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Re: parameters and pipelines (revised)

From: Norman Walsh <ndw@nwalsh.com>
Date: Fri, 06 Jul 2007 14:40:46 -0400
To: public-xml-processing-model-wg@w3.org
Message-ID: <87y7httlxt.fsf@nwalsh.com>
/ ht@inf.ed.ac.uk (Henry S. Thompson) was heard to say:
| We've got several gaps in this area at the moment.
| 1) The current draft doesn't say anything about how you get access to
|    parameters passed to pipelines 'from outside';
| 2) The current draft doesn't say anything about how you get access to
|    parameters passed to pipelines when invoking them by name;
| 3) There is an ednote in the current draft [1] raising the question of
|    whether steps which declare no parameter inputs get the pipeline
|    parameters by default.
| Here are some suggestions about how we deal with this, based in part
| on IRC discussion with Norm (but don't assume he agrees with any of
| this):
| 1) Sets of parameter name/value pairs may be externally specified in
|    two ways, which I'll call 'named' and 'anonymous':
|    named: a named collection of name/value pairs
|    anonymous: an un-named collection of name/value pairs
|    For example
|  > xproc -params xslt1 x=1 y=1 -params xslt2 x=2 y=2 -params debug=no pipe.xpl
|    might be one way to produce two named sets and one anonymous set
|    p:pipeline allows parameter inputs (a parameter input is a static
|    error on any other container).
|    It's a dynamic error if a named parameter set is specified
|    externally for a pipeline which does not have a parameter input of
|    the same name.  An unnamed parameter set is always allowed.
|    Steps can access named parameter sets in the obvious way, e.g.
|      <p:input port='[some param port]'>
|       <p:pipe step='pipe' port='xslt1'/>
|      </p:input>
|    Pipelines with _no_ parameter inputs declared get an anonymous one
|    for free, and implementations MUST make the anonymous set available
|    via it in this case.


|    It is implementation-defined what happens to the anonymous set in
|    the presence of explicit parameter input declarations (e.g. they go
|    to first one, last one, it's an error).

I think it's also implementation-defined what happens to parameters
that specify a named set when the pipeline doesn't have a parameter
input port with that name. It ought to be an error, I think, but I'm
not sure we can force implementors not to just ignore them or do
something else.

|    Steps can access the unnamed parameter set using defaulting:
|      <p:input port='[some param port]'/>
|    [As for ordinary inputs, it's a static error to attempt to bind to
|     an undeclared parameter port.]

Yes, I think this is right, though I'm having trouble keeping the
distinction between:

     <p:input port="parameters"/>
     <p:input port="source"/>

  (where "parameters" are the default parameters and "source" is the
  default readable port)

and this:


clear in my head. I keep thinking there shouldn't *be* a distinction.

|    It's a static error to include p:parameter as the child of a step
|    unless it has exactly one declared parameter port.

Yes, I think that's what we should do for V1.

| 2) A pipeline invoked by name gets its parameters in an analogous way:
|    2a) Iff the pipeline was defined with one or more parameter input
|        ports, the invocation may include a binding for those ports:
|        <p:pipeline type='my:pipe'>
|         . . .
|         <p:input port='xslt1' kind='parameter'/>
|         . . .
|        </p:pipeline>
|        <my:pipe>
|         . . .
|         <p:input port='xslt1'>
|          <p:inline>
|           <c:parameter name='x' value='3'/>
|          </p:inline>
|         </p:input>
|         . . .
|        </my:pipe>


|    2b) Explicit use of p:parameter in a named pipeline invocation
|         i) is a static error if the pipeline declares more than one
|            parameter input;
|         ii) feeds parameters into a declared parameter input if the 
|             pipeline declares exactly one;
|         iii) feeds parameters into an anonymous parameter input otherwise.
|        <p:pipeline type='my:pipe'>
|          <p:xquery>
|           <p:input port='parameters'/>
|           . . .
|          </p:xquery>
|          . . .
|         </p:pipeline>
|         <my:pipe>
|          <p:parameter name='forXQuery' value='17'/>
|          . . .
|         </my:pipe>

Yes, I think that's the right way to go.

|     2c) A pipeline which declares no parameter input ports 'inherits'
|         the anonymous parameter set when invoked by name.  Parameters
|         specified in this way shadow those coming via 2b.iii above,
|         i.e. it treats the explicit ones as overridable defaults.

