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Subject: Comments on the new view of xdi graph (was: Re: [xdi] Minutes: XDITC Telecon Thursday 1-2PM PT 2010-09-16)


Dear Drummond, All,

Thank you for your excellent work performed in Whistler, which has  
been of much help for better understanding XDI addressing issues.

I started from it and have elaborated some thoughts, which are  
hereafter expressed as comments and questions on your minutes.

Looking forward to hear your opinions,

Best Regards,
Giovanni

Def. Quota "Drummond Reed" <drummond.reed@xdi.org>:

>
> Giovanni asked the question: what does
>
> =drummond/+friend/$
>
> identify?
>
> He said that since =drummond/+friend can have any number of objects (e.g.,
> =markus, =bill, etc.), what specifically it means when you use the object
> identifier $, i.e., when you make the statement =drummond/+friend/$.
>
> Drummond answered that the very precise semantics now proposed for $ (along
> with * and ! when used alone as XDI subjects or objects) means that they
> represent datatypes, specifically datatypes of XDI statements, which
> Drummond calls XDI supertypes.
>
>  * $ represents the root of statements in any context that identify
> subcontexts.
>  * * represents the root of statements in any context that identify
> relations between that context as an XDI subject and other contexts as XDI
> objects.
>  * ! represents the root of statements in any context that identify
> properties of that context whose object is an XDI literal.
>
> So Drummond's answer to Giovanni's question is that =drummond/+friend/$ is a
> precise metagraph statement that identifies the node in the metagraph that
> is the root of all XDI statements that identify the members of the set
> =drummond/+friend. In other words, it identifies the set of all of of
> Drummond's friends.
>
> Giovanni: I cannot see this for the moment at least. To me there are only
> arcs labelled "+friend" which point to different nodes (=markus, =john,
> etc.)
>
> Drummond said it would help to the graph using our proposed new notation to
> illustrate his answer.
>
> Note: after the call, Drummond did this and concluded that
> =drummond/+friend/$ is equivalent to =drummond+friend, i.e., it is an arc
> labelled +friend from the subject =drummond to a subcontext. The identifier
> of the subcontext is $, or "self". This is the XDI equivalent of an RDF
> blank node, since it can now be the subject of other XDI statements, however
> this node is in fact identified by the combination the subject (=drummond)
> and the predicate (+friend), so the address of the node is =drummond+friend.
>
> This means that semantically =drummond/+friend/$ is semantically equivalent
> to =drummond/+friend/=drummond+friend. Another way to put this is that is it
> a true statement to say:
>
>     =drummond+friend/$is/(=drummond/+friend/$)
>
>

1) Hmm... I can't see that

> This means that semantically =drummond/+friend/$ is semantically equivalent
> to =drummond/+friend/=drummond+friend.

I pointed out many times that =drummond/+friend/=drummond+friend is  
not a semantically valid statement and I thought we had already agreed  
to reject it from our specs. Maybe you simply meant that  
=drummond/+friend/$ is semantically equivalent to =drummond+friend?

2) Now, if "=drummond/+friend/$ is equivalent to =drummond+friend",  
and $/=drummond/$ is equivalent, at least in my understanding, to  
=drummond, why do we need to use these triples?

I mean, those blank nodes that you want to address with the $  
notation, they have already an address: =drummond, =drummond+friend.  
This even works for =drummond+friend=markus (which I have always read  
as "that =markus in the context of =drummond+friend") and for which I  
cannot figure out how it could be expressed using the $ notation  
(=drummond/+friend/=markus/$ ????).

The simple concatenation of =drummond, +friend and =markus  
"subjectifies" those contexts, and represents a way to assign them an  
address. Isn't it?

3) As I have understood that the primary reason to use the $ notation  
is that we need a way to identify the current context ("this  
context"). I.e.:

> {
>        "*": {
>                "$is": [
>                        "=!1234.5678.aaaa.bbbb",
>                        "=example",
>                        "=web*example"
>                ],
>                "$is$a": [
>                        "+person$"
>                ],
>                "+friend": [
>                        "=markus",
>                        "=mary",
>                        "=joejohnson"
>                ]
>        },
> }

then, why couldn't we simply declare this once for all at the  
beginning of the document and then use the "conventional" RDF-like  
notation in the following? In other words:

> {
>        "*": {
>                "$is": ["=my.example"]
>        },
>
>        "=my.example": {
>                "$is": [
>                        "=!1234.5678.aaaa.bbbb",
>                        "=example",
>                        "=web*example"
>                ],
>                "$is$a": [
>                        "+person$"
>                ],
>                "+friend": [
>                        "=markus",
>                        "=mary",
>                        "=joejohnson"
>                ]
>        },
> }

What do you think?

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