GNU bug report logs - #20087
'gensym' is not guaranteed to return a fresh symbol

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Message #17 received at 20087 <at> debbugs.gnu.org (full text, mbox):

From: rain1 <at> openmailbox.org
To: 20087 <at> debbugs.gnu.org
Subject: Re: bug#20087: gensym
Date: Tue, 22 Mar 2016 11:21:05 +0000

On 2016-03-22 05:24, Mark H Weaver wrote:
> ludo <at> gnu.org (Ludovic Courtès) writes:
>> ‘gensym’ returns interned symbols, but the algorithm to determine the
>> new symbol is simplistic and predictable.
>> 
>> Thus, one can arrange to produce a symbol before ‘gensym’ does, 
>> leading
>> ‘gensym’ to return a symbol that’s not fresh (in terms of ‘eq?’), as 
>> is
>> the case with the second call to ‘gensym’ here:
> 
> rain1 <at> openmailbox.org writes:
>> I agree, this goes against the main assumption people have about
>> gensym. I was able to reproduce the bug.
>> 
>> Here's a patch to libguile/symbol.c which fixes this behavior by
>> incrementing the gensym counter in a loop until it creates a fresh
>> symbol.
> 
> I've considered this idea in the past, but it only avoids collisions
> with symbols that have been interned before the gensym.  It does not
> avoid collisions with symbols interned *after* the gensym.  Obviously,
> there's no way to avoid such collisions.

Thanks for looking over the patch I sent!

One expects of gensym to create a fresh symbol, something not EQ? to any 
symbol that already exists. It is an important property to be able to 
rely on and this patch achieves that.

About symbols interned after, would that refer to something like this:

------------------------
scheme@(guile-user)> (define a (gensym "x"))
scheme@(guile-user)> a
$1 = x280
scheme@(guile-user)> (eq? a (string->symbol "x280"))
$2 = #t
------------------------

In most lisps gensym creates an uninterned symbol. I think that would 
stop the previous giving #t. I could write a patch for this if wanted.

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