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*Subject*: **Re: [xdi] Proposal for separation of subject/predicate and parent-subject/child-subject semantics**

*From*:**Markus Sabadello <markus.sabadello@xdi.org>***To*: Giovanni Bartolomeo <giovanni.bartolomeo@uniroma2.it>*Date*: Wed, 21 Apr 2010 18:11:47 +0200

So if I send the following XDI message to an endpoint:

=markus

$add

/

=markus

"markus.sabadello@gmail.com"

and then I send this XDI message:

=markus

$get

/

=markus+email

What will be the XDI response?

Markus

On Wed, Apr 21, 2010 at 11:10 AM, Giovanni Bartolomeo <giovanni.bartolomeo@uniroma2.it> wrote:

Hello Markus and Drummond,

thanks for your replies. I think there is however some confusion which I want to clarify once for all. I'm not saying that +a/+b is resolution equivalent to +a+b. I think we are now all agreeing on this (at least I think :-).

Logical equivalence is a different concept. It means that if +a/+b is resolvable then also +a+b must be. Note that they DO NOT resolve to the SAME node (this is instead resolution equivalence).

The requirement I'm talking about (if +a/+b is resolvable then also +a+b must be and viceversa) is needed to ensure consistency in my semantic model. For instance, having in a graph

=example

$has

will allow to deal with =example+email as the set of emails which =example owns (=example+email), and consequently make assertions on them (pls refer to http://lists.oasis-open.org/archives/xdi/201004/msg00004.html for details on this, btw I can clarify them in next phc). This is also highlighted in point 1) of http://lists.oasis-open.org/archives/xdi/201003/msg00014.html.

Note that resolution of certain statements is needed, even if not asserted in the graph, but inferred using a reasoner (again pls refer to my mail for details) in order to implement what we have discussed in

http://lists.oasis-open.org/archives/xdi/201001/msg00047.html

=bill.barnhill: We can get the graph containing statements about any statement in the graph, and that graph will only exist if and only if the statement is in the graph. put more simply (+a/+b) is resolvable, or R(+a/+b) = True. Generalizing this we see that if R(X) = True then R((X)) = True, meaning and (X) are logically equivalent

Drummond: It seems the import of what you are saying is that +a <=> (+a)

giovanni: resolution = your statement is true; fault = your statement is false

=bill.barnhill: but not +a = (+a)

Drummond: Yes!

Drummond: A $is statement in an XDI graph establishes what Bill calls "resolution equivalence"

where we used symbol <=> to address logical equivalence, and = to address resolution equivalence.

Now my point is: either the TC has a different semantic model to propose for XDI (in this case, I would be glad to know more and discuss about it), or the TC wants to discard semantic consistency for XDI. If the latter is the case, then, frankly, I think the TC is underestimate a tremendously powerful feature which could instead change the whole concept of web applications in next few years. Please let me know your opinion about it.

Thank you,

Giovanni

Def. Quota "Markus Sabadello" <markus.sabadello@xdi.org>:

What I'm afraid of is that people will never understand resolution

equivalence, and that it would be difficult to implement at the storage and

messaging levels of an XDI stack.

Can't we just have logical equivalence without resolution equivalence?

On the logical level a high level application/reasoner/etc can infer lots of

stuff based on some $is, $has, $is$a, etc rules we make up, but I'm really a

bit scared of having to implement such extra XDI "magic" just for plain XDI

storage and messaging..

Markus

On Tue, Apr 20, 2010 at 5:21 PM, Giovanni Bartolomeo <

giovanni.bartolomeo@uniroma2.it> wrote:

Dear Drummond, All,

thank you for this proposal - two main thoughts from my side:

1) it is FUNDAMENTAL that if +a/+b is resolvable then also +a+b MUST be,

and viceversa. We used the symbol <==> to say this, +a/+b <==> +a+b.

Obviously the stored document can only contain one of the two, e.g. the

document only contains +a/+b, but +a+b MUST be resolvable as well.

This is about the relationship between asserted and inferred statements

which I illustrated in the second part of my email

http://lists.oasis-open.org/archives/xdi/201004/msg00004.html - we didn't

discuss this yet because we ran out of time last week, but it would be good

to address this in next phc.

