PRINT-OBJECT for built-in classes

[Edi Weitz]

Thomas,

That was the piece of the puzzle I had missed.

Thanks,
Edi.




On Sat, Aug 29, 2015 at 12:04 PM, Thomas Burdick <thomas at burdick.fr> wrote:
> Hi Edi,
>
> That certainly reads to me as though the intent is for print-object to be
> the decision point (at least in cases where *print-pretty* is false).
>
> However, as far as I understand 11.1.2.1.2:
>
> Except where explicitly allowed, the consequences are undefined if any of
> the following actions are performed on an external symbol of the COMMON-LISP
> package:
>
> [...]
>
> 19. Defining a method for a standardized generic function which is
> applicable when all of the arguments are direct instances of standardized
> classes.
>
>
> I think the much-feared "consequences are undefined" phrasing there means
> the implementation is free to act as though the method you defined is in
> fact defined, but completely ignore it when doing dispatch. (or reformat
> your hdd or make monkeys fly or whatever).
>
> -Thomas
>
>> From: edi at weitz.de
>> Date: Sat, 29 Aug 2015 11:10:15 +0200
>> To: pro at common-lisp.net
>> Subject: PRINT-OBJECT for built-in classes
>
>>
>> The following "works" in three different CL implementations I tried
>> while in three others the way complex numbers are printed doesn't
>> change.
>>
>> ? (defmethod print-object ((obj complex) stream)
>> (format stream "#< ~A + ~A * I >"
>> (realpart obj) (imagpart obj)))
>> #<STANDARD-METHOD PRINT-OBJECT (COMPLEX T)>
>> ? #c(1 1)
>> #< 1 + 1 * I >
>>
>> My understanding of 22.1.2 of the standard is that each Lisp MUST have
>> a PRINT-OBJECT method for complex numbers. The question then is
>> whether I'm allowed to redefine it like above (I think I am) and/or
>> whether an implementation is allowed to accept this redefinition
>> without a warning but then to ignore it (which, as I said, is what
>> happens in three respectable Lisps).
>>
>> Thanks,
>> Edi.
>>
>> [Note: There's no point in talking me out of this as I don't actually
>> want to do it anyway. It's just an example and I'm only interested in
>> what exactly is governed by the standard. Complex numbers are also
>> just an example. I'm interested in PRINT-OBJECT for built-in classes
>> in general.]
>>