Subject: Re: bug#25295: Acknowledgement (26.0.50; Represent eieio objects using object-print in backtraces and edebug)

From: [email-masked]
Date: Sat, 31 Dec 2016 14:23:18 -0500

Eric Abrahamsen <[email-masked]> writes:

> On 12/31/16 00:48 AM, [email-masked] wrote:
>> Eric Abrahamsen <[email-masked]> writes:
>>
>> I think the only way to integrate `object-print' with the existing
>> `print' functions, would be to make it follow the same protocol.  That
>> is, currently `object-print' is really `object-to-string', it should be
>> changed (or perhaps a new function (e.g., `print-object') would be a
>> better idea, so as not to break existing code too much) to accept a
>> PRINTCHARFUN argument, and print to it.
>
> The problem is that pretty much all of the printing happens at the C
> level. Whole lisp structures are sent directly to C, and it's the C code
> that recurses through them and decides how to print everything it finds
> inside. Lisp code never gets a chance (except in a few very specific
> situations).
>
> For example: when an error is raised, `backtrace--print-frame' gets all
> the contents of the error as a single argument. It simply punts that to
> `prin1', and then it's done. There's no chance to pick apart that single
> argument and see if there is an object inside. `eval-expression'
> essentially does the same thing.
>

You would need to add a branch in print_object to detect eieio objects,
same as for the other alternative.

>>> Personally, I'd be willing to lose the ability to customize object
>>> representations with `object-print', if it meant that print_object could
>>> produce a #<obj notation for eieio objects. That would mean writing a 
>>> C test like INSTANCEP or what have you.
>>>
>>
>> That's easier, of course, but a non-customized representation would be
>> pretty uninformative.
>
> Having looked at the code, I'm not too optimistic about achieving the
> ideal solution. Getting eval-expression and backtraces to stop exploding
> seems like enough for now.