On Tue, 28 Jun 2011 19:53:02 +0200, Ian Hickson <i...@hixie.ch> wrote:
On Tue, 28 Jun 2011, Philip Jägenstedt wrote:
On Mon, 27 Jun 2011 07:53:53 +0200, John Giannandrea <jgi...@google.com>
wrote:
>
> In the user feedback from the schema.org proposal, which uses
> microdata as its syntax, we have seen several use cases that would
> seem to require multiple itemtypes per itemscope.
>
> Currently the microdata spec only allows one itemtype which defines
> the meaning of the vocabulary for subsequent itemprops.
>
> Allowing an arbitrary list of itemtypes would not be desirable because
> then a user agent would have to have knowledge of the type
> vocabularies in order to parse the page.
Nothing needs to be known about the vocabulary in order to handle
itemtype currently, at least if by "user agent" you mean browsers and
the DOM API. In other words, allowing multiple types wouldn't be a
problem here.
That depends how multiple types are done. If all the non-URL properties
are assumed to be those defined by the first type, then it's fine. But if
you want to be able to use terms from either vocabulary, you'd need
vocabulary knowledge to be able to preprocess the data, which would be
counter to microdata's design goals.
(For example, suppose you have types A and B and A defined property
'foo'.
You have:
<div itemscope itemtype="A B">
<span itemprop="foo">...</span>
</div>
<div itemscope itemtype="B A">
<span itemprop="foo">...</span>
</div>
If type "B" later has a property "foo" defined as well, the meaning of
existing pages changes, as does the necessary processing for generic
processors that just want to tag each item with unambiguous properties
(e.g. the vocabulary-agnostic Microdata to RDFa conversion algorithm).
(s/RDFa/RDF/)
Indeed, multiple types doesn't work at all if you want to mix different
types. I was assuming that the use case was to extend types, kind of like
http://schema.org/Person/Governor. However, it doesn't work all that well
even in that case, since there's no way to know which type is the
extension of the other and which properties exist only on the extended
type.
--
Philip Jägenstedt
Core Developer
Opera Software
