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Radical Parameters Proposal

From: Alex Milowski <alex@milowski.org>
Date: Thu, 24 May 2007 13:15:46 -0700
Message-ID: <28d56ece0705241315q32a15200w2dc9a838488e80aa@mail.gmail.com>
To: public-xml-processing-model-wg <public-xml-processing-model-wg@w3.org>

I feed like we lead ourselves into a deep dark road that
leads into an abyss.  We have some basic requirements
for parameters:

   1. We need to be bind static values at the step-level.

   2. We need to be able to select values for parameters from
       input ports (pipeline or results of other steps).

We've solved the above with the status quo document.

We've also found that:

  1. Different steps in the same "context" (e.g. sibling steps) may
      having conflicting parameter names and values.

   2. We need to be able to pass parameter values in from
       external to the pipeline.

   3. Some would like the ability to produce a parameter name/value
       documents from steps within the pipeline (e.g. use XSLT to
       generate step parameter bindings).

With the status quo, (1) is a problem in that simple parameters
passed in from "the command line" will conflict with the status
quo.  The pipeline author has to do something with options to
make that work with the status quo.

Also, (2) has the consequence in the status quo document that
if you pass in a parameter that for "step A" that unknowingly a
parameter to "step B" that changes how "step B" works, you're
going to get a different result than you might expect.  That's a big
problem (or, at least, I think so).

We just don't have a strong use case that supports (3) (see
the requirements doc [1]).  We could certainly imagine that feature
being useful and imagine more complicated versions of our
use cases, but we haven't and doing so is adding work.

Given all this, here's a radical proposal:

1. We drop in-scope parameters and cascading of parameter values.

2. Parameters can only be bound locally on the step invocation.

3. Options still work as they do and so you can always use
    an option to pass in a parameter value by:

      <p:parameter name="foo" select="$bar"/>

4. We allow the name to be set from a expression (syntax to
    be "discussed):

     <p:parameters match="/params/param" name="@name" select="@value"/>

5. p:group no longer needs p:parameter as there isn't any in-scope

Keep in mind that you can still bind parameters to XPath
expressions against ports.  That means you can pass
a document to a pipeline with parameter name/value pairs
of a syntax of you own choice and then select the values
out of that document.

What this means is that:

   * if a step takes a parameter, you need to author that binding into
     the pipeline.

   * you can only pass arbitrary parameters via an input to
     the pipeline or a constructed result from some step in the pipeline.

   * the author has ultimate control over parameter bindings and so
     no command-line switch is ever going to make strange things

   * if you have two XSLT steps with a need for different parameter
     bindings, then you are just fine because you *have* to
     specify parameter bindings.

Our main use case is XSLT and most parameters to transformations
are "feature switches" or additional datums.  As such, it isn't unreasonable
to say, for V1, that you just have to know the transformation that you
are embedding and if you want to expose a parameter to the user of
the pipeline you either must use an option or bind the parameter value
to a expression against an input port of the pipeline.

I think this leaves the door open to future expansion and control over
parameters and parameter sets.

I did say it was radical and feel free to call me "insane".  :)

[1] http://www.w3.org/TR/xproc-requirements/#use-cases

--Alex Milowski
"The excellence of grammar as a guide is proportional to the paucity of the
inflexions, i.e. to the degree of analysis effected by the language

Bertrand Russell in a footnote of Principles of Mathematics
Received on Thursday, 24 May 2007 20:16:00 UTC

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