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Re: What class of grammars is an =?UTF-8?Q?XPath=3F?=

  • From: rjelliffe <rjelliffe@allette.com.au>
  • To: <xml-dev@lists.xml.org>
  • Date: Fri, 28 Jan 2011 04:28:53 +1100

Re:  What class of grammars is an =?UTF-8?Q?XPath=3F?=
 On Thu, 27 Jan 2011 14:19:41 +0000, Michael Kay <mike@saxonica.com> 

> Ah, you have interpreted the question quite differently from me (and
> perhaps correctly?) I thought it was looking at XPath as a grammar in
> which the sentences being described are document instances.

 Yes.  I did not make myself very clear.I blame the humidity. The syntax 
 of XPath is not my interest.

 XPaths could be considered kinds of catapillar expressions, or indexed 
 grammars, or branching grammars. And probably some combination of them: 
 indexed catapillar pebble grammars? I am interested in whether anyone 
 has done any formal work recently on this. (I suppose if it does not 
 have an adequate formal characterization at the current state of the 
 art, it is in the position of SGML/DTDs in the 80s and 90s, waiting for 
 the right economic moment to become a carrot attractive to academia.)

 That the language may be open is no problem:  if the XPath is   "/x"  
 then it will include simple <x/> or <x><a/></x> or anything with 
 top-level element x.

 The particular expression that Michael brought up -- 
 sum(//order/@value) > 826 -- certainly implies an open language (the 
 document must have at least one //order/@value). There is no necessary 
 difference between the grammar for data content and for elements, so 
 actually the language would consist of all documents that have at least 
 one //order/@value where all //order/@values contain numbers.

 It is possible to


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