Re: [Monetdb-developers] Monetdb-developers Digest, Vol 4, Issue 13

Hi Jens, I read your recursion proposal, but I do not really understand the need or goal for such a syntax extension: - is't the default XQuery function recursion more powerful? - why not confine oneself to have an *internal* algebraic closure operator, if that is such a useful abstraction? Nor do I understand the need for the focus on nodes and set fixpoint semantics. If the result expression is a sequence of nodes you may define this to be duplicate-free and in document order, but it is less obvious why atomic sequences should be treated like that. Order is a first-class citizen in XQuery, and atomic types are also first-class. And what about element construction? Isn't it obvious given the recursive nature of XML that people may want to build XML documents with recursion -- so the choice to union all results always is limiting. I can imagine that *some* XQuery recursion patterns could be translated to this fixpoint operator. Tail recursion is the big "success story" here (given your node-union fixpoint semantics it will actually only be possible in rare cases). However, given the fact that XQuery has for-loops, it is quite unlikely in the first place that in the real world people will use tail-recursion for iteration (this may distinguish XQuery from "purer" functional languages). I conclude that this extension per-se does not help in translating recursion in XQuery for the algebra backend at all (nor is it its goal), rather places a second recursive vehicle beside it. I am completely puzzled to hear such proposals from the usually XQuery-standard-respecting community in Garching. Finally, what exactly is "hacky" about the milprint_summer recursion approach? Maybe you mean with "hacky" that it does not try to eliminate recursion, and thus only works if the target language offers recursion. The reality is that only trivial recursion patterns can be eliminated, so this limitation will apply always. And you may forget, that a lot of recursion in milprint_summer *is* eliminated, as all calls inside a for-loop are reduced to one thanks to "loop-lifting" (maybe we could even "sell" this idea in the FP community). So actually it surely is not as primitive/hacky as the implementation of recursion in say Galax. Defining myself as an engineer, I am aware of the limitations of my knowledge and understanding of such formal issues. ready to be educated.. Peter -----Original Message----- From: monetdb-developers-bounces@lists.sourceforge.net [mailto:monetdb-developers-bounces@lists.sourceforge.net] On Behalf Of monetdb-developers-request@lists.sourceforge.net Sent: Saturday, September 23, 2006 12:14 AM To: monetdb-developers@lists.sourceforge.net Subject: Monetdb-developers Digest, Vol 4, Issue 13 Send Monetdb-developers mailing list submissions to monetdb-developers@lists.sourceforge.net To subscribe or unsubscribe via the World Wide Web, visit https://lists.sourceforge.net/lists/listinfo/monetdb-developers or, via email, send a message with subject or body 'help' to monetdb-developers-request@lists.sourceforge.net You can reach the person managing the list at monetdb-developers-owner@lists.sourceforge.net When replying, please edit your Subject line so it is more specific than "Re: Contents of Monetdb-developers digest..." Today's Topics: 1. Upcoming feature: Recursion in Pathfinder (algebra) (Jens Teubner) 2. configure & *_config.h include files (Stefan Manegold) 3. Re: configure & *_config.h include files (Stefan Manegold) 4. Error with the Java Client (Jim Foley) 5. Re: Error with the Java Client (Fabian Groffen) ---------------------------------------------------------------------- Message: 1 Date: Fri, 22 Sep 2006 14:11:03 +0200 From: Jens Teubner Subject: [Monetdb-developers] Upcoming feature: Recursion in Pathfinder (algebra) To: monetdb-developers@lists.sourceforge.net Cc: Loredana Afanasiev Message-ID: <20060922121103.GF11762@notekemper14.informatik.tu-muenchen.de> Content-Type: text/plain; charset=us-ascii Hi all, this is to keep you informed what we are currently working on in Munich. Pathfinder's purely relational approach so far cannot handle *recursion* in XQuery expression in a sensible way (recursion in the W3C XQuery language is possible by means of user-defined recursive function). The milprint_summer branch includes some hacky implementation (with help of MIL PROCs) that provides recursion nevertheless. Together with people from UVA (Loredana Afanasiev in particular), we are currently working on support for recursion in an algebraic manner. A generic handling of recursion seems quite difficult. The W3C XQuery language syntax does not pose any restrictions on the style of recursion that can be described. The common approaches to deal with recursion in relational systems, in contrast, cannot cope with all these variants of recursion. A second problem is that the kind of recursion used in a given query (e.g., tail recursion) used in a given query seems quite tricky to infer (and a consistent treatment of this case in the algebraic compiler adds another challenge). Hence, we decided to go for a limited recursion feature in Pathfinder first. We may later on try to add a more generic handling. We are currently implementing a *syntax extension* to Pathfinder that allows to describe a limited notion of recursion. The new construct with $var seeded by expr1 recurse expr2 will have the following semantics: 1. expr1 is evaluated once and then used as a seed to initiate the recursion. $var is bound to the result of expr1 in the first recursion step. 2. Then, expr2 is evaluated over and over. Variable $var is visible in expr2 and for each recursion step will be bound to the union of all items (nodes) obtained in the previous recursion steps. (The results of all recursion steps are collected into the overall result using set semantics.) 3. Recursion terminates if a recursion step does not contribute any new items (nodes) to the overall result (i.e., we reached a fixpoint). (Remark: There still is some discussion if (for performance reasons) each recursion step should only observe the *new* items obtained in the previous recursion step, or whether the entire result set collected so far should be observable in expr2.) The `union' semantics of this new recursion construct implies that we can do recursion only on *nodes*, not on atomic values. This quite directly matches the requirements of the UVA group (and others) that are looking for a "transitive closure" operator. Note that the above syntax extension that we are about to introduce does not interfere with any existing constructs. So our changes will not affect any (existing) valid XQuery expression. Motivation: There is a high demand for transitive closure functionality in the group at UVA (i.e., Loredana and others), but also in other research groups (as I was told). Transitive closure is, e.g., a vital component of an XPath dialect known as "Regular XPath". Regards from Garching Jens -- Jens Teubner Technische Universitaet Muenchen, Department of Informatics D-85748 Garching, Germany Tel: +49 89 289-17259 Fax: +49 89 289-17263 Software is like sex: It's better when it's free -- Linus B. Torvalds