Re: Declaring Lisp function types

[Andrea Corallo <acorallo@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Andrea Corallo <acorallo@gnu.org>
>> Cc: Eli Zaretskii <eliz@gnu.org>,  emacs-devel@gnu.org,
>>   stefankangas@gmail.com,  adam@alphapapa.net,  arthur.miller@live.com
>> Date: Wed, 01 May 2024 16:57:58 -0400
>>
>> Stefan Monnier <monnier@iro.umontreal.ca> writes:
>>
>> > Thanks Eli for reminding me: as mentioned earlier, I think we should
>> > clarify what are the possible consequences if a type does not faithfully
>> > reflect the behavior of the function.  Examples of questions that one
>> > should be able to answer based on the doc:
>> >
>> > - Can an incorrect type affect Emacs' behavior (other than performance)?
>> > - And if so, can it cause a crash?
>> > - What if the type is/was right but an advice does
>> >   not obey the original type of the advised function?
>> >
>> > The doc doesn't need to answer those questions directly, but it should
>> > describe the possible effects of type annotations in enough details that
>> > we can answer those questions.
>>
>> Thanks, I think it should be more clear now.  I don't think we want to
>> be over specific there as what the implementation does when the
>> 'promise' of the declaration is not respected might change in the
>> future.
>
> The text you added says an incorrect type produces undefined behavior.
> Is that true for all alternatives of calling the function:
> interpreted, byte-compiled, and native-compiled, or only with some of
> these?

AFAIK only native-compiled ATM.  Do you think we should specify that?

> "Undefined behavior" sounds scary, and I wonder whether it is really
> accurate.  If the practical expressions of this UB are only some
> specific ones, perhaps we should mention them?  For example, UB
> generally means your entire system could be brought down, but I very
> much doubt that this could happen due to incorrect type declaration,
> could it?

There are probably very few cases that can cause this (ATM I could think
only of one) but if one pokes at that is actually not that difficult:

(defun foo ()
  (declare (type (function () cons)))
  3)

(defun bar ()
  (car (foo)))

Compiling this code and calling car crashes my Emacs.

PS We might want to control this in the future adding a compilation
parameter 'safety'.

  Andrea