[foaf-dev] foaf:OnlineAccount owl:disjointWith foaf:Document ? [was strawpoll: migrate from foaf:holdsAccount to foaf:account?]

[Toby Inkster]

Let us examine this in the context of another relationship:

	# Bob is Alice's brother.
	<http://alice.example.com/> a foaf:Document ;
		xfn:sibling <http://bob.example.com/> .
	<http://bob.example.com/> a foaf:Document .

This document-to-document relationship (used in XFN) jars with a lot of
people. However, it is a perfectly valid way of modelling the
relationship between Alice and Bob, provided it is used consistently:
provided that the xfn:sibling term is specifically defined as linking
from document to document, such that the "owners" (whatever that means)
of the documents are siblings; provided it's never used with, say, a
foaf:Person as the domain or range.

Similarly,

	<#cygri> foaf:account <http://twitter.com/cygri> .

is a perfectly consistent way of modelling account ownership, provided
that we're clear that foaf:account is defined as linking not from a
person to (the rather abstract idea of) an account, but as linking from
a person to a document associated with that account. (We can even call
that document a foaf:OnlineAccount, provided again that we are
consistent with that definition.)

That definition is consistent and usable, however we have to ask whether
it's a better way of modelling accounts than the alternative, which
would be:

	<#cygri> foaf:account [
		foaf:accountProfilePage <http://twitter.com/cygri>
		] .

The former is clearly more concise, and thus presumably "easier" for
people to use. The latter has the advantage that separate data can be
provided for the account and the page. Ian's example of the creation
date for the account and page being different may be contrived, but it's
still conceivable that such a thing could happen. e.g. an existing
instant messaging service could at some point start providing profile
pages for its users - the document creation date and the account
creation date would then differ.

Perhaps a solution might be that we keep foaf:holdsAccount and introduce
a foaf:account property too...

	foaf:holdsAccount
		rdfs:domain foaf:Agent ;
		rdfs:range foaf:OnlineAccount .
	foaf:account
		rdfs:domain foaf:Agent;
		rdfs:range foaf:AccountProfileDocument ;
		rdfs:subPropertyOf foaf:page .
	foaf:AccountProfileDocument
		owl:disjointWith foaf:OnlineAccount ;
		rdfs:subClassOf foaf:Document .
	foaf:accountProfilePage
		rdfs:domain foaf:OnlineAccount ;
		rdfs:range foaf:AccountProfileDocument ;
		rdfs:subPropertyOf rdfs:seeAlso .
		
We could then add some OWL2 rules to tell us that this:

	?person foaf:holdsAccount [ foaf:accountProfilePage ?page ] .

implies this:

	?person foaf:account ?page .

In this diagramme I drew some time ago:

http://buzzword.org.uk/2009/foaf-Person-OnlineAccount-Document.png

The arc labelled "???" would be accountProfilePage and the label
"homepage" would become "account". (Ignore the bidirectionality of the
arrow heads -- I don't know why I did that.)

-- 
Toby A Inkster
<mailto:mail at tobyinkster.co.uk>
<http://tobyinkster.co.uk>