Relax p:variable restrictions

Hi folks,

In line with our focus on usability, I think we should relax the restriction
that p:variable can only occur at the top a subpipeline. We did it so that
implementors would not have to analyze every XPath expression for variable
references. I propose new rules:

1. p:variable can go anywhere a step can go
2. p:variables are lexically scoped in the obvious way
3. It's an error if the xpath-context of a p:variable relies, directly
   or indirectly, on the output from any step that uses the variable
4. It's an error if reordering the steps in the pipeline moves any step
   into the "shadow" of a different variable with the same name
   (breaks the lexical scoping)
5. p:variables are invisible for the purposes of computing the default
   readable port

Assume for the moment that p:identity has a 'ref' option. Consider:

  <p:variable name="ex:foo" select="1">
    <p:xpath-context><p:empty/></p:xpath-context>
  </p:variable>

  <p:identity name="one" ref="{$ex:foo}"/>

  <p:variable name="ex:foo" select="2">
    <p:xpath-context><p:empty/></p:xpath-context>
  </p:variable>

  <p:identity name="two" ref="{$ex:foo}"/>

The identity named 'one' sees ref=1, the identity named 'two' sees ref=2.

  <p:variable name="ex:foo" select="1">
    <p:xpath-context><p:pipe name='two'/></p:xpath-context>
  </p:variable>

  <p:identity name="one" ref="{$ex:foo}"/>

  <p:variable name="ex:foo" select="2">
    <p:xpath-context><p:empty/></p:xpath-context>
  </p:variable>

  <p:identity name="two" ref="{$ex:foo}"/>

Error: The first ex:foo indirectly depends on the output of the
identity step that uses it.

  <p:variable name="ex:foo" select="1">
    <p:xpath-context><p:empty/></p:xpath-context>
  </p:variable>

  <p:identity name="one" ref="{$ex:foo}">
    <p:input port="source">
      <p:pipe step="two"/>
    </p:input>
  </p:identity>

  <p:variable name="ex:foo" select="2">
    <p:xpath-context><p:empty/></p:xpath-context>
  </p:variable>

  <p:identity name="two" ref="{$ex:foo}">
    <p:input port="source">
      <p:inline><doc/></p:inline>
    </p:input>
  </p:identity>

Error. Step 'one' has to be "moved" to after 'two'. That moves it into an area
with a different lexical $ex:foo.

                                        Be seeing you,
                                          norm

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