GNU bug report logs - #21855
eq?

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Andy Wingo <wingo <at> pobox.com>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#21855: closed (eq?)
Date: Fri, 24 Jun 2016 15:32:02 +0000

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Your message dated Fri, 24 Jun 2016 17:31:11 +0200
with message-id <87vb0yeh8g.fsf <at> pobox.com>
and subject line Re: bug#21855: eq?
has caused the debbugs.gnu.org bug report #21855,
regarding eq?
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


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From: Atticus <atticus0 <at> posteo.org>
To: bug-guile <at> gnu.org
Subject: eq?
Date: Sat, 07 Nov 2015 13:58:48 +0100

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).

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From: Andy Wingo <wingo <at> pobox.com>
To: <tomas <at> tuxteam.de>
Cc: Atticus <atticus0 <at> posteo.org>, 21855-done <at> debbugs.gnu.org
Subject: Re: bug#21855: eq?
Date: Fri, 24 Jun 2016 17:31:11 +0200

On Sun 08 Nov 2015 11:23, <tomas <at> 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

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