GNU bug report logs - #14039
Bug in with-fluids semantics

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From: Stefan Israelsson Tampe <stefan.itampe <at> gmail.com>
To: Daniel Hartwig <mandyke <at> gmail.com>
Cc: 14039 <at> debbugs.gnu.org
Subject: bug#14039: Bug in with-fluids semantics
Date: Sat, 23 Mar 2013 16:41:15 +0100

I would expect
(k) to be the same. Otherwise fluids would not mix well with undo redo sematics.
But I do understand that people might have decided that it should work
like this.
and knowing the this semantic, one can fix the problem. If the
semantics is correct
I really can't find an example where it is useful though. Do you have
an example?

/Stefan

On Sat, Mar 23, 2013 at 3:51 PM, Daniel Hartwig <mandyke <at> gmail.com> wrote:
> On 23 March 2013 18:41, Stefan Israelsson Tampe <stefan.itampe <at> gmail.com> wrote:
>> Consider this simple exmple with fluids and reodos via propmts,
>>
>> (define (f x)
>>   (let ((s (make-fluid 0)))
>>      (with-fluids ((s 0))
>>         (let lp ((i 0))
>>            (cond ((>= i 100) (fluid-ref s))
>>                  ((= i 50) (abort-to-prompt 'tag) (lp (+ i 1)))
>>                  (else (fluid-set! s (+ (fluid-ref s) i))
>>                        (lp (+ i 1))))))))
>>
>> (define k (call-with-prompt 'tag (lambda () (f 1)) (lambda (k . l)
>> k)))
>>
>> Then we will get in guile-2.0 pretty resent git version
>> scheme@(guile-user)> (k)
>> $1 = 4900
>> scheme@(guile-user)> (k)
>> $2 = 8575
>>
>
> What values do you expect from successive calls to K?
>
>> The reason is that when the with-fluid returns normally it does a full
>> swap. It should only do half a swap e.g. restore the old value of the
>> fluid and not store the current which is of non use because it can not
>> be reached anymore and it contaminates the continuation k.
>
> K captures S, a fluid, along with the dynamic extent.  There is only a
> single dynamic extent to which K resumes, and only one value
> associated to the fluid S within that.  Subsequent calls to K do not
> generate a new dynamic extent, so it makes sense that modifications to
> the fluids value persist.  This example behaives as expected according
> to my understanding of fluids.
>
> Am I missing something?

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