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

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Stefan Israelsson Tampe <stefan.itampe <at> gmail.com>
Subject: bug#14039: closed (Re: bug#14039: Bug in with-fluids semantics)
Date: Sat, 23 Mar 2013 19:10:02 +0000

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Your bug report

#14039: Bug in with-fluids semantics

which was filed against the guile package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 14039 <at> debbugs.gnu.org.

-- 
14039: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=14039
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Andy Wingo <wingo <at> pobox.com>
To: Stefan Israelsson Tampe <stefan.itampe <at> gmail.com>
Cc: 14039-close <at> debbugs.gnu.org
Subject: Re: bug#14039: Bug in with-fluids semantics
Date: Sat, 23 Mar 2013 20:07:31 +0100

On Sat 23 Mar 2013 11:41, Stefan Israelsson Tampe <stefan.itampe <at> gmail.com> writes:

> 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.

That's not how fluids work, semantically: for better (I think) or for
worse (you think).  We cannot change this.

A
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From: Stefan Israelsson Tampe <stefan.itampe <at> gmail.com>
To: bug-guile <at> gnu.org
Subject: Bug in with-fluids semantics
Date: Sat, 23 Mar 2013 11:41:29 +0100

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

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.

/Stefan

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