Many back ends are quite similar.
nnml is just like
nnspool, but it allows you to edit the articles on the server.
nnmh is just like
nnml, but it doesn't use an active file,
and it doesn't maintain overview databases.
nndir is just like
nnml, but it has no concept of “groups”, and it doesn't allow
It would make sense if it were possible to “inherit” functions from back ends when writing new back ends. And, indeed, you can do that if you want to. (You don't have to if you don't want to, of course.)
All the back ends declare their public variables and functions by using a
To inherit functions from other back ends (and allow other back ends to inherit functions from the current back end), you should use the following macros:
(nnoo-declare nndir nnml nnmh)
nndir has declared here that it intends to inherit functions from
defvar, but registers the variable as a public server variable. Most state-oriented variables should be declared with
In addition to the normal
defvar parameters, it takes a list of
variables in the parent back ends to map the variable to when executing
a function in those back ends.
(defvoo nndir-directory nil "Where nndir will look for groups." nnml-current-directory nnmh-current-directory)
This means that
nnml-current-directory will be set to
nndir-directory when an
nnml function is called on behalf
nndir. (The same with
defunand takes the same parameters. In addition to doing the normal
defunthings, it registers the function as being public so that other back ends can inherit it.
(nnoo-map-functions nndir (nnml-retrieve-headers 0 nndir-current-group 0 0) (nnmh-request-article 0 nndir-current-group 0 0))
This means that when
nndir-retrieve-headers is called, the first,
third, and fourth parameters will be passed on to
nnml-retrieve-headers, while the second parameter is set to the
(nnoo-import nndir (nnmh nnmh-request-list nnmh-request-newgroups) (nnml))
This means that calls to
nndir-request-list should just be passed
nnmh-request-list, while all public functions from
nnml that haven't been defined in
nndir yet should be
Below is a slightly shortened version of the
nndir back end.
;;; nndir.el --- single directory newsgroup access for Gnus ;; Copyright (C) 1995,1996 Free Software Foundation, Inc. ;;; Code: (require 'nnheader) (require 'nnmh) (require 'nnml) (require 'nnoo) (eval-when-compile (require 'cl)) (nnoo-declare nndir nnml nnmh) (defvoo nndir-directory nil "Where nndir will look for groups." nnml-current-directory nnmh-current-directory) (defvoo nndir-nov-is-evil nil "*Non-nil means that nndir will never retrieve NOV headers." nnml-nov-is-evil) (defvoo nndir-current-group "" nil nnml-current-group nnmh-current-group) (defvoo nndir-top-directory nil nil nnml-directory nnmh-directory) (defvoo nndir-get-new-mail nil nil nnml-get-new-mail nnmh-get-new-mail) (defvoo nndir-status-string "" nil nnmh-status-string) (defconst nndir-version "nndir 1.0") ;;; Interface functions. (nnoo-define-basics nndir) (deffoo nndir-open-server (server &optional defs) (setq nndir-directory (or (cadr (assq 'nndir-directory defs)) server)) (unless (assq 'nndir-directory defs) (push `(nndir-directory ,server) defs)) (push `(nndir-current-group ,(file-name-nondirectory (directory-file-name nndir-directory))) defs) (push `(nndir-top-directory ,(file-name-directory (directory-file-name nndir-directory))) defs) (nnoo-change-server 'nndir server defs)) (nnoo-map-functions nndir (nnml-retrieve-headers 0 nndir-current-group 0 0) (nnmh-request-article 0 nndir-current-group 0 0) (nnmh-request-group nndir-current-group 0 0) (nnmh-close-group nndir-current-group 0)) (nnoo-import nndir (nnmh nnmh-status-message nnmh-request-list nnmh-request-newgroups)) (provide 'nndir)