emacs: Improve the definition and use of `notmuch-fcc-dirs'.

Re-work the declaration and definition of `notmuch-fcc-dirs'. The
variable now allows three types of values:

- nil: no Fcc header is added,

- a string: the value of `notmuch-fcc-dirs' is the name of the
  folder to use,

- a list: the folder is chosen based on the From address of the
  current message using a list of regular expressions and
  corresponding folders:

     ((\"Sebastian@SSpaeth.de\" . \"privat\")
      (\"spaetz@sspaeth.de\" . \"OUTBOX.OSS\")
      (\".*\" . \"defaultinbox\"))

  If none of the regular expressions match the From address, no
  Fcc header will be added.

emacs/notmuch-maildir-fcc.el

@@ -16,6 +16,7 @@
 ;; To use this as the fcc handler for message-mode,
 ;; customize the notmuch-fcc-dirs variable
 
+(eval-when-compile (require 'cl))
 (require 'message)
 
 (require 'notmuch-lib)
@@ -23,41 +24,40 @@
 (defvar notmuch-maildir-fcc-count 0)
 
 (defcustom notmuch-fcc-dirs "sent"
- "Determines the maildir directory to save outgoing mails in.
+ "Determines the maildir directory in which to save outgoing mail.
 
- If set to non-nil, this will cause message mode to file your
- mail in the specified directory (fcc).
+Three types of values are permitted:
 
- It is either a string if you only need one fcc directory or a
- list if they depend on your From address (see example).
+- nil: no Fcc header is added,
 
- In the former case it is a string such as \"INBOX.Sent\".
+- a string: the value of `notmuch-fcc-dirs' is the name of the
+  folder to use,
 
- In the fancy setup, where you want different outboxes depending
- on your From address, you supply a list like this:
+- a list: the folder is chosen based on the From address of the
+  current message using a list of regular expressions and
+  corresponding folders:
 
-     ((\"defaultinbox\")
-      (\"Sebastian Spaeth <Sebastian@SSpaeth.de>\" . \"privat\")
-      (\"Sebastian Spaeth <spaetz@sspaeth.de>\" . \"OUTBOX.OSS\")
-     )
+     ((\"Sebastian@SSpaeth.de\" . \"privat\")
+      (\"spaetz@sspaeth.de\" . \"OUTBOX.OSS\")
+      (\".*\" . \"defaultinbox\"))
 
- The outbox that matches a key (case insensitive) will be
- used. The first entry is used as a default fallback when nothing
- else matches.
+  If none of the regular expressions match the From address, no
+  Fcc header will be added.
 
- In all cases, a relative FCC directory will be understood to
- specify a directory within the notmuch mail store, (as set by
- the database.path option in the notmuch configuration file).
+In all cases, a relative FCC directory will be understood to
+specify a directory within the notmuch mail store, (as set by
+the database.path option in the notmuch configuration file).
 
- You will be prompted to create the directory if it does not exist yet when 
- sending a mail.
-
- This function will not modify the headers if there is a FCC
- header, but will check that the target directory exists."
+You will be prompted to create the directory if it does not exist
+yet when sending a mail."
 
  :require 'notmuch-fcc-initialization
  :group 'notmuch
-)
+ :type '(choice
+	 (const :tag "No FCC header" nil)
+	 (string :tag "A single folder")
+	 (repeat :tag "A folder based on the From header"
+		 (cons regexp (string :tag "Folder")))))
 
 (defun notmuch-fcc-initialization ()
   "If notmuch-fcc-directories is set,
@@ -67,44 +67,66 @@
     (setq message-fcc-handler-function
           '(lambda (destdir)
              (notmuch-maildir-fcc-write-buffer-to-maildir destdir t)))
-    ;;add a hook to actually insert the Fcc header when sending
+    ;; add a hook to actually insert the Fcc header when sending
     (add-hook 'message-header-setup-hook 'notmuch-fcc-header-setup))
 
 (defun notmuch-fcc-header-setup ()
-  "Adds an appropriate fcc header to the current mail buffer
+  "Add an Fcc header to the current message buffer.