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

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From: ludo <at> gnu.org (Ludovic Courtès)
To: Mark H Weaver <mhw <at> netris.org>
Cc: rain1 <at> openmailbox.org, 20087 <at> debbugs.gnu.org
Subject: bug#20087: gensym
Date: Thu, 24 Mar 2016 09:45:38 +0100

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

> 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?

Oh indeed, I guess I had overlooked that.

Ludo’.

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