Re: Escapes in Backquoted Strings

Daniel Diaz Fri, 25 Mar 2011 09:57:34 -0700

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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Re: Escapes in Backquoted Strings

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