Re: Should mode commands be idempotent?

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> It shouldn't be needed: the idempotence should emerge naturally from the
>> way the code is written, rather than being the result of special tests
>> to detect that particular situation.
> I'm not too sure: take the example of visual-line-mode: how do you make that
> idempotent without explicitly checking whether the mode has already
> been activated?

In this case I'd typically do something like the patch below,


        Stefan


diff --git a/lisp/simple.el b/lisp/simple.el
index a3d1cc3864..ed0a29bbcc 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -7053,22 +7053,21 @@ visual-line-mode
   :lighter " Wrap"
   (if visual-line-mode
       (progn
-	(set (make-local-variable 'visual-line--saved-state) nil)
-	;; Save the local values of some variables, to be restored if
-	;; visual-line-mode is turned off.
-	(dolist (var '(line-move-visual truncate-lines
-		       truncate-partial-width-windows
-		       word-wrap fringe-indicator-alist))
-	  (if (local-variable-p var)
-	      (push (cons var (symbol-value var))
-		    visual-line--saved-state)))
+	(unless visual-line--saved-state
+	  ;; Save the local values of some variables, to be restored if
+	  ;; visual-line-mode is turned off.
+	  (dolist (var '(line-move-visual truncate-lines
+		         truncate-partial-width-windows
+		         word-wrap fringe-indicator-alist))
+	    (if (local-variable-p var)
+	        (push (cons var (symbol-value var))
+		      visual-line--saved-state))))
 	(set (make-local-variable 'line-move-visual) t)
 	(set (make-local-variable 'truncate-partial-width-windows) nil)
 	(setq truncate-lines nil
-	      word-wrap t
-	      fringe-indicator-alist
-	      (cons (cons 'continuation visual-line-fringe-indicators)
-		    fringe-indicator-alist)))
+	      word-wrap t)
+        (add-to-list 'fringe-indicator-alist
+                     (cons 'continuation visual-line-fringe-indicators)))
     (kill-local-variable 'line-move-visual)
     (kill-local-variable 'word-wrap)
     (kill-local-variable 'truncate-lines)