2) About the following statements (in which the symbol <==> is used in a

somehow different way than in point 1)

+a/$has/+b <==> +a/+b

+a/+/+b <==> +a+b

+a/$/$b <==> +a$b

+a/=/=b <==> +a=b

+a/@/@b <==> +a@b

+a/*/*b <==> +a*b

+a/!/!b <==> +a!b

+a/()/(b) <==> +a(b)

note that the statements on the left side in their own give origin to

metastatements like +a/$has/+, +a/$has/$, +a/$has/() which in turns are

logically equivalent to +a+, +a$, +a() whose semantics is totally unknown to

me.

Kind Regards,

Giovanni

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

Per my action item from Thursday's telecon, following is more about my

analysis and proposal for solving the "$has semantics problem" once

and for all.

*** ANALYSIS ***

My first conclusion, on which I think we all agree, is that while +a+b

might imply +a/+b and vice versa, that does not mean that: a) they

have the same semantics, or b) that they identify the same XDI graph.

Indeed, since one is an XDI subject and the other adds an XDI

predicate, by definition they cannot identify the same XDI graph.

To illustrate in English, most English verbs can be made into nouns by

putting them in their infinitive form (“to be”, “to run”, “to shout”).

But “run” and “to run”, while clearly linked semantically, do not have

the same meaning. They cannot be interchanged with each other.

Thus my second conclusion is that none of the three XDI graph examples

we were discussing on our telecon two weeks ago were in fact

equivalent. They were each different XDI graphs that had similar but

not identical semantics.

My third conclusion is that this means that we do in fact need clearly

distinguished ways of semantically and syntactically of expressing

that an XDI subject (e.g., +a) : 1) has a child subject (e.g., +a+b),

and 2) has a predicate (e.g., +a/+b).

My fourth conclusion was that this is the problem we have been

struggling with for the last year: by using $has (with or without

variants like $has$a) to express both parent subject/child subject

relationships (+a+b) and subject/predicate relationships (+a/+b), we

have wrapped ourself in a classic Gordian knot.

*** PROPOSAL ***

Once the problem was framed this way, it became much easier (for me

anyway) to see that a clean solution would be to dedicate the $has

predicate to only one semantic – either the parent subject/child

subject relationship or the subject/predicate relationship. Of those

two options, I strongly prefer dedicating the $has verb to expressing

the subject/predicate relationship (+a/+b), since this is the simpler

and more intuitive to understand, and mirrors the semantics of the $a

predicate, which expresses predicate/object relationships.

If $has is dedicated to that expression, then we simply need a

different predicate (or set of predicates) to express parent

subject/child subject relationships.

For that, an obvious answer is to use XRI delimiters as predicates.

This is intuitive since it is syntactically required to have a

delimiter between XRI subsegments, and each of the six XRI delimiters

(not counting parentheses for cross-references) has its own

XRI-defined "semantics", e.g., * means reassignable identifier, !

means persistent identifiers, etc.

So to summarize using +a and +b notation:

+a/$has/+b <==> +a/+b

+a/+/+b <==> +a+b

+a/$/$b <==> +a$b

+a/=/=b <==> +a=b

+a/@/@b <==> +a@b

+a/*/*b <==> +a*b

+a/!/!b <==> +a!b

+a/()/(b) <==> +a(b)

Semantically, it would still be true that +a+b/$is$a/+b (and the

inverse that +b/$a/+a+b). And while +a+b would imply that +a/+b is

valid, and vice versa, the existence of one in an XDI graph would not

imply the existence of the other.

So, if =example wanted to do a query to determine if +a+b and +a/+b

both existed in an XDI graph, it would be:

=example

$get

/

+a

$has

+b

+

+b

I have been trying out this proposal in sample XDI documents for the

past week and it seems to work very well. Please consider and

experiment with it and post your thoughts. We'll put it on the agenda

for next week's telecon.

=Drummond

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**Follow-Ups**:**Re: [xdi] Proposal for separation of subject/predicate and parent-subject/child-subject semantics***From:*Drummond Reed <drummond.reed@xdi.org>

**References**:**Proposal for separation of subject/predicate and parent-subject/child-subject semantics***From:*Drummond Reed <drummond.reed@xdi.org>

**Re: [xdi] Proposal for separation of subject/predicate andparent-subject/child-subject semantics***From:*"Giovanni Bartolomeo" <giovanni.bartolomeo@uniroma2.it>

**Re: [xdi] Proposal for separation of subject/predicate and parent-subject/child-subject semantics***From:*Markus Sabadello <markus.sabadello@xdi.org>

**Re: [xdi] Proposal for separation of subject/predicate andparent-subject/child-subject semantics***From:*"Giovanni Bartolomeo" <giovanni.bartolomeo@uniroma2.it>

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