Re: subpipelines, Vnext and extension elements redux

/ ht@inf.ed.ac.uk (Henry S. Thompson) was heard to say:
[...]
|  <p:when test="p:step-available('my:compound')">
|   <p:identity/>
|
|   <my:compound>
|    ....
|   </my:compound>
|  </p:when>
|
| How does an implementation which _doesn't_ implement my:compound know
| whether the "primary outputs *must* be consumed" rule is violated or
| not?  Clearly it can't, since my:compound may have an arbitrarily
| complex syntax before you get to, say, a subpipeline which might have
| a step which bound to the relevant output.

It might (I say might) be sufficient to add a rule that it's a static
error for an unknown compound step to participate in an implicit
binding.

| So, we appear to have at least four choices:
|
|  1) Add a p:declare-compound-step, and try to specify what constraints
|    _don't_ hold of subpipelines with unimplemented compound steps in
|    them;
|
|  2) Loosen the syntax so that unknown elements are assumed to be
|     unimplemented compound steps and are ignored;
|
|  3) Go back to the idea of extension namespaces, and treat unknown
|     elements _in an extension namespace_ as unimplemented compound
|     steps;
|
|  4) Accept that there is no backward-compatible way to introduce
|     new/extension compound steps, and therefore that they will cause
|     static errors in implementations which don't know about them.
|
| I guess after all this I prefer (4), on the grounds that (1) is just
| too messy, (2) gives up too much, (3) doesn't allow for new compound
| steps _in the pipeline language_ in a backward-compatible way, and
| anyway, the chances of a workaround being available which would enable
| one to write backwards-compatible pipelines using new/extension
| compound steps is so small that there's no point in buggering with the
| language to make that possible.

I think that's where we wound up back when we decided to take
pfx:other-compound-step out of 4.7, 

  http://www.w3.org/TR/2007/WD-xproc-20070706/#p.other

| Phew!
|
| _If_ we accept this analysis and its conclusion, I think I know what
| 2.1 and 4.7 should look like . . .

Great! That's what the editor loves to hear :-)

                                        Be seeing you,
                                          norm

-- 
Norman Walsh <ndw@nwalsh.com> | So, are you working on finding that bug
http://nwalsh.com/            | now, or are you leaving it until later?
                              | Yes.