Re: [core] Link target attributes in CoRAL (was: Review of CoRAL)

[Christian M. Amsüss <[e-mail]>]

Hi,

On Mon, Nov 05, 2018 at 12:57:10PM +0100, Klaus Hartke wrote:
> > * ibd: Then, it can say something like "MUST NOT occur in a CoRAL
> >   document as attributes, but are expressed as link relations, nested
> >   links and link relations, respectively" to indicate that their
> >   information is not lost in the transition.
> 
> This is true for "rel" and "anchor", but not for potential new
> attributes defined in the future. I don't think CoRAL should try to
> accommodate those, so I'd just forbid all of them (without giving a
> mapping to CoRAL) as it is now.

Non-target link attributes are something I'd discourage in general;
would you, when someone does define new non-target attributes, encourage
them to specify how they'd be expressed in CoRAL?

(RDF would typically do some form of reificiation there, or provide an
intermediate anonymous node).

> Since in CoRAL it's possible to make statements about link targets
> (here: the literal "Überschrift"), something like this would look
> natural to me:
> 
>       #using iana = <http://www.iana.org/assignments/relation/>
>       #using attr = <http://TBD2/>
> 
>       iana:terms-of-service </tos> {
>          attr:title "Nutzungsbedingungen" { attr:hreflang "de" }
>          attr:title "Terms of use"        { attr:hreflang "en" }
>       }
> 
>    <=>
> 
>       </tos>; rel=terms-of-service;
>          title*=UTF-8'de'Nutzungsbedingungen;
>          title*=UTF-8'en'Terms%20of%20use

That's kind of neat and kind of scary. The scary comes from the
implications of

    hosts </present> {
        attr:title "Gift" { attr:hreflang "en" }
        attr:title "Geschenk" { attr:hreflang "de" }
    }
    hosts </posion> {
        attr:title "Poison" { attr:hreflang "en" }
        attr:title "Gift" { attr:hreflang "de" }
    }

which may be read to imply that there is a string "Gift" that is both
English and German and describes both resources.

I think it can still work b/c none of that would survive a naïve
round-trip to RDF, and a converter would need to make

    </present> attr:title "Gift"@en, "Geschenk"@de .
    </poison> attr:title "Poison"@en, "Gift"@de .

out of it because RDF statements can't have literals in their subject,
but that issue would need to be well-understood, and literals would need
to be described along the lines of "Two literals are only interchangable
if they have the same literal value *and* all their properties are
identical", where it is not possible to give a literal properties
outside of where it is defined (for that would create a different
literal).

> Another, related question is how to express link target attributes
> containing multiple, white-space separated values.

Yes; I have a similar proposal for that in micrurus[1], but it does not
go as far as converting to URIs or numerics yet.

[1]: https://gitlab.com/chrysn/micrurus/blob/master/README.rst#special-provisions-for-link-format-conversions

> If there's a guarantee that there won't be any new link target
> attributes in the future (for RFC 6690 link serializations), then we
> could skip the whole mapping topic and just define a new, more natural
> set of link relation types:

I don't think we need a guarantee for that. We can, as they pop up, for
some funny-string-valued attributes define equivalent more semantic
attributes, each with their own mapping. So a general CoRAL processor
you could encounter both `attr: rt="x y"` or `split-attr:rt rt:x,
split-attr rt:y` and needs to know of the equivalence.

A profile like links-CoRAL could then say that types need to be
expressed using the split attributes.

I would, in general, discourage defining new white-space-separated
attributes, and in particular I already do so in RD, for the RD would
need to know that an attribute is such a one for usable lookup.

As for the prefix, attr:rt, attr:if and attr:ct have registries, so we
could treat non-URI bare values in analogy to link relation types.
(Those produce currently uncompressible URIs, leading to profile-defined
URIs in what will be another thread).

Best regards
Chrisian

-- 
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