From: Andy Wingo <wingo@pobox.com>
To: <tomas@tuxteam.de>
Cc: Atticus <atticus0@posteo.org>, 21855-done@debbugs.gnu.org
Subject: bug#21855: eq?
Date: Fri, 24 Jun 2016 17:31:11 +0200 [thread overview]
Message-ID: <87vb0yeh8g.fsf@pobox.com> (raw)
In-Reply-To: <20151108102323.GA5299@tuxteam.de> (tomas@tuxteam.de's message of "Sun, 8 Nov 2015 11:23:23 +0100")
On Sun 08 Nov 2015 11:23, <tomas@tuxteam.de> writes:
> On Sat, Nov 07, 2015 at 01:58:48PM +0100, Atticus wrote:
>> So I wanted to try out gnu guix and thus make myself more familiar with
>> guile first. While running some tests I encountered a problem/bug with eq?:
>>
>> $ guile -v
>> guile (GNU Guile) 2.1.1
>>
>> $ guile
>> scheme@(guile-user)>
>> (define (multirember a lat)
>> (cond
>> ((null? lat) '())
>> ((eq? (car lat) a) (multirember a (cdr lat)))
>> (else (cons (car lat) (multirember a (cdr lat))))))
>>
>> scheme@(guile-user)> (multirember '(a b) '(x y (a b) z (a b)))
>> $1 = (x y z)
>>
>> So why does guile return (x y z)? I expected (x y (a b) z (a b)). I know
>> eq? should only be used with symbols (and thus this example is more
>> theoretical) but nevertheless the return value is not right, since (eq?
>> '(a b) '(a b)) returns #f (Btw same in guile 2.0.11).
>
> Hm. As far as I know (eq? '(a b) '(a b)) is not *guaranteed* to evaluate
> to #f. The implementation might be free to re-use things it "knows" to be
> constant (I might be wrong, though).
Tomas is correct; within one compilation unit, constant literals will be
deduplicated. That means that within one compilation unit, (eq? '(a b)
'(a b)) will indeed be #t.... yarggghhhh.... but:
scheme@(guile-user)> (eq? '(a b) '(a b))
$1 = #f
scheme@(guile-user)> ,optimize (eq? '(a b) '(a b))
$2 = #f
Evidently the optimizer is doing the compare at compile-time, which it
is allowed to do, and at compile-time the values are actually distinct.
I will see if I can fix that. However Tomas' logic is impeccable :)
Closing as things are all working fine, I think.
Cheers,
Andy
prev parent reply other threads:[~2016-06-24 15:31 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-07 12:58 bug#21855: eq? Atticus
2015-11-08 10:23 ` tomas
2015-11-08 13:30 ` Atticus
2015-11-08 13:38 ` tomas
2015-11-09 7:57 ` Atticus
2016-06-24 15:31 ` Andy Wingo [this message]
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