 |
 |
I also couldn't come up with a non awkward way to put it without going into the issues of scope and datasets (hence the weasel words). Blank nodes are syntactic lemon juice. On Nov 18, 2014 1:36 PM, "[log in to unmask]" <[log in to unmask]> wrote: > > Blank node identifiers are essentially variables (in SPARQL they are an > alternative syntax for variables)… > > I dont think this is wrong, but in my experience it is more common to > think of blank nodes as specifically existentially-bound variables with a > scope that is the document in which they are found. So the triple: > > _:x :favoriteEggType :Chicken . > > makes the assertion, "There exists a thing, to which we will refer as > '_:x', but only inside this document, the :favoriteEggType of which is > :Chicken". If we see in the same document: > > _:y a :Farmer . > _:y :hates _:x . > > We can read "There exists a thing, to which we will refer as '_:y', but > only inside this document, which is a :Farmer, and which :hates the thing > to which, only in this document, we are referring to as '_:x'". > > Or if we see: > > _:x :fears _:z . > > And _:z doesn't appear as the subject of a triple in our document, we can > read "There is a thing that, just inside this document, we will call '_:z', > and the :Weasel that inside this document we call '_:x' :fears '_:z'", and > so on. Because of the lack of a unique name assumption for RDF, it is not > defined whether the :Weasel :hates the :Farmer, or someone else entirely. > > That is all rather awkward reading, and this is a well-known complaint > about blank nodes. Existential qualifiers are useful, but in mass they can > become confusing. It is like listening to a story told by someone who > cannot remember anyone's name. Often RDF isn't meant for human consumption > anyway, but analogous problems occur in machine processing. For example, as > Thomas Berger remarked: > > > In practice this matters when one wants to add or remove individual > statements or subgraphs from graphs: When the graphs or subgraphs have > blank nodes as their origin, you usually can't. > > Often you can't because it is very difficult to calculate exactly what > changes you are making in the possible interpretations of the graph. Simon > Spero's example shows that: we don't, as he says, know how many :Weasels we > actually have. And once we are no longer in the scope of our original > document, we can only refer to a :Weasel by some kind of query. If the > attributes of :Weasels don't support queries that will identify them > uniquely, we more-or-less lose track of them. This is bad if, for example, > we discover new information about our :Weasels and would like to record it > in a useful way. > > There are occasionally good reasons to use blank nodes, but here: > > http://richard.cyganiak.de/blog/2011/03/blank-nodes-considered-harmful/ > > are some cautionary remarks about them from Richard Cyganiak, one of the > editors of the RDF standards. > > --- > A. Soroka > The University of Virginia Library > > On Nov 18, 2014, at 12:06 PM, Simon Spero <[log in to unmask]> wrote: > > > On Nov 18, 2014 11:13 AM, "Joseph Kiegel" <[log in to unmask]> > wrote: > > > > > > > > In RDF 1.1 Concepts and Abstract Syntax, section 3.4, we find: "Blank > node identifiers . are always locally scoped to the file or RDF store, and > are not persistent or portable identifiers for blank nodes". [...] Isn't > it true, then, that blank node identifiers, which are valid at Library A, > are not defined when they get to Library B? This seems like a problem. > > > > > > Is the use of blank nodes consistent with BIBFRAME's function as a > carrier? > > > > What the specification means is that a blank node _:x that refers to > some thing in an RDF file transferred from A to B may not refer to the > same thing except during in the one use of that file. > > > > It may not be the name of the thing in the stores at A *or* B, and if > the same file is ingested twice, it could refer to two different things > that happen to have the same values for the stated properties. > > > > Blank node identifiers are essentially variables (in SPARQL they are an > alternative syntax for variables)... > > > > Suppose we have the following file: > > --------------- > > _:x rdf:type :Weasel. > > _:x :favoriteEggType :Chicken . > > ---------------- > > > > This says that there is something that is a Weasel and whose favorite > type of egg is Chicken. > > > > If we see this twice, we cannot tell how many chicken pickin Weasels we > have. > > > > A different file could use _:x to refer to some Chicken. > > > > Simon > > >