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

[Eric Abrahamsen <eric@ericabrahamsen.net>]

I'm working on changing Gnus servers into structs, and replacing the
nnoo.el architecture with generic functions, and while I think I've got
a workable roadmap I'm running into some practical confusions. Stefan
I'm ccing you directly because I suspect you're the only one who knows
the answers to my questions :)

The approach is this: Change the server interface functions in
gnus-int.el into generic functions. Provide a generalizer that
recognizes current Gnus servers, and dispatches to the current function
definitions (using `gnus-get-function' and all that). Gradually change
the in-tree backends to be defined with cl-defstruct, and use normal
dispatching-on-struct for those. Eventually the legacy generalizer would
be left in place just to deal with old-style out-of-tree backends.

As an example, here's what `gnus-request-list' currently looks like:

--8<---------------cut here---------------start------------->8---
gnus-int.el:
(defun gnus-request-list (gnus-command-method)
  (funcall (gnus-get-function gnus-command-method 'request-list)
	   (nth 1 gnus-command-method)))

nnimap.el:
(deffoo nnimap-request-list (&optional server)
 <get IMAP group list>)
--8<---------------cut here---------------end--------------->8---

Afterward it would look like this:

--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)))

nnimap.el:
(cl-defmethod gnus-request-list ((server gnus-server-imap))
  <get IMAP group list>)
--8<---------------cut here---------------end--------------->8---

The nnimap version will dispatch on the `gnus-server-imap' defstruct,
that happens automatically. What I need is to be able to write a
generalizer/specializer that will recognize a legacy server (if calling
it "legacy" is annoying I can find something else) and dispatch on it.
The docstring of `cl-generic-define-generalizer' is gnomic, though I
found some better information in the docstring of
`cl-generic-generalizers', and looked at some of the existing
generalizer definitions.

Here's what I've worked up so far:

--8<---------------cut here---------------start------------->8---
(cl-generic-define-generalizer gnus-legacy-server-generalizer
  90 (lambda (name &rest _) `(???)
  (lambda (tag &rest _) (when (eq (car-safe tag) 'gnus-legacy-server)
			  (list tag))))

(cl-defmethod cl-generic-generalizers :extra "gnus-legacy-server" (thing)
  "Dispatch on old-style Gnus server definitions."
  (when wut?
    (list gnus-legacy-server-generalizer)))
--8<---------------cut here---------------end--------------->8---

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. I just don't know where that test is supposed to go,
or why.

I understand this will probably be inefficient and ugly, but I hope that
before too long it would be a rare case that the generalizer is checked
at all.

Thanks,
Eric