Re: [xsl] Are XPath expressions parsed using compiler parsingtechniques?

Subject: Re: [xsl] Are XPath expressions parsed using compiler parsingtechniques?
From: "Alain Couthures alain.couthures@xxxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>
Date: Tue, 10 May 2022 05:59:19 -0000
 Hi Roger,
The second version of Fleur, my own XQuery engine written in Javascript,
is also including its own handwritten parser. This one is a Javascript
port from the XSLT stylesheet of old XSLTForms versions which parsed
XPath expressions into Javascript instructions. It can be tricky for a
homemade parser to consider, first, "if" as an element name, then, "if(a
eq b)" as a function call, then "if(a eq b) then" as the beginning of an
If-then-else statement, because "if" is not a keyword.
About errors detection, it is quite helpful for developers to locate each
error within the corresponding source text. This location collect has to
be implemented within the parser.
Static errors are to be detected at parser/compiler step. Some are
grammatical errors and parsing will detect them. Others are due to type
errors: after parsing, some kind of evaluation has to be performed not
with values but with types (sequence types, actually), and sometimes
union of types, to avoid most type checks at runtime. It is also surely
possible to evaluate static expressions, such as 3 + 5, and to normalize
values, such as 7.00. For optimising the compiler output, it is
interesting to consider that atomization is useless on atoms. For
runtime, considering position predicates, such as [1], is a common
Dynamic errors handling requires links to source within the result of the
parser/compiler. It is also required for a step debugger.

  Le 10/05/2022 00:42, Michael Kay mike@xxxxxxxxxxxx <xsl-list-service@lists.> a C)crit :

  The XPath parser used in Saxon is a hand-written hybrid of a
  recursive-descent parser and a precedence parser.
  The very first version was actually derived from James Clark's xt
  parser, and over the years it evolved out of all recognition, but
  without ever being redesigned from scratch. I have no idea if
  throwing it away and starting again with a generated parser would
  give any benefits. I suspec that having a hand-written parser gives
  us more control over diagnostics and error recovery, and it also
  enables us to support multiple grammars (different versions of XPath
  and XQuery, plus XSLT patterns) within a single parsing framework.
  XPath (and even more so XQuery) has a lot of ad-hoc rules for
  resolving ambiguities, for example the rule that in the expression (/
  or /*), "or" parses as an element name, not as a binary operator (I
  don't think this ambiguity was even discovered for many years after
  XPath 1.0 was published). Again, I think it's probably easier to
  implement such ad-hoc rules with a hand-written parser. But someone
  who understands a particular parser generator well could probably
  find a way to do it.
  Michael Kay Saxonica

    On 9 May 2022, at 21:35, Roger L Costello costello@xxxxxxxxx <xsl-list-service@lists.> wrote:
    Roger wrote:
    >> Are XPath expressions parsed using >> compiler parsing
    Michael Kay responded:

      Yes, of course

    Hi Michael, does that mean each time Saxon encounters a place in
    an XSLT program where an XPath expression is expected, Saxon
    sends the expression into an XPath parser which tokenizes the
    expression, parses it into a syntax tree, and then traverses the
    tree to evaluate the expression? Did you use a parser generator
    to auto-generate the parser? If yes, which parser generator did
    you use? If you didn't use a parser generator, why not?

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