bug#24186: setlocale can't be localised

[Zefram]

Ludovic Courtes wrote:
>That wouldn't help with the "setlocale" issue you describe per se, but
>this would address such use cases in a different way.
>
>WDYT?

Yes, explicit locale objects and locale parameters to relevant functions
are a good thing.  In general, the model of a global locale state is
broken, at least by threading, so some advance beyond the setlocale system
is necessary.  Note the new(er) "uselocale" system in libc, which gives
a per-thread locale state, fixing the biggest problem with setlocale.
Some form of that could also be mapped into Guile; it would be reasonable
to have a fluid that determines the locale to use where not overridden
by an explicit parameter.

All of that is welcome, but, as you say, doesn't deal with the actual
problem I identified with setlocale.  One can expect that setlocale will
continue to be used for the foreseeable future, and it needs to be shorn
of its unwanted side effects.

-zefram

bug#24186: setlocale can't be localised

[Ludovic Courtès]

Hi,

Zefram  skribis:

> (define (call-with-locale cat val body)
>   (let ((oldval #f))
>   (dynamic-wind
> (lambda () (set! oldval (setlocale cat)) (setlocale cat val))
> body (lambda () (setlocale cat oldval)
>
> (define (day-of-week-string)
>   (strftime "%A" (localtime (current-time 
>
> (define (day-of-week-string-for-locale loc)
>   (call-with-locale LC_TIME loc day-of-week-string))
>
> ;; user-locale is application-specific code defined elsewhere
> (define (day-of-week-string-for-user user)
>   (day-of-week-string-for-locale (user-locale user)))

This does not really answer your question, but (ice-9 i18n) provides
first-class locale objects, which avoid the whole global locale issue
(info "(guile) Internationalization").

Currently important procedures such as ‘strftime’ or SRFI-19’s
‘date->string’ cannot use such locale objects, though.  I think it would
make sense to add an optional locale object argument to ‘srftime’ and
‘date->string’ (though we should create a (srfi srfi-19 gnu) module for
that to make it clear that this is a GNU extension.)

That wouldn’t help with the ‘setlocale’ issue you describe per se, but
this would address such use cases in a different way.

WDYT?

Ludo’.

bug#24186: setlocale can't be localised

[Andy Wingo]

On Tue 09 Aug 2016 14:15, Zefram  writes:

> I wrote:
>>is my first time compiling a Guile myself.  It's failing on a missing
>>library for which Debian supplies no package.
>
> Turns out there was a package.  It was complaining about a lack of
> "bdw-gc", and Debian doesn't have anything of that name, but it does
> have it under the name "libgc".  So I've now got 2.1.3 running.
>
> All of the code in my day-of-week-string-for-locale sketch works exactly
> the same on 2.1.3 as it did on 2.0.

Tx for this testing and thanks again for these excellent bug reports :)

Andy

bug#24186: setlocale can't be localised

[Zefram]

I wrote:
>is my first time compiling a Guile myself.  It's failing on a missing
>library for which Debian supplies no package.

Turns out there was a package.  It was complaining about a lack of
"bdw-gc", and Debian doesn't have anything of that name, but it does
have it under the name "libgc".  So I've now got 2.1.3 running.

All of the code in my day-of-week-string-for-locale sketch works exactly
the same on 2.1.3 as it did on 2.0.

-zefram

bug#24186: setlocale can't be localised

[Andy Wingo]

Greets :)

Firstly, just to make sure that we are getting things right in 2.2 (and
if not we need to know), would you mind testing with the latest 2.1.x
release?

Also, do you have a concrete program whose behavior you expect to be
different?  I appreciate the call-with-locale example but I'm not sure
what is going wrong so having an example invocation would be useful.

On Mon 08 Aug 2016 18:32, Zefram  writes:

> The setlocale *write* operation also affects port encoding (actually
> the default port encoding fluid and the encoding of currently-selected
> ports), and that seems to be an intentional change, but it also breaks
> the above code.

I believe that the intention (for better or for worse) is that calling
`setlocale' with 2 arguments changes the "default port encoding".  Like,
the next port you open will have the encoding specified by the
`setlocale', if you don't change it explicitly later.  But I don't think
it should change the encoding of already-open ports, should it?

Andy

bug#24186: setlocale can't be localised

[Zefram]

In Guile 1.8 it was possible to localise the effect of a setlocale
operation, but in Guile 2.0 it's no longer possible by natural use of the
locale API.  This loss of a useful facility is either a bug or something
that needs to be discussed in the documentation.

In Guile 1.8 one could perform a temporary setlocale for the execution of
some piece of code, and revert its effect by another setlocale on unwind.
This looks like:

(define (call-with-locale cat newval body)
  (let ((oldval #f))
(dynamic-wind
  (lambda () (set! oldval (setlocale cat)) (setlocale cat newval))
  body (lambda () (setlocale cat oldval)

Some difficulty arises from this being temporally scoped, where dynamic
or lexical scoping would be nicer, but in single-threaded programs it
works pretty well.  The C setlocale(3) API, after which Guile's setlocale
is modelled, is obviously designed to enable this kind of mechanism: the
read operation reports all relevant state, and the write operation with
the old value sets it all back as it was.  It is critical to this ability
that the read operation does indeed report all the state that will be set.

In Guile 2.0, the setlocale function no longer corresponds so closely to
the C setlocale(3), and this critical guarantee has been lost.  I have
previously reported in bug#22910 that the setlocale read operation
has a side effect on port encoding, and obviously that interferes with
the above code, but actually there's still a problem if that's fixed.
The setlocale *write* operation also affects port encoding (actually
the default port encoding fluid and the encoding of currently-selected
ports), and that seems to be an intentional change, but it also breaks
the above code.  The setlocale read operation doesn't report the encoding
of the currently-selected ports, so doesn't represent everything that
setlocale will set.  The setlocale write operation is not even capable
of setting the port encodings independently: it sets all three to the
encoding nominated by the locale selected for LC_CTYPE purposes.

I think adding this extra effect to setlocale was a mistake.  It doesn't
fit the locale API.  If the extra effect is removed, that would resolve
this problem.

If you really want setlocale to have this effect, then something needs to
be done to address the ability that has been lost.  The documentation
certainly needs to describe the effect on port encoding, which it
currently doesn't.  (There is a mention of some interaction with the
%default-port-encoding fluid in the documentation of that fluid, but it
doesn't match reality: it doesn't say that setlocale writes to the fluid.)
It also ought to specifically warn that the setlocale save-and-restore
dance that works in C doesn't work here.  It should explain what needs
to be done by library functions that want to achieve a localised locale
change.  Are they entirely forbidden to use setlocale?  Are they expected
to manually save and restore port encodings around setlocale calls?
(This is complicated by set-port-encoding! not accepting #f as an encoding
value, despite it actually being a permitted value for the encoding slot.)
Some example code equivalent to the above call-with-locale would be
useful.

-zefram