Re: Escapes in Backquoted Strings

[Jan Burse]

Hi

Thank you, this explains a lot. So we have here
a feature (atom_no_escape) and a bug (X = `\`
not accepted).

Bye

Daniel Diaz schrieb:
>   Hello Jan,
>
> sorry for the late reply...
>
> There is a flag to control the meaning of backquoted strings. By default
> it is an atom without interpreting escape sequences.
> But this can be changed (for more info see
> http://gprolog.org/manual/html_node/gprolog045.html#toc178).
>
> Here is an example:
>
> | ?- current_prolog_flag(back_quotes,X).
>
> X = atom_no_escape
>
> yes
> | ?- X=`\x61\`.
>
> X = '\\x61\\'
>
> yes
> | ?- set_prolog_flag(back_quotes,atom).
>
> yes
> | ?- X=`\x61\`.
>
> X = a
>
>
> Hope this helps
>
> Daniel
>
>
> Le 19/03/2011 18:32, Jan Burse a écrit :
> > Dear All
> >
> > I am working with
> >     GNU Prolog 1.3.1
> >     By Daniel Diaz
> >     Copyright (C) 1999-2009 Daniel Diaz
> >
> > It seems that escapes in backquoted strings are
> > detected, but not converted.
> >
> > Normal single quotes:
> >
> >   ?- X = 'abc'.
> >   X = abc
> >   | ?- X = '\''.
> >   X = ''''
> >   ?- X = '\x61\'.
> >   X = a
> >
> > Back quotes:
> >
> >   ?- X = `abc`.
> >   X = abc
> >   ?- X = `\'`.
> >   X = '\\'''
> >   ?- X = `\x61\`.
> >   X = '\\x61\\'
> >
> > It seems that the escapes are not converted. But
> > they are detected during parsing of backquoted
> > strings, at least the first escaped character is
> > always expected:
> >
> >   ?- X = `\`.
> >   .
> >   uncaught exception: error(syntax_error('user_input:10 (char:8)
> >   expression expected or previous operator needs brackets'),read_term/3)
> >
> > But the closing escape character of a noctal or
> > hexadecimal constant seems not to be not expected:
> >
> >   ?- X = `\x61`.
> >   X = '\\x61'.
> >   ?- X = `\0`.
> >   X = '\\0'
> >
> > From my understanding of ISO standard for back quoted strings,
> > the same escaping rules should hold as for single quoted
> > strings. I am referening to non quote char (* 6.4.2.1 *).
> >
> > Is this a bug or a feature?
> >
> > Bye
> >
> >
> >
> >
> >
> > _______________________________________________
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> >
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>
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