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11.7.2.4 Writing New Back Ends

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 editing articles.

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 package called nnoo.

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
This macro declares the first parameter to be a child of the subsequent parameters. For instance:
          (nnoo-declare nndir
            nnml nnmh)

nndir has declared here that it intends to inherit functions from both nnml and nnmh.

defvoo
This macro is equivalent to defvar, but registers the variable as a public server variable. Most state-oriented variables should be declared with defvoo instead of defvar.

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 of nndir. (The same with nnmh.)

nnoo-define-basics
This macro defines some common functions that almost all back ends should have.
          (nnoo-define-basics nndir)

deffoo
This macro is just like defun and takes the same parameters. In addition to doing the normal defun things, it registers the function as being public so that other back ends can inherit it.
nnoo-map-functions
This macro allows mapping of functions from the current back end to functions from the parent back ends.
          (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 value of nndir-current-group.

nnoo-import
This macro allows importing functions from back ends. It should be the last thing in the source file, since it will only define functions that haven't already been defined.
          (nnoo-import nndir
            (nnmh
             nnmh-request-list
             nnmh-request-newgroups)
            (nnml))

This means that calls to nndir-request-list should just be passed on to nnmh-request-list, while all public functions from nnml that haven't been defined in nndir yet should be defined now.

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)