procedures. Example:

(define sc2-b (sxlet (
    (*root* tree)
    (sxml:child
      (lambda (test-pred?)
        (lambda (nodeset)
          ((node-trace "child axis (out)")
            ((sxml:child test-pred?)
              ((node-trace "child axis (in)") nodeset)))))))
  sc1))
(define x-b (eval sc2-b (interaction-environment)))
(pp (x-b tree))

  Now the transcript looks so:

-->child axis (in) :(*top* (AAA (BBB) (CCC) (BBB) (BBB) (DDD (BBB))
(CCC)))
-->child axis (out) :((AAA (BBB) (CCC) (BBB) (BBB) (DDD (BBB)) (CCC)))
-->child axis (in) :(AAA (BBB) (CCC) (BBB) (BBB) (DDD (BBB)) (CCC))
-->child axis (out) :((CCC) (CCC)) ((CCC) (CCC))

  So I've affected XPath evaluation without changing Scheme code.
It is not a superior archievement, but it may be quite useful.



-- 
Oleg Paraschenko


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