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RE: [SCXML] Question regarding the meaning and purpose of type 'location expression in namelist and param location attributes

From: Jim Barnett <jim.barnett@genesys.com>
Date: Tue, 28 Oct 2014 14:20:02 +0000
To: Ate Douma <ate@douma.nu>, "www-voice@w3.org" <www-voice@w3.org>
Message-ID: <57A15FAF9E58F841B2B1651FFE16D281020F0BF8@GENSJZMBX03.msg.int.genesyslab.com>
Ate,
  I think that clarifications that you propose make sense, and I will add them to the spec.  I think that it also makes sense to add a further clarification that the "write back" or "application" is done "as if by <assign>".  For the XPath data model that would mean that it was done as "replacechildren".   Applications that want to do another kind of assignment can always do it in two steps:  write the returned value to a temporary location and then do an explicit <assign>.

- Jim

-----Original Message-----
From: Ate Douma [mailto:ate@douma.nu] 
Sent: Tuesday, October 28, 2014 10:02 AM
To: www-voice@w3.org
Subject: Re: [SCXML] Question regarding the meaning and purpose of type 'location expression in namelist and param location attributes

On 28-10-14 10:39, David Junger wrote:
> Le 28 okt 2014 à 03:31, Ate Douma <ate@douma.nu> a écrit :
>>
>> AFAICT the purpose of the location attribute is only to yield the 
>> value at the specified location, not the location itself. But then: 
>> why not just and only use the expr attribute always, which should be 
>> giving the exact same result, if it targets a datamodel location?
>
> Yes, the location expression itself matters when you use 'location' 
> rather than 'value'. It is tied to the use of empty <finalize/> when 
> receiving events back from the invocation: in that case, params sent 
> back with the name assigned to of one of the locations on starting the 
> invocation will have their value copied into that location. I don't 
> recall that there is any other use, and that one is pretty marginal. 
> But the definition clearly states that a location expression can be 
> any valid left-hand-side value in the ECMAScript datamodel. The tests do not currently check for full conformance with this.
> They should use a different name and location, for starters. And they 
> should check that params with a name that wasn't associated with a 
> location in the <invoke> are not copied.
>
> . or we could just drop this pretty complex, obscure, and marginal feature.

Thanks David, that indeed is an obscure but valid use-case, at least for now :)

This use-case and the (only?) reason for using the location attribute clearly can use so more explicit wording in the specification IMO: in the context of the <param> element, as well as the main body of the description of the <invoke> element.
Right now the only place this seems to be described is in the <finalize> element.

I suggest at least changing the description for the <param> location attribute as something like:

   "A location expression [...] that specifies the location in the datamodel to retrieve the value from, and write back a value in case of processing returned data from an invoked child session with an empty <finalize/> in its <invoke> element".

And something similar should be added to the description of the <invoke> namelist attribute.

What also needs some further clarification is how a returned value from the invoked session should be 'applied' back to the original location, in case of the xpath datamodel. The operation seems to be similar like <assign>, but (for
xpath) should it use type 'replace' or 'replacechildren'?
For <assign> the default is 'replacechildren', but either might make sense for <finalize/> handling. The specification says nothing about that though.

Kind regards, Ate

>
> David
>
Received on Tuesday, 28 October 2014 14:20:34 UTC

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