[Monetdb-developers] XRPC syntax

Jens Teubner jens.teubner at in.tum.de
Fri Oct 27 12:40:07 CEST 2006

Hi Peter, hi Jennie,

On Tue, Oct 24, 2006 at 09:13:00PM +0200, Ying Zhang wrote:

> I found the formal definition of XQueryD grammar (BTW. it's called
> XQueryD, not DXQ.  DXQ is something else.) in "A Framework for XML-Based
> Integration":
>   ExprSingle ::= "execute at" <URL> [ "xquery" { ExprSingle } | ... ]

this syntax actually looks okay to me as well.

The reason: XQuery is crafted in a way that it uses as few keywords as
possible.  To achieve that, the lexical analyzer (a) often recognizes
patterns that are larger than a single word (matching "execute at"
instead of "execute" removes the ambiguity with a child step searching
for an `execute' element) and (b) switches parsing states depending on
the symbols that the lexer sees.

What we have here is that the lexical analyzer reads an expression.
After reading an expression, it will *always* wait for an operator
(e.g., an arithmetic operator, a sequence constructor, but also `as
type', etc.).  In this state it is safe to recognize a lot more keywords
without making them ambiguous with, e.g., path steps.  So if we throw in
the `xquery' here, the state machine of the lexical analyzer should run
fine again.  `xquery' will be recognized as a keyword whenever an
operator is expected, but not otherwise.

Peter, Pathfinder's parser is strictly separated as it is supposed to be
in a well-designed parser.  There is a lexical analyzer, which is
*independent* of the parser (done with bison in Pathfinder).  Being
unaware of any context information, our lexical analyzer thus cannot
know that it currently is somewhere between the two components of an
"execute at" clause.

If you prefer the above syntax, I could implement that.  It should be an
easy fix.  Just let me know.


Jens Teubner
Technische Universitaet Muenchen, Department of Informatics
D-85748 Garching, Germany
Tel: +49 89 289-17259     Fax: +49 89 289-17263

No scientist is ever right, they just
can't be proved wrong at the time!
                         -- Richard Feynman

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