- From: Peter Coppens <pgp.coppens@pandora.be>
- Date: Sat, 20 Dec 2003 20:25:56 +0100
- To: "'Michael Brundage'" <xquery@comcast.net>, <www-ql@w3.org>

Michael, Thanks for your reply. Perhaps you can educate me on where I went wrong, because it indicates a fundamental misunderstanding of how to read BNF's from my side, and I would certainly like to avoid that in the future. I always thought that the EBNF (A.1) and the precedence rules (A.4) contain "duplicate" information. In other words the non-terminals of the EBNF or chosen so that they "reduce" in a way that the operator priorities are implemented as expected. A.4 also says that in case of doubt (the same level), the rules are applied left to right (left associative I believe it is called in "grammar" lingo). In other words, and in this case it does not matter, but it illustrates what I want to say 1 + 2 - 3 means ( 1 + 2 ) - 3 The same reasoning led me to believe that "some .... or ...." has to be interpreted as "(some ....) or (....)" and not as "some ... or ...." Can you indicate where my failure is. As a side remark, I am not sure I get the relevance of your explanation on the relative priorities of "and" and "or", but perhaps that is "the/my problem". Thanks for your help, Peter -----Original Message----- From: Michael Brundage [mailto:xquery@comcast.net] Sent: Saturday, December 20, 2003 7:37 PM To: pgp.coppens@pandora.be; www-ql@w3.org 2 Subject: Re: Precedence rules for QuantifiedExpr - OrExpr - AndExpr Hi Peter, I think you're misreading the spec. Both expressions parse as for $x in (1,2,3) where some $y in (1,2) satisfies ((1 = $y) OP ($y =1)) return $x where OP is "or" or "and". The "and" operator has a precedence level immediately greater than "or" -- there is no intervening expression. Quoting from section A.1 of the current (Nov 12, 2003) draft: [55] OrExpr ::= AndExpr ( "or" AndExpr ) [56] AndExpr ::= InstanceofExpr ( "and" InstanceofExpr)* So for example, A or B and C parses as A or (B and C) Hope this helps, Michael Brundage xquery@comcast.net Writing as Author, XQuery: The XML Query Language (Addison-Wesley, 2004) Co-author, Professional XML Databases (Wrox Press, 2000) not as Technical Lead Common Query Runtime/XML Query Processing WebData XML Team Microsoft On 12/19/03 7:36 AM, "Peter Coppens" <pgp.coppens@pandora.be> wrote: > All, > > Looking at the XQuery spec, I am somewhat surprised by the > consequences of the precedence rules for QuantifiedExpr - OrExpr and > AndExpr > > What I mean is: > > Take the query > > for $x in (1,2,3) > where > some $y in (1,2) satisfies 1 = $y and $y = 1 return $x > > Which, I think, is equivalent to > > for $x in (1,2,3) > where > some $y in (1,2) satisfies (1 = $y and $y = 1) return $x > > But now take the query > > for $x in (1,2,3) > where > some $y in (1,2) satisfies 1 = $y or $y = 1 return $x > > > Which, I think, is equivalent to > > for $x in (1,2,3) > where > (some $y in (1,2) satisfies 1 = $y) or $y = 1 > return $x > > I find that rather confusing. > > So I guess I have the following questions > > (1) is the above interpretation correct? > (2) is this a deliberate choice and if yes, are there any motivations for > that decision that can be shared? > (3) would it not be possible to add an extra level of precendence where the > OrExpr comes to sit between QuantifiedExpr and AndExpr, or would that > propagate to have other side effects? > > Thanks, > > Peter >

Received on Saturday, 20 December 2003 14:33:20 UTC