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

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Message #23 received at 20087 <at> debbugs.gnu.org (full text, mbox):

From: Mark H Weaver <mhw <at> netris.org>
To: ludo <at> gnu.org (Ludovic Courtès)
Cc: rain1 <at> openmailbox.org, 20087 <at> debbugs.gnu.org
Subject: Re: bug#20087: gensym
Date: Wed, 23 Mar 2016 13:55:13 -0400

ludo <at> gnu.org (Ludovic Courtès) writes:

> Mark H Weaver <mhw <at> netris.org> skribis:
>
>> If we cannot eliminate the possibility of collisions, and we cannot
>> avoid intentional collisions, what can we do?  I think the best we can
>> hope for is to significantly reduce the probability of _unintentional_
>> collisions, perhaps by starting the gensym counter at a large number.
>
> I’m not sure if that would help.
>
> One thing that could help avoid unintentional collisions is to
> automatically add whitespace before the number, such that:
>
>   (gensym "x") => #{x 123}#

I think this is a good idea.

>> The other thing we can do is to clearly document these inherent problems
>> with gensym, so that they will not be misused for jobs for which they
>> are not appropriate.
>
> I think we should add a sentence to that effect in the manual.

It turns out the manual already has the following text in the ‘gensym’
entry, which I think is sufficient.

     The symbols generated by ‘gensym’ are _likely_ to be unique, since
  their names begin with a space and it is only otherwise possible to
  generate such symbols if a programmer goes out of their way to do so.
  Uniqueness can be guaranteed by instead using uninterned symbols
  (*noteSymbol Uninterned::), though they can’t be usefully written out
  and read back in.

What do you think?

      Mark

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