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Re: Turtle syntax: Please align base URI with RFC 3986 & 3987

From: David Booth <david@dbooth.org>
Date: Wed, 29 May 2013 01:10:00 -0400
Message-ID: <51A58DA8.7030806@dbooth.org>
To: Markus Lanthaler <markus.lanthaler@gmx.net>
CC: public-rdf-comments@w3.org
Hi Markus,

I think my reply back to Andy addresses everything that you brought up:
If not, let me know and I'll clarify further.


On 05/28/2013 06:57 PM, Markus Lanthaler wrote:
> On Monday, May 27, 2013 5:36 AM, David Booth wrote:
>>>> That would require a relative URI specified in
>>>> @base to be resolved using "Reference Resolution", which is
>>>> specified in section 5 of RFC 3986.  But the result of
>>>> "Reference Resolution" is "a
>>>> string matching the <URI> syntax rule of Section 3", and the <URI>
>>>> production *allows* a fragment identifier.
>>> And why should that be a problem?
>> Because a base URI as defined in RFC 3986 does not permit a fragment
>> identifier.  Therefore, if @base specified a relative URI which was
>> resolved using RFC3986 "Reference Resolution" then the result could
>> contain a fragment identifier.  Thus, a Turtle "base URI" could contain
>> a fragment identifier, whereas an RFC 3986 "base URI" does not permit a
>> fragment identifier.
> No, that's not correct. Even if the base contains a fragment identifier the
> result of resolving any relative IRI (even the empty string "") will result
> in a URI which does not contain the fragment identifier. Thus it really
> doesn't matter. The fragment identifier will be ignored in any case.
>>> It is aligned with the two RFCs. There might be a case where you
>>> can't resolve a relative @base as the document itself has no IRI
>>> but that's the
>>> same problem as not being able to resolve relative IRIs anywhere else
>>> in such a document.
>> If it is aligned with RFC 3986 and 3987 then the alignment certainly is
>> not very visible.  I spent quite a lot of time trying to track it down,
>> and finally concluded that nothing in the Turtle spec requires Turtle's
>> notion of a base URI (which AFAICT is specified using @base) to be an
>> absolute-IRI as defined in those RFCs.  Can you please point me to the
>> exact wording that requires a Turtle base URI to be an absolute-IRI?
> @base enables to establishment of the base URI, it is not the final URI. If
> base contains a relative IRI it is resolved against the document's URI or
> the application supplied base to obtain the final base URI.
>> [...]
>> Notice that it only references RFC3986 sections 5.1.2 and 5.1.3, which
>> only talk (vaguely) about where the base URI might come from.  Those
>> sections do not constrain the base URI to be an absolute-URI.  It is
>> the
>> beginning of RFC3986 section 5.1 that constrains a base URI to be an
>> absolute-URI, and that portion is *not* referenced by the Turtle spec.
> Yes, in the end you need an absolute URI otherwise you can't resolve
> relative ones. There a number of "layers" where the base might come from.
> @base -> document URI -> application supplied. I'm writing this mail offline
> so I can't give you the exact section in the RFC, but that's explained there
> as well.
>> The last sentence of that second paragraph in Turtle section 6.3 does
>> say "Each @base directive sets a new In-Scope Base URI, relative to the
>> previous one", and I guess that sentence is the justification for why
>> you and Andy are saying that @base can specify a relative URI.  But
> Yes
>> knowing that RFC3986 requires a base URI to be an absolute-URI, I had
>> understood that sentence to mean "Each @base directive sets a new
>> In-Scope Base URI, [in relation to] to the previous one", i.e., it is
>> new in relation to the previous one.  I had no idea it was suggesting
>> that @base could specify a relative URI.
>> Bottom line:
>>    - This stuff is not at all clear in the current wording.
> I find that quite clear and in-line with what, e.g., HTML does. Can you
> suggest some concrete wording which would make it clearer?
>>    - If @base is permitted to specify a relative IRI then: (a) an
>> explanation should be added to explain how that relative IRI is
>> converted into an absolute-IRI (including what happens to any fragment
>> identifier that the relative IRI contains); and (b) Turtle will not be
>> aligned with SPARQL in this regard.
> The RFC's explain how a relative IRI can be resolved against a base to an
> absolute IRI. @base does nothing special here. Isn't referencing the RFC
> enough?
>>    - If @base is NOT permitted to specify a relative IRI then the Turtle
>> spec should make clear that @base must specify an absolute-IRI, in
>> alignment with SPARQL.
> That's not the case.
>> I was not aware that HTML allowed base URIs to be relative, but, it
>> seems more important to align Turtle with SPARQL than with HTML.  Plus
>> it would also be simpler.
> What's the advantage of such a restriction? If someone wants to use absolute
> URIs that's fine. It doesn't add any complexity because the URI resolution
> algorithm have to be implemented in any case.
> --
> Markus Lanthaler
> @markuslanthaler
Received on Wednesday, 29 May 2013 05:10:28 UTC

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