It's subtle, but I think that's what "does the right thing" most often.

|    3) If you want to combine the anonymous set with explicit
|       parameters, use the p:parameters step:
|       <p:parameters name="mixin">
|        <p:input port='parameters'/>
|        <p:parameter name="foo" value="baz"/>
|       </p:parameters>
|       . . .
|       <p:xslt>
|        <p:input port="parameters">
|          <p:pipe step="mixin" port="result"/>
|        </p:input>
|       <p:xslt>
|       Combining the anonymous set with named sets is a sin, but you
|       can do it using _two_ p:parameters steps.

It's not a sin, exactly, there just isn't any syntax for it because the
only way to get the anonymous ones is by defaulting the binding.

| Open questions:
|  A) Should <p:input kind='parameter' .../> as a child of p:pipeline be
|     purely a declaration, i.e. be necessarily empty, or should we
|     allow it to have content, in which case how do we treat that
|     content -- merge it with external input, ignore it if there's any
|     external input, . . .?

I'd be happy if it was just a declaration. If it isn't, then its
binding should be treated like the other input bindings, only used if
there's no binding given by the caller.

|  B) There's a covert assumption in the current spec., unchanged by any
|     of the above, that the API from the runtime to step
|     implementations will have a way of accessing parameters.  Since
|     parameters are declared, this access could take port name as an
|     argument, or it could just be undifferentiated as to port name,
|     that is, it's just "give me all the parameter bindings you have
|     for this instance of this step".  I don't suppose we _have_ to say
|     anything about this, but we could choose to say e.g. that
|     implementations _should_ provide access by port name, or at least
|     indicate what port particular parameter settings arrived via. . .
|     As long as we allow more than one parameter port per step (and I
|     think we should), I have some inclination to encourage the
|     provision of access to them by port name.

Yes, I think that makes sense.

|  C) Is the shadowing specified in (2c) above the right way around?  I
|     think it is, noting that if you _really_ want to override the
|     values coming 'from outside', you can do so on any step which
|     accesses the anonymous set.

I think (2c) is the right way around for the default case. Whatever
way we make it, it'll be wrong half the time :-)

| Finally question (3) above becomes, in the terms of the proposal in
| (1) and (2) above, "Under what circumstances should the runtime
| deliver the anonymous parameter set when a step implementation asks
| for its parameters?"
|     Possible answers:
|       a) Always;
|       b) Only if a parameter port has been explicitly bound to it (as
|          in the final example under (1) above) (and that port is asked
|          for);
|       c) If a parameter port has been explicitly bound to it (and is
|          asked for), or if some parameter ports have not been bound at
|          all (and all parameters are asked for? and any unbound
|          parameter port is asked for? and there is only one parameter
|          port declared and unbound?).
|     I believe Alessandro favours (b), and I favour (c).  The
|     difference is manifested in the case of a simple p:xslt step -- if
|     the pipeline is minimal, i.e.
|      <p:pipeline>
|       <p:xslt>
|        <p:input port="transform">
|         <p:document href="..."/>
|        </p:input>
|       </p:xslt>
|      </p:pipeline>
|      and I invoke it with some parameters (either 'from outside', or
|      by invoking it by name), does the XSLT step see the parameters?
|      I think users will expect it to, and I think they're right.
|      On proposal (c) above, if you want to _protect_ a step from
|      parameters, you would write e.g.
|      <p:pipeline>
|       <p:xslt>
|        <p:input port='parameters'>
|         <p:empty/>
|        </p:input>
|       </p:xslt>
|      </p:pipeline>
|      thereby forestalling the delivery of the anonymous set.

I think that all works. Thanks, Henry!

                                        Be seeing you,

Norman Walsh <ndw@nwalsh.com> | One of the great misfortunes of mankind
http://nwalsh.com/            | is that even his good qualities are
                              | sometimes useless to him, and that the
                              | art of employing and well directing
                              | them is often the latest fruit of his
                              | experience.-- Chamfort

Received on Friday, 6 July 2007 18:56:31 GMT

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