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Re: Increased lookahead requirements in the Turtle draft

From: Eric Prud'hommeaux <eric@w3.org>
Date: Sat, 2 Nov 2013 08:34:03 -0400
To: David Robillard <d@drobilla.net>
Cc: public-rdf-comments@w3.org
Message-ID: <20131102123401.GA4798@w3.org>
* David Robillard <d@drobilla.net> [2013-02-17 17:43-0500]
> Hi,
> 
> I recently got a bug report from a user who's encountered dots in
> prefixed names in "Turtle" found in the wild which my parser does not
> yet support.  So, I looked at the draft towards implementing this.
> 
> Unfortunately it looks like a can of worms for a simple
> recursive-descent parser.  The previous specification could be
> implemented with 1 character of lookahead, but I don't think this one
> can.
> 
> Since a PrefixedName can contain a dot, while reading a PrefixedName if
> the next character is a dot, it is ambiguous whether or not the dot is
> part of the PrefixedName or the end of a statement.  To determine this,
> you need to check whether or not the next-next character is a valid
> PrefixedName character, and until this is known, neither the dot nor the
> next character can be 'eaten'.
> 
> The significance is that *1* character of "lookahead" isn't really
> lookahead, you just need a peek().  Anything greater requires some kind
> of real lookahead implementation, or at least some crafty case-specific
> kludges to get around it.
> 
> This is not necessarily a spec problem, and two character lookahead is
> not an onerous requirement in general, but compared to 1 it is.  I just
> thought it was worth mentioning that there is a considerable new
> implementation requirement here.  I will have to pay a price in
> throughput for this as well.
> 
> It's clear, though, that dots in prefixed names are desirable.  Ideally,
> tokens, including the delimeters (i.e. '.' and ';'), would be whitespace
> delimited, so reading a PrefixedName would simply stop when whitespace
> is encountered and this problem would not exist.  Perhaps not realistic
> given existing practice, but it would certainly be nice.

Many apologies for the response time on this. Thank you for your
comment and I hope you enjoyed implementing Serd. As to the request
for a required whitespace character after numeric literals, the RDF
Working Group believes that introducing a requirement for whitespace
before '.' and ';' will break a considerable fraction of the deployed
Turtle and introduce unfortunate incompatibilities with SPARQL and
Notation3. If you are content with this resolution, please reply with
"[RESOLVED]" in the begging of the Subject:.


> Cheers,
> 
> -dr



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Received on Saturday, 2 November 2013 12:34:37 UTC

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