Re: Rename `eww' to `web'

[Juanma Barranquero]

;;; dispatcher.el --- generic command dispatcher  -*- lexical-binding: t -*-

;; Copyright (C) 2013 Free Software Foundation, Inc.

;; Author: Juanma Barranquero <address@hidden>
;; Maintainer: FSF
;; Keywords:

;; This file is part of GNU Emacs.

;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.

;;; Commentary:

;; Defines a new macro `define-dispatcher' to create generic commands.
;; Generic commands are ones (like web, mail, news, encrypt, irc, etc.)
;; that can have different alternative implementations where choosing
;; among them is exclusively a matter of user preference.

;; (define-dispatcher COMMAND) creates a new interactive command
;; M-x COMMAND and a customizable variable dispatcher-COMMAND-alternatives.
;; Typically, the user will not need to customize this variable; packages
;; wanting to add alternative implementations should use
;;
;; ;;;###autoload (push '("My impl name" . my-impl-symbol)
dispatcher-COMMAND-alternatives


;;; Code:

(defgroup dispatcher nil
  "Generic dispatcher"
  :group 'applications)

(defmacro define-dispatcher (command &rest customizations)
  "Define new command `COMMAND'.
The variable `dispatcher-COMMAND-alternatives' will contain
alternative implementations of COMMAND, so that running
`C-u M-x COMMAND' will allow the user to chose among them.
CUSTOMIZATIONS, if non-nil, should be composed of alternating
`defcustom' keywords and values to add to the declaration of
`dispatcher-COMMAND-alternatives' (typically to add new groups)."
  (let* ((command-name (symbol-name command))
         (varalt-name (concat "dispatcher-" command-name "-alternatives"))
         (varalt-sym (intern varalt-name))
         (varimp-sym (intern (concat "dispatcher--" command-name
"-implementation"))))
    `(progn

       (defcustom ,varalt-sym nil
         ,(format "Alist of alternative implementations for the `%s' command.

Each entry must be a pair (ALTNAME . ALTFUN), where:
ALTNAME - The name shown at user to describe the alternative implementation.
ALTFUN  - The function called to implement this alternative."
                  command-name)
         :type '(alist :key-type string :value-type function)
         :group 'dispatcher
         ,@customizations)

       (defvar ,varimp-sym nil "Internal use only.")

       (defun ,command (&optional arg)
         ,(format "Run generic command `%s'.
If used for the first time, or with interactive ARG, ask the user which
implementation to use for `%s'.  The variable `%s'
contains the list of implementations currently supported for this command."
                  command-name command-name varalt-name)
         (interactive "P")
         (when (or arg (null ,varimp-sym))
           (let ((val (completing-read
                       ,(format "Select implementation for command
`%s': " command-name)
                       ,varalt-sym nil t)))
             (unless (string-equal val "")
               (customize-save-variable ',varimp-sym
                                        (cdr (assoc-string val ,varalt-sym))))))
         (if ,varimp-sym
             (funcall ,varimp-sym)
           (message ,(format "No implementation selected for commmand `%s'"
                             command-name)))))))

(provide 'dispatcher)

;;; dispatcher.el ends here

dispatcher.el
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