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

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Message #8 received at 14039 <at> debbugs.gnu.org (full text, mbox):

From: Daniel Hartwig <mandyke <at> gmail.com>
To: Stefan Israelsson Tampe <stefan.itampe <at> gmail.com>
Cc: 14039 <at> debbugs.gnu.org
Subject: Re: bug#14039: Bug in with-fluids semantics
Date: Sat, 23 Mar 2013 22:51:05 +0800

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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