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Re: defaulting

From: Henry S. Thompson <ht@inf.ed.ac.uk>
Date: Thu, 28 Jun 2007 15:26:29 +0100
To: Norman Walsh <ndw@nwalsh.com>
Cc: public-xml-processing-model-wg@w3.org
Message-ID: <f5b1wfwi296.fsf@hildegard.inf.ed.ac.uk>

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Norman Walsh writes:

Richard and Henry write:

> |   It is _not_ an error to have no default outputs (this is a change).
>
> I don't think that's a change. Unless you meant to write "It _is_ an
> error to have no default outputs."

It _is_ a change (see err:XS0015 below), but I should have been clearer.

At the moment we have:

 * If there is a preceding sibling step element:
   
   - If that preceding sibling has exactly one output port, or an
     output port designated as the default, then that output port
     becomes the default readable port.

   - Otherwise, the default readable port is undefined.

and further down

  It is a static error (err:XS0015) to specify default="no" on the
  p:output of a step which has exactly one output. In other words, if
  any step or step has exactly one output, that output is always the
  default output.

On our proposal, this would change to say simply

 * If there is a preceding sibling step element: 
    
   - If that preceding sibling has an output port designated as the
     default, then that output port becomes the default readable port.
 
   - Otherwise, the default readable port is undefined. 

and

 If any stepa has exactly one output, and it does not specify
 'default="no"', that output is treated as the default output.

> |   The default input of a step is bound to the default readable port if
> |   it is not otherwise bound;
> |
> |   Non-default inputs are only bound to the default readable port if
> |   you call for that to happen by writing
> |    <p:input port="not-the-default-input-port"/>
>
> So it's an error to leave a non-default input unspecified? What's the
> value of this change?

Unbound secondary inputs are detected, instead of getting silently bound.

> |  That is, p:store _has_ no default output.
>
> Bleh. I suppose I can live with it, but I'd be happier, personally, if
> pipeline authors creating pipelines with no output were required to
> end with a p:sink step.

Further to the change discussed at more length above, it will be
possible to have various steps with no default output.  Any of them
will do.  I think p:store really shouldn't have a default output.

ht
- -- 
 Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
                     Half-time member of W3C Team
    2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
            Fax: (44) 131 650-4587, e-mail: ht@inf.ed.ac.uk
                   URL: http://www.ltg.ed.ac.uk/~ht/
[mail really from me _always_ has this .sig -- mail without it is forged spam]
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Received on Thursday, 28 June 2007 14:26:41 GMT

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