GNU bug report logs - #77725
31.0.50; Add support for types accepted by `cl-typep' to cl-generic?

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From: David Ponce <da_vid <at> orange.fr>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 77725 <at> debbugs.gnu.org, Eli Zaretskii <eliz <at> gnu.org>
Subject: bug#77725: 31.0.50; Add support for types accepted by `cl-typep' to cl-generic?
Date: Sat, 26 Apr 2025 11:50:31 +0200

On 2025-04-25 21:37, Stefan Monnier wrote:
> [ Just tooting my horn, nothing to see.  ]
> 
>>> Here is a test case with a recursion:
>>>
>>> (cl-deftype T1 ()
>>>    "Root type.
>>> Check if passed object is a subtype of T1. I.e., if T1 is present in
>>> object type parents."
>>>    `(satisfies ,(lambda (o) (memq 'T1 (gtype-of o)))))
>>
>> Hmm... I can see that it could be handy if you don't know how to
>> characterize type T1 other than as the "sum" of its subtypes.
>> But this seems rather circular.  Have you seen such things out in
>> the wild?
> 
> BTW, this form of circularity is called an *impredicative* definition:
> the above definition does not directly refer to T1 (as in the case of
> a recursive definition), instead it refers (via the definition of
> `gtypes-of`) to the set of all types, to which T1 happens to belong.
> 
> As it happens, the notion of *type* was invented/introduced by Bertrand
> Russel (early 20th century) specifically in order to try and disallow
> the kind of paradoxes (like Russel's paradox or the paradox of the
> barber) that impredicativity makes possible.
> 
> OK, you can go back to your regularly scheduled ELisp hacking now,

Thank you!  I always appreciate enriching my knowledge :-)

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