Re: thoughts on metadata

> I think the current proposal for meta data is inadequate for the current
> and future needs because it is not part of the syntax of the language, its
> proposed is too limited, and in the current form it cannot be given a
> semantics.

Right.  That is why I also proposed the "directives" [1] (even though 
the term is probably not appropriate; so if you have anything better). 
The examples you mentioned are actually good examples of the kinds of 
things directives can be used for.  So, we have the current list of 
examples for the use of directives:

These are the ones we I think we really want to have in BLD:

- import of RIF rule sets
- reference to external data sources (e.g. RDF)
- reference to external data models (e.g. RDFS/owl)

These are things to keep in mind for extensions of BLD:

- sorting of query answers
- priorities
- preferences

If we would need variables in directives, extensions to BLD can extend 
the notion of "directive".

Note that still the same discussion (we've had already several times) 
applies: whether we want to have a generic syntax element "directives" 
or whether we want to extend the syntax for each of the possible directives.

Best, Jos

[1] http://www.w3.org/2005/rules/wiki/Annotations

> 
> Here are some concrete problems.
> 
> 1. Meta data can be attached to various parts of the rules, not just rules
>    themselves, and this metadata can affect the semantics.
>    A simple example is specifying that the output to a query must be sorted.
>    This changes the semantics, since the answer is not a set any more
>    but a list.
> 
> 2. Some important types of rule systems are based on prioritized defaults
>    (defeasible, courteous, preference LP).
> 
>       - Here metadata is part of the syntax and of semantics.
>       - Typically metadata consists of rule labels and priority or
>         preference information.
>       - Metadata items are often *terms with variables* and not just strings.
> 
> 3. The current proposal does not offer any obvious or natural way for the
>    metadata to be processable by a (possibly different) rule set.
> 
>    
> 
> 
> 	--michael  
> 

-- 
                          debruijn@inf.unibz.it

Jos de Bruijn,        http://www.debruijn.net/
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