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Re: Composability

From: Norman Walsh <ndw@nwalsh.com>
Date: Wed, 06 Jun 2007 12:23:54 -0400
To: public-xml-processing-model-wg@w3.org
Message-ID: <87ps4959wl.fsf@nwalsh.com>
/ ht@inf.ed.ac.uk (Henry S. Thompson) was heard to say:
| Jeni Tennison writes:
|
|> Norman Walsh wrote:
|>> / Jeni Tennison <jeni@jenitennison.com> was heard to say:
|>> [...]
|>> | and call this new pipeline in the same way as before:
|>> |
|>> |   <my:matching-documents xmlns:my="http://www.example.com/ns/jeni">
|>> |     <p:option name="test" value="/my:foo[@bar = $bar]" />
|>> |   </my:matching-documents>
|>> |
|>> | The value '/my:foo[@bar = $bar]' is passed as the value of the test
|>> | option to <jt:matching-documents> as a string. But in my pipeline,
|>> | where <p:matching-documents> is called, there's no binding for the
|>> | 'my' prefix, nor for the $bar in-scope option, so the pipeline fails.
|>> Perhaps we should allow options to be passed into called
|>> pipelines. At
|>> the moment, it's only forbidden by the special rules for constructing
|>> the p:pipeline environment. We could fix that.
|>> Then, if $bar is in scope for the pipeline that calls
|>> my:matching-documents, it would be in scope for the step that
|>> evaluates it. Options would be passed down.
|>
|> But then they shouldn't be options. The distinguishing feature of
|> options is that they are predefined, and if you're passing in name/value
|> pairs from the calling pipeline, then those pairs can't be known in advance.
|
| I'm not clear why that's at issue here.  We're talking about the
| availability of option _bindings_ for discharging variable references
| in XPaths, for which we've already established that all in-scope
| option bindings are available.  The only question is, should named
| pipelines be different from other steps in what constitutes the
| in-scope bindings.  I agree with Norm that I can't see any reason why
| there should be a difference, so the '$bar' part of your example
| should work just fine.

The problem is that pipelines are defined externally so lexical scoping
doesn't work.

Sure, this works:

  <p:pipeline>
    <p:option name="bar" value="value"/>

    <p:group>
      <p:option name="foo" select="$bar"/>
    </p:group>
  </p:pipeline>

But if the p:group is instead a pipeline, it looks like this:

  <p:pipeline>
    <p:import href="some-library.xml"/>
    <p:option name="bar" value="value"/>

    <px:pipeline/>
  </p:pipeline>

  ...some-library.xml...

  <p:pipeline name="pipeline" namespace="px:">
    <p:option name="foo" select="$bar"/>
  </p:pipeline>

I think I expect the reference to $bar to be an error in that
external pipeline.

| Namespaces are a different question, but my _inclination_ is to say
| that we should at least consider what is from the _user_ perspective a
| very simple statement: prefixes in XPath expressions in attribute
| values in XProc pipeline documents are expanded wrt the namespace
| bindings of their parent EII.  What implementations have to do to
| achieve this is their problem.  At first blush, doesn't seem too bad
| to me -- options whose values are XPaths (oops, here comes option
| value typing again :-) have to carry their namespace binding context
| with them.

That's what my implementation does and I haven't yet constructed an
example where it doesn't work, but I haven't had time to try very hard
either.

                                        Be seeing you,
                                          norm

-- 
Norman Walsh <ndw@nwalsh.com> | Everything should be made as simple as
http://nwalsh.com/            | possible, but no simpler.

Received on Wednesday, 6 June 2007 16:24:04 GMT

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