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Re: Literal quote character causes exception.

From: Paulo Moura
Subject: Re: Literal quote character causes exception.
Date: Tue, 6 Oct 2009 15:54:42 +0100

On 2009/10/06, at 13:18, Nicolas Pelletier wrote:


On Tue, Oct 6, 2009 at 19:46, Daniel Diaz <address@hidden paris1.fr> wrote:

after looking carefully at the ISO standard, there is no bug here. ISO says
the syntax is:

0'<single quoted char>

where <single quoted char> is a character that could appear in a quoted string. So if you want to use ' inside either you have to write it twice
('') or to escape it \'

You should use one of the valid forms: 0''' or 0'\'

I see... Yes, on re-reading, I was wrong.

I don't have the standard with me right now, but I seem to remember that the ('') and \' is only used within quoted atoms.

I agree the result is not very nice...
In 1.3.2 I will relax the lexer to also accept 0'' (to remain compatible
with other Prolog systems).

Thank you. Compatibility is indeed how I ran into this case: Logtalk
includes in its contributions directory a XML parser, which loaded
fine when used over SWI Prolog, but failed to load when user over GNU
Prolog. Running further tests now with XSB shows there are varying
interpretations on what the standard says...

Unfortunately, yes. Nevertheless, GNU Prolog is the first compiler I have found that chokes on 0''. Relaxing the lexer seems the sensible thing to do until the standard is clarified/rectified/updated.

Best regards,


Paulo Jorge Lopes de Moura, PhD
Assistant Professor
Dep. of Computer Science, University of Beira Interior
6201-001 Covilhã, Portugal

Office 3.18  Ext. 3276
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Research:  <http://logtalk.org/> Blog: <http://blog.logtalk.org/>

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