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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)

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