I totally agree, modulo a bunch of corrections.
On 2007-07 -31, at 18:43, Story Henry wrote:
I think the point of Tim's post was that really he thinks that the
only things that are literals are strings.
Friendly correction: I don't think the only literals *are*
strings, but I point out that the existing
system can be defined (modeled?) using just strings and datatype
properties.
The RDF system could have been just strings and properties.
This was in response to Garret's complaint that RDF datatypes had no
rhyme nor reason.
I wanted to point out that one could look at them quite cleanly.
[] xsd:integer "123" .
can be written in shorthand as
"123"^xsd:integer .
(see the N3 tutorial)
Because everybody in the xml world was going crazy about xsd
datatypes they wanted this more complex notion of literals. To
satisfy them the ^^ notation was added.
123 = "123"^^xsd:integer,
"123"^xsd:integer,
123 .
is a nice shortcut in N3.
Yes. To be precise,
123 and 123"^^xsd:integer are alternative syntaxes for the same thing.
In my scheme when xsd:integer is an rdf property, then it also holds
that
"123"^integer = "123"^^xsd:integer
(or as a triple, "123"^^xsd:integer xsd:integer "123".)
The RDFS definition at http://www.w3.org/TR/rdf-schema/#ch_literal
is at first circular ("the class of literal values") and then
nonexistent for some literals ("This specification does not define
the class of plain literals."). The RDF Primer explanation at
http://www.w3.org/TR/rdf-concepts/#section-Literals is more
helpful: A literal is a resource identified by a lexical
representation, which representation may be "more convenient or
intuitive" to use instead of a URI.
So at the end of the day, a literal is simply a resource that is
easy to refer to using a string of characters. That's all well and
good, but why should that affect my model? Why is a resource an
instance of another class (rdfs:Literal) just because I like to
identify it by a lexical representation?
You may have a good point there. There is a difference between a
string and a URI though that is worth keeping in mind. Namely you
have to look at the world to find out if the following is true or
false
Friendly correction: There is a difference between a [string used as
a] Symbol and a [string used as a] Literal in the language.
I understand that 'symbol' is the right term for the term <http////
whatever> and I understand 'litera' maybe weird but RDF for the term
"http://whatever";. What we are distinguishing here is the role in
the RDF (n3 or whatever) syntax.
<uri1> = <uri2> .
where uri1 and uri2 are two different strings.
But you don't need to look at the world to tell that the following
is false
"uri1" = "uri2" .
you know they are different just by doing a character by character
comparison.
yes
Now you may say that really that's just because they are a list of
characters, and you could refer to each character as a URI, and
then well your point would stand. So you could have something like
( <http://unicode.org/char/u> <http://unicode.org/char/r> <http://
unicode.org/char/i> <http://unicode.org/char/1> ) = ( <http://
unicode.org/char/u> <http://unicode.org/char/r> <http://unicode.org/
char/i> <http://unicode.org/char/2> ) .
There you would have to know something about the referents of each
of these characters to tell that it can't be true.
Still it is convenient to have literals, you have to admit. Because
when you see one, you know how to deal with it immediately. And we
are engineers, so we do like to have some conveniences.
Take for example the resource identified by the URI <http://
example.org/presidents/GeorgeWBush>. This resource may have an
rdf:type of foaf:Person. (I could assert all sorts of other RDF
statements about this resource, but will decline to do so at this
time.) Is this resource an instance of the class rdfs:Literal? No?
Why not?
But wait---if I decide that it's easier to represent this resource
using a string, I could create the resource "George W.
Bush"^^foaf:Person.
Please have a closer look at N3, or else we will keep repeating the
same points. Tim BL would say that
the above is more clearly stated as:
"George W. Bush"^foaf:Person .
which is
[] foaf:Person "George W. Bush" .
And as you can see that is not what you meant. foaf:Person is a
class and not a relation.
(think of relations as arrows going between things as depicted here
http://blogs.sun.com/bblfish/resource/Syntax-Semantics-Photo.jpg )
Also to refer to people via a String is not helpful, because they
can have different names. Since the
following is necessarily false
"George W. Bush" = "George Bush" .
you would not be able to make the assertions we all want to make.
In RDF we think of it this way:
[] foaf:name "George W. Bush",
"George Bush" .
That is ok. A person can have two name relations different
strings. URIs are Universal Names.
Strings are resources too, but they refer to themselves.
That is an important and useful enough difference to make a special
case for the semantics.
Henry
