bug#38252: 27.0.50; Gnus server definitions and generic function specializers

[Eric Abrahamsen <eric@ericabrahamsen.net>]

On 11/17/19 21:03 PM, Stefan Monnier wrote:
>> I'm ccing you directly because I suspect you're the only one who knows
>> the answers to my questions :)
>
> I doubt it, there's a lot of people around here more familiar with
> CLOS-style programming than I.
>
>> --8<---------------cut here---------------start------------->8---
>> gnus-int.el:
>> (cl-defgeneric gnus-request-list (server)
>>   "Docs and stuff.")
>>
>> (cl-defmethod gnus-request-list ((server gnus-server-legacy))
>>   (funcall (gnus-get-function server 'request-list)
>> 	   (nth 1 server)))
>
> Why not just:
>
>     (cl-defmethod gnus-request-list (server)
>       (funcall (gnus-get-function server 'request-list)
>     	       (nth 1 server)))
>
> which means "use it as a fallback".  The downside is that it will be
> used for non-legacy servers if there is no specific implementation for
> that server, but presumably you can detect it and signal an appropriate
> error somewhere inside gnus-get-function.
>
>> What I'm trying to do is fairly simple: if the argument is a list, and
>> the head of the list is a symbol that can be assoc'd into
>> `nnoo-definition-alist', and the second element is a string, then it's a
>> gnus-legacy-server.
>
> Why not just use a `cons` specializer?
>
> IIUC all the non-legacy servers will use structs, so you don't need to
> use a specializer that's so specific that it has to check "if the
> argument is a list, and the head of the list is a symbol that can be
> assoc'd into `nnoo-definition-alist', and the second element is
> a string".

Okay, perhaps I was overthinking this. I don't want to assume that the
fallback is an old-style server (we might want to use the fallback for
other purposes), but you're right that 'cons is pretty much sufficient.
If anyone feeds a cons value into a Gnus server interface function that
*isn't* a server definition, that falls into "serves you right"
territory.

But just so I don't feel like I wasted my afternoon: how *would* one
write the generalizer I'm talking about?

Thanks,
Eric