Re: Escapes in Backquoted Strings

Hi,

can you give me the exact error message for X = `\`
?

If it is

| ?- X = `\`.

uncaught exception: error(syntax_error('user_input:1 (char:8)
expression expected or previous operator needs
brackets'),read_term/3)

it is due to the fact \ is also an operator and thus needs to be put
inside ( ) as an argument of another operator (here it is =). Use
instead:

| ?- X = (`\`).

X = \

Daniel

Le 25/03/2011 21:38, Jan Burse a écrit :

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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