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Re: Tag-less literals and literals with empty tags

From: Alan Ruttenberg <alanruttenberg@gmail.com>
Date: Wed, 6 Jul 2011 13:12:19 -0400
Message-ID: <CAFKQJ8=uBR3avkx9p46E=mvOka_xWEsgrXsH+hE9OGiS0kt+wA@mail.gmail.com>
To: Jakob Voss <jakob.voss@gbv.de>
Cc: public-esw-thes@w3.org
On Wed, Jul 6, 2011 at 7:38 AM, Jakob Voss <jakob.voss@gbv.de> wrote:
> Antoine Isaac wrote:
>
>>> Alan Ruttenberg wrote:
>>> without a language tag it is a string, with a language tag it is a
>>> pair of strings. The set of plain literals without language tags is
>
>>> *not* the set of pairs (string , "").
>
> The set of pairs (string , "") is not RDF. This is like comparing an RDF
> Triple with a poem and asking whether two can be equal or not.

Not quite. It is like comparing a triple to a generalized triple in
http://www.w3.org/TR/owl2-profiles/#Reasoning_in_OWL_2_RL_and_RDF_Graphs_using_Rules.
Language tags are a subset of string, and so the set (string,string)
is a generalization of value space (string, language tag).

> The RDF data model does not allow such thing as "an empty language tag", so
> it is fruitless to discuss about the meaning of it in context of RDF. The
> SPARQL specification contains explicitly "Note that the RDF data model does
> not include literals with an empty language tag.":
> http://www.w3.org/TR/rdf-sparql-query/#func-lang

This wouldn't be normative, but this is:
http://www.ietf.org/rfc/rfc3066.txt and as SPARQL correctly notes
language tags can not be empty.
Language-Tag = Primary-subtag *( "-" Subtag )
Primary-subtag = 1*8ALPHA
Subtag = 1*8(ALPHA / DIGIT)

RDF MT says: Throughout this document we use the term 'character
string' or 'string' to refer to a sequence of Unicode characters, and
'language tag' in the sense of RFC 3066

>> But for other RDF syntaxes, it's maybe not so clear. And I'm not sure
>> it's "so clear" even for the RDF/XML situation: digging this was
>> painful. Whichever of the alternatives is right (a tag-less literal is
>> equivalent to a literal with empty language tag, or not) an extra line
>> in one of the RDF specs would be handy!
>
> That's mixing apples and oranges. The RDF spec is clear - there are no empty
> language tags. The specifications of RDF syntaxes may less clear, but then
> it's the particular problem of a RDF syntax. By the way I would avoid any
> discussion about any RDF issues that refers to examples in RDF/XML, unless
> you talk about RDF/XML and only RDF/XML.

Yup.

>
> Jakob
>
> --
> Jakob Vo▀ <jakob.voss@gbv.de>, skype: nichtich
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>
Received on Wednesday, 6 July 2011 17:13:07 GMT

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