`define-alternatives' bugfix and UI enhancement

[bpt/emacs.git] / lisp / simple.el

diff --git a/lisp/simple.el b/lisp/simple.el
index ae18ae6..8ff6b9c 100644 (file)
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -7654,10 +7654,20 @@ warning using STRING as the message.")
 ;; ;;;###autoload (push '("My impl name" . my-impl-symbol) COMMAND-alternatives
 
 (defmacro define-alternatives (command &rest customizations)
-  "Define new command `COMMAND'.
-The variable `COMMAND-alternatives' will contain alternative
-implementations of COMMAND, so that running `C-u M-x COMMAND'
-will allow the user to chose among them.
+  "Define the new command `COMMAND'.
+
+The argument `COMMAND' should be a symbol.
+
+Running `M-x COMMAND RET' for the first time prompts for which
+alternative to use and record the selected command as a custom
+variable.
+
+Running `C-u M-x COMMAND RET' prompts again and overwrite the
+previous choice.
+
+The variable `COMMAND-alternatives' contains an alist with
+alternative implementations of COMMAND.
+
 CUSTOMIZATIONS, if non-nil, should be composed of alternating
 `defcustom' keywords and values to add to the declaration of
 `COMMAND-alternatives' (typically :group and :version)."
@@ -7688,13 +7698,19 @@ contains the list of implementations currently supported for this command."
          (interactive "P")
          (when (or arg (null ,varimp-sym))
            (let ((val (completing-read
-                       ,(format "Select implementation for command `%s': " command-name)
-                       ,varalt-sym nil t)))
+                      ,(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))))))
+              (when (null ,varimp-sym)
+                (message
+                 "Use `C-u M-x %s RET' to select another implementation"
+                 ,command-name)
+                (sit-for 3))
+              (customize-save-variable ',varimp-sym
+                                       (cdr (assoc-string val ,varalt-sym))))))
          (if ,varimp-sym
-             (funcall ,varimp-sym)
+             (call-interactively ,varimp-sym)
            (message ,(format "No implementation selected for command `%s'"
                              command-name)))))))