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Subject: Re: [xdi] Link Contracts revised

Hey Giovanni,

I think the challenge we have here is to try and align the theoretical / scientific background (the XDI metagraph model) with practical, intuitive patterns that are easy to use. I don't know if that can always be done. I assume when Drummond wrote those link contract examples, he probably tried to make it as simple as possible :)

From a practical perspective, I want to be able to easily write and evaluate link contracts, e.g. I want a list of all subjects to which a link contract grants permissions.

First of all, I don't think $has should sometimes produce subjects with ( ), and sometimes not. It seems in your examples the parentheses get added "automatically".

For your solution #1, I think to produce subject =drummond+friend$contract$sig(=web*markus) I would have to construct my link contracts like this:


And for your solution #2, to produce subject =drummond+friend$contract(=web*markus$sig), I think I would have to do this:


Both patterns look a bit more complicated than the original, but I guess they would work too.. Not sure what makes most sense from a theoretical perspective.. I'm basically happy with anything that can be parsed and understood by an XDI endpoint that has to evaluate the link contracts.


On Mon, Nov 9, 2009 at 3:46 AM, Giovanni Bartolomeo <giovanni.bartolomeo@uniroma2.it> wrote:

I was revising link contracts and I've noted the following two issues:

We have this example:

               =web*markus     <-- Drummond adds Markus to this contract -->

Thus a new subject is created:


ISSUE#1: since $has is left associative (+a+b+c <==> ((+a/+b)/+c)), see mail "Key implications of new metagraph $has definition" (May 4, 2009) this statement is parsed as =web was part of the composite subject =drummond+friend$contract$sig=web, and not part of =web*markus.
To solve this, my proposal is to use xref, as follow:


ISSUE#2: we have now the following entailment (+x+y==>+x+y/$is$a/+y):


this sounds a bit odd. My proposal is to change the original statement as follow

               =web*markus$sig <-- Drummond adds Markus to this contract -->

This way we have the new composite subject =drummond+friend$contract(=web*markus$sig) which correctly entails


There would be more to say about the relationship btw $contract$sig and =drummond+friend$contract(=web*markus$sig), OO people (like myself) could see inheritance and polimorphism applied here, but for the moment just stop here, waiting for your comments.

Kind Regards,

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