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

[Eric Abrahamsen]

On 11/17/19 23:36 PM, Stefan Monnier wrote:
>> But just so I don't feel like I wasted my afternoon: how *would* one
>> write the generalizer I'm talking about?
>
> Ah, that part.
> I guess It could look like (guaranteed 100% untested):
>
>     (defvar gnus--legacy-server-tag (make-symbol "gnus-legacy-server"))
>     (cl-generic-define-generalizer gnus-legacy-server-generalizer
>       90 (lambda (varname &rest _)
>            `(and (consp ,varname)
>                  (symbolp (car ,varname))
>                  (assq (car ,varname) nnoo-definition-alist)
>                  (stringp (cadr ,varname))
>                  gnus--legacy-server-tag))
>       (lambda (tag &rest _)
>         (when (eq tag gnus--legacy-server-tag) (list tag))))
>     
>     (cl-defmethod cl-generic-generalizers ((_ (eql gnus-legacy-server)))
>       "Dispatch on old-style Gnus server definitions."
>       (list gnus-legacy-server-generalizer))

That's perfect, thanks. I don't need tested code, I just needed to
understand the logic of which bit is responsible for what. I wonder if
it would be useful to have an example of this, either in the manual or
cl-generic.el code comments? 

There's still a lot of work to do before struct servers and cons servers
can coexist within Gnus, but it's nice to have this piece of the puzzle
solved.