GNU bug report logs - #17825
broken set! in iteration

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Message #15 received at 17825 <at> debbugs.gnu.org (full text, mbox):

From: Mark H Weaver <mhw <at> netris.org>
To: Alírio Eyng <alirioeyng <at> gmail.com>
Cc: 17825 <at> debbugs.gnu.org
Subject: Re: bug#17825: broken set! in iteration
Date: Sat, 21 Jun 2014 17:29:05 -0400

Mark H Weaver <mhw <at> netris.org> writes:

> tags 17825 notabug
> close 17825
> thanks
>
> Alírio Eyng <alirioeyng <at> gmail.com> writes:
>
>> I can't see why the second code doesn't work like the first:
>>
>> (use-modules (srfi srfi-1))
>> (define D '(((3 4))))
>> (let ((r 1))
>>        (set! D (append D '(())))
>>        (display D)(newline)
>>        (set-car! (drop D r) (car (drop D (- r 1)))))
>> (let ((r 2))
>>        (set! D (append D '(())))
>>        (display D)(newline)
>>        (set-car! (drop D r) (car (drop D (- r 1)))))
>> output:
>> (((3 4)) ())
>> (((3 4)) ((3 4)) ())
>>
>> (use-modules (srfi srfi-1))
>> (define D '(((3 4))))
>> (map (lambda (r)
>>        (set! D (append D '(())))
>>        (display D)(newline)
>>        (set-car! (drop D r) (car (drop D (- r 1)))))
>>      '(1 2))
>> output:
>> (((3 4)) ())
>> (((3 4)) ((3 4)) ((3 4)))
>
> This code mutates literal lists, which is not allowed.  Specifically,
> this code calls 'set-car!' on pairs that come from a literal list,
> namely '(()).  The optimizer assumes that this will never happen, and
> generates code based on that assumption.

Actually, this isn't even a question of optimization.  It is simply the
fact that '(()) returns the same pair every time, and the 'set-car!'
mutates it to '((3 4)).  In some future version of Guile, we hope to
detect this error somehow, perhaps by keeping literals in read-only
memory.

(list '()), on the other hand, returns a freshly-allocated pair each
time it is called.

      Mark

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