double_quotes Prolog flag incorrect and cannot be modified

[UWN]

?- current_prolog_flag(double_quotes, V).

V = chars ? ;

no
?- writeq("a").
[97] % unexpected - expected [a]
yes

?- set_prolog_flag(double_quotes, codes).

no, unexpected.

The flag double_quotes was introduced by ISO as a changeable flag in 7.11.2.5 meaning (6.3.7):

If the Prolog flag double_quotes has a value chars, a
double quoted list token dql containing L double
quoted characters is a list l with L elements, where
the N-th element of the list is the one-char atom whose
name is the N-th double quoted character of dql.
....

[UWN]

Previously reported 2015-01-31 as #29.

[UWN]

Actually, already reported 2011-01-24 as #18

[triska]

@jfmc: If possible, could you please have a look at this issue? For the upcoming ISO SC 22 meeting, I would like to add Ciao to the Unicode test cases, and these tests make the most sense when double_quotes is set to chars since they are about Unicode characters in quoted context.

It seems this issue is confined to the lexer, which only would have to read double-quoted strings as characters (instead of codes), no semantic or textual changes are necessary in programs, the parser or any other parts of the engine. Is there anything else that blocks this?

[jfmc]

Dear @triska, I am away from my development computer this week. Effectively this is an easy change that should not take too much to fix and knowing that it is required for your tests, we will give it much higher priority.