Hi Peter,

I can of course wait for Jens to reply, but I guess it doesn't hurt to add my 2ct ...

p.a.boncz wrote:

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?

The idea, as I understand it, is to have an "internal algebraic closure operator" (or fixpoint operator). The problem is that XQuery function recursion is too powerful. It is an unsolved problem how to translate arbitrary function recursion to algebra expressions with a fixpoint operator. The quickest way to (1) have some support for recursion, that (2) we know how to translate into an algebra and execute, is to extend the syntax with special recursion construct and disallow any function recursion. This is, I guess, exactly how I would approach it. First restrict to manageable proportions, then try to extend.

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.

The restrictions on set fixpoint semantics and nodes is, I believe, to simply confine first to simple things that the research community researched before. I believe that we first need to check what known techniques apply here, and only then see if we can extend these techniques so that they are better suitable to XQuery.

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 don't agree. The syntax extension does help in translating recursion in XQuery to the algebra backend, because it is a construct for which it is clear how to translate it to a fixpoint operator. "Real-world people" will of course want to write queries with all kinds of weird recursions in them. But "real-world users" will also want their queries to be executed. I think that with a syntax extension it is much easier for "real-world people" to understand what is supported and what is not. If they can squeeze their recursion into this syntax, it is supported. If they cannot, it is not supported. Otherwise, you would have to document which patterns are supported, supply detailed and concise error messages, etc. etc.

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.

How is recursion eliminated by loop-lifting or function inlining? Any recursion would still remain after this, won't it?

Defining myself as an engineer, I am aware of the limitations of my
knowledge and understanding of such formal issues.

Let's not forget that (any variant of) SQL only has a very restricted form of recursion as well. Recursion in a database systems is simply an unsolved problem. In the context of XQuery nothing less.

ready to be educated..

Peter

Just my 2cts,
Maurice.

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