Do symbols need to be EQ?

Kenneth Tilton ken at tiltontec.com
Fri Jul 3 08:21:54 UTC 2015


On Fri, Jul 3, 2015 at 3:09 AM, Edi Weitz <edi at weitz.de> wrote:

> Just out of curiosity and without any relevance in practise:
>
> Is there one place in the standard where it is explicitly said that
> two symbols which are the "same" symbol must be "identical"?  I know
> that there are a couple of examples where this is implied, but
> formally the examples aren't part of the standard, right?
>
> The EQ dictionary entry for example shows this example:
>
>   (eq 'a 'a)  =>  true
>
> and then it continues with this note (emphasis mine): "Symbols that
> print the same USUALLY are EQ to each other because of the use of the
> INTERN function."
>
> And the entry for INTERN is actually the closest I could find in terms
> of clarification because it says that if a symbol of a specified name
> is already accessible, _IT_ is returned -- which sounds like object
> identity to me.
>
> But how does this fit into the picture?
>
>   CL-USER 1 > (defparameter *s* 'foo)
>   *S*
>   CL-USER 2 > (unintern 'foo)
>   T
>   CL-USER 3 > (defparameter *s2* 'foo)
>   *S2*
>   CL-USER 4 > (eq *s* *s2*)
>   NIL
>
> *S* has lost its home package and is thus not EQ to *S2*, sure, but
> how do we explain this in terms of object identity?  Has the UNINTERN
> operation changed the identity of *S* which once was the one and only
> CL-USER::FOO but can't be anymore because this role is now occupied by
> *S2*?
>
> Did I miss some clarifying words in the standard?  Did I just manage
> to confuse myself?
>

I think you manged to confuse yourself. unintern of course did not change
the identity of *s* (by which we are meaning the symbol bound to *S*) --
identity is identity is identity. Unintern did, however, change the package
 of *s*, so (as one side-effect) a new symbol of the same name in the same
package is a new object (identical to nothing at birth).

Perhaps the problem is confusing the levels of abstraction offered by (a)
EQ and (b) object identity. The latter is a very simple idea. EQ, as you
adroitly demonstrated, worries about all sorts of things, including a
symbol's package.

my2 anyway.

-kt



> Thanks,
> Edi.
>
> PS: The UNINTERN entry warns about side effects which could harm
> consistency, so maybe this is what they meant?
>
>


-- 
Kenneth Tilton
54 Isle of Venice Dr
Fort Lauderdale, FL 33301

ken at tiltontec.com
http://tiltontec.com
@tiltonsalgebra

646-269-1077

"In a class by itself." *-Macworld*
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