Hacking at injection

Hi folks,

I’ve spent a little time this weekend hacking around the edges of the
injection problem. Here’s an example of what I’ve come up with so far:

<p:injectable xmlns:p="http://www.w3.org/ns/xproc"
              xmlns:c="http://www.w3.org/ns/xproc-step">

  <p:input step="/p:declare-step/p:identity[1]" port="source"
           message="Document sent to identity[1] on source"/>

  <p:input step="//p:identity" port="source"
           base-uri="pipe.xpl"
           message="Document sent to identity on source for pipe"/>

  <p:output step="/*/p:wrap-sequence" condition="$opt-class = 'b'">
    <doc class="{$opt-class}">{.}</doc>
  </p:output>

  <!-- also p:step for when a step runs -->
</p:injectable>

The idea is that you can put an “inspector” on a particular input port
or on a particular output port. (The comment at the bottom aludes to a
p:step injectables as well, for when a step executes; I’m less
confident about that one, purely on implementation grounds.)

In order to insert something, you have to be able to point to it.
Pointing with step names doesn’t work because you need to have
something that’s unique across the whole pipeline, not just in a
particular scope. Rather than inventing a new kind of step addressing
language, I just went with XPath.

If you don’t specify a base URI, the injector applies to all pipelines
(which is fine because usually there’s only one anyway).

The message can be either an attribute or content. If it’s an
attribute, it’s an AVT so you can inspect the in-scope variables at
the point of insertion. It’s an error if you refer to variables that
aren’t in scope at that point.

If the message is content, it’s like p:inline, it expands the TVT.

Condition let’s you specify when you’d like the injectable to “fire”.

There’s still lots of unanswered questions, like where does the output
go?

I’m implementing this by literally inserting steps into the graph.
That complicates the graph, which could change behavior, but it means
the injectables can refer to different variables than the steps.

                                        Be seeing you,
                                          norm

-- 
Norman Walsh
Lead Engineer
MarkLogic Corporation
Phone: +1 512 761 6676
www.marklogic.com