RE: compound steps and primary inputs

Compound steps, such as p:for-each or p:group (even p:pipeline) are also
their own declarations. That means that the p:input/p:output elements
you specify on a compound step determine the signature of the compound
step (and what will be the primary input/output ports, if any).
 
p:for each does not allow you to specify an input port. Unless you say
otherwise, it will read documents from the implicit readable input port
and for each of the documents it executes the subpipeline. In each
iteration, the document is available via the "curent" input port. This
input port is created by the XProc processor and is only visible to the
subpipeline.
 
Compound steps that don't declare (or can't declare) an input port are
just wrappers around the subpipeline, and the data from the implicit
readable input port just "flows through", directly to the subpipeline.
 
Regards,
Vojtech


________________________________

	From: xproc-dev-request@w3.org [mailto:xproc-dev-request@w3.org]
On Behalf Of James Garriss
	Sent: Wednesday, September 24, 2008 2:54 PM
	To: XProc Dev
	Subject: Re: compound steps and primary inputs
	
	
	I understand what a primary input port is.  What I don't know is
how I know that p:for-each has only one input.  If I look at a step like
p:filter, it's obvious from it's declaration (I'm catching on to the
terms!) that it has only one input port:
	
	<p:declare-step type="p:filter">
	     <p:input port="source"/>
	     <p:output port="result" sequence="true"/>
	     <p:option name="select" required="true"/>
<!-- XPathExpression -->
	</p:declare-step>
	
	But p:for-each doesn't seem to have a declaration.  It looks
sorta like BNF/parser-token-stuff to me.  So what tells me that
p:for-each has only one input?
	
	<p:for-each
	  name? = NCName>
	    ((p:iteration-source? &
	      (p:output |
	       p:log)*),
	     subpipeline)
	</p:for-each> 
	
	James Garriss
	http://garriss.blogspot.com
	
	
	
	
________________________________

	From: Norman Walsh <ndw@nwalsh.com>
	Date: Tue, 23 Sep 2008 16:45:22 -0400
	To: XProc Dev <xproc-dev@w3.org>
	Subject: Re: compound steps and primary inputs
	Resent-From: XProc Dev <xproc-dev@w3.org>
	Resent-Date: Tue, 23 Sep 2008 20:46:27 +0000
	
	James Garriss <james@garriss.org> writes:
	
	> Do all compound steps have primary inputs?  I ask because I
can't tell by
	> reading the WD if p:for-each has a primary input or not.  I'm
probably
	> missing a key word somewhere.
	
	The p:for-each step has only one input and the rules say 
	
	  [Definition: If a step has a document input port which is
explicitly
	  marked "primary='true'", or if it has exactly one document
input
	  port and that port is not explicitly marked "primary='false'",
then
	  that input port is the primary input port of the step.]
	
	By the second clause, the input port of p:for-each is primary.
	
	> When I use it in Calabash 0.6.1, it behaves as if it does:
	>
	> <p:declare-step xmlns:p="http://www.w3.org/ns/xproc">
	>     <p:input port="source">
	>         <p:document href="BookStore.xml"/>
	>     </p:input>
	>     <p:output port="result"/>
	>     <p:filter select="/BookStore/Book[Date>'1970']"/>
	>     <p:for-each>
	>         <p:output port="result"/>
	>         <p:identity/>
	>     </p:for-each>
	> </p:declare-step>
	>
	> The output of filter, a sequence of documents
(<Book>...</Book>), is being
	> inputted (is that a word?) into p:for-each.
	
	That's right. And that's what you want, isn't it?
	
	> Appreciate any insights!
	
	1. You only need to use p:filter if some part of the expression
you want
	to test against is computed by the pipeline. 
	
	2. A p:for-each that contains only an identity step is a little,
uh,
	redundant.
	
	I believe that you could simplify the pipeline above to just
this:
	
	<p:declare-step xmlns:p="http://www.w3.org/ns/xproc">
	    <p:input port="source">
	        <p:document href="BookStore.xml"/>
	    </p:input>
	    <p:output port="result"/>
	    <p:identity select="/BookStore/Book[Date>'1970']"/>
	</p:declare-step>
	
	                                        Be seeing you,
	                                          norm
	
	-- 
	Norman Walsh <ndw@nwalsh.com> | If we lived alone in a
featureless
	http://nwalsh.com/            | desert we should learn to place
the
	                              | individual grains of sand in a
moral or
	                              | aesthetic hierarchy.--Michael
Frayn