XML Processing Model WG

Meeting 42, 2 Nov 2006


See also: IRC log


Norm, Rui, Mohamed, Alex, Henry, Alessandro, Paul, Michael, Andrew, Richard


Accept this agenda?

-> http://www.w3.org/XML/XProc/2006/11/02-agenda.html


Accept minutes from the previous meeting?

-> http://www.w3.org/XML/XProc/2006/10/26-minutes.html


Next meeting: telcon 9 Nov 2006

No regrets given.

Review of open action items

A-13-01: continued

A-41-01: completed

A-41-02: completed

A-41-03: completed

A-41-04: continued

Technical agenda

Review of the 27 Oct draft

-> http://www.w3.org/XML/XProc/docs/ED-xproc-20061027/

Henry: It needs to be possible to specify if parameters are required or optional.

<scribe> ACTION: Norm to address optional/required parameters in the next draft. [recorded in http://www.w3.org/2006/11/02-xproc-minutes.html#action01]

Discussion of select vs. match semantics

Richard: I certainly want to be able to stream, but I'm not convinced that we have to restrict to match patterns because you've got to do some analysis anyway. It doesn't seem to be more difficult one way or the other.
... You've got to be able to not stream in general anyway.
... An implementation is bound to have to do most of this work anyway.

Henry: I don't have as much of a concern about that as I do about the interpretation of bare names.
... I find it odd to write //foo when I want the foo's. I'd have expected "foo" to do that.

Richard: I agree that that's odd, OTOH, there are things that are hard to write as match patterns.

Norm: I think we should use select on input and therefore select everywhere.

Richard: That argument seems much less appealing in the viewport case.
... It's very much like an XSLT identity transform with a single pattern.

Henry: I agree, but I also agree that the consistency argument is a good one.

Richard: We have to do something special in the viewport case anyway, because we can only process the top most match.
... We could do something different there.

Michael: Is the use of match patterns an unquestioned win or only an illusory win for streaming.
... Some people seemed convinced that mattered for streaming, others didn't agree.

Richard: I agree you have to do a fair amount of analysis to be as efficient, but I think you're going to have to do that anyway.
... Something I keep meaning to do is write some code to split a general XPath expression into streamable and non-streamable parts.
... It's just an example of locality of reference.

Alex: One of the things I'm thinking about here that the conservative thing to do is say that it's match patterns.
... I'm concerned that people will push a lot of things into expressions on inputs that are maybe the wrong way to do things in a pipeline.

Richard: I'm not quite sure I see this. Even if we have select expressions, we can only have select expressions that return node sets and probably only elements.
... The only difference in practice that I can see is that there are some things that are difficult to express with match.
... Those are annoying, but maybe they aren't common enough to matter.

Henry: Example?

Richard: How would you write the last chapter?

Michael: //chapter[last()]

Richard: That works if they are all siblings of the same book

Michael: Ok then I need ... (scribe missed)

Richard: To do it as a match //chapter[not(following::chapter)]
... That's legal but unbelievably expensive in most implementations
... There's a reasonably efficient expression for it, but not as a match pattern.

<richard> (//chapter)[last()]

Richard: I raise this case because someone really wanted to do that here recently
... It was in a program that only used match patterns.

Alex: I like match better and I still think it's the conservative thing to do.

Murray: I think that what I've heard is that some want to do select and others want to do match.
... The issue is that we can't do both because it's too much of a burden on implementations.

Richard: No, if you could do select then you don't need match.

Norm expresses concern that giving users both ways of doing it makes everything harder and more confusing.

<MSM> [MSM silently agrees with Norm: choose one, don't make the user choose.]

Straw poll: for input and for-each, and leaving aside viewport for a moment, do you prefer select semantics or match semantics.

Results: 7 for select, 3 for match, one for allowing users to choose either at their discretion

Norm: Is there anyone who objects to accepting select semantics as the consensus view.

None heard.

What about viewport?

Alex: If we have select semantics, I still think that match is the right semantic for viewport.

Norm: Viewport is special because we only want the outer-most subtree.

Alex: I think the match semantics work really well here.

Richard: I agree.

Norm: I suppose I could live with viewport being different, but it's not my first choice.
... I still prefer to say that it's select with a special rule.

Alex: But that special rule is really hard to figure out.
... Well, I suppose if the ancestor is in the results....

Richard: I've written numerous programs to do this and it always seems natural to have a match pattern and not recurse after you match.
... Some programs have had to recurse, but that's not always a clear option. But for viewport, it's just impossible.
... Sometimes what you want in that case is to recurse on the output of the viewport.
... I suppose it could work the other way around, do the inner ones first and the move outward.

Norm: Let's not do that one.

Richard: Right, that's my point. Once you start to consider recursion, there are just too many options.

Henry: I think the core is, how is a bare name interpreted.

Straw poll: for viewport, do you prefer select semantics or match semantics.

Results: 4 select, 5 for match; Henry asserts Micheal would have voted match if he was still here.

Paul: If we're going to go with select on some things and match on others, they better be different attribute names.

Norm: anyone object to accepting as consensus that viewport will have match semantics.

None heard.

<scribe> ACTION: Norm to update the draft to reflect the chosen semantics for selection on input, for-each, and viewport. [recorded in http://www.w3.org/2006/11/02-xproc-minutes.html#action02]

Henry: The interpretation is slightly different in the case of an input port on a parameter. Certain things are legal there that aren't legal elsewhere and the interpretation is different.

<ht> concat('a', /foo)

Any other business

Norm: When do we publish another public draft?

Norm proposes December

Alex/Henry: Let's do it next Friday

Some discussion of when the publishing blackout period for the AC meeting begins

Proposal: new public working draft on 17 November

Norm wants to answer the declare-* or not question before 17 November, expects to devote next meeting to that.

No objections to a new public WD on 17 November.


Summary of Action Items

[NEW] ACTION: Norm to address optional/required parameters in the next draft. [recorded in http://www.w3.org/2006/11/02-xproc-minutes.html#action01]
[NEW] ACTION: Norm to update the draft to reflect the chosen semantics for selection on input, for-each, and viewport. [recorded in http://www.w3.org/2006/11/02-xproc-minutes.html#action02]
[End of minutes]

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$Date: 2006/11/03 12:02:54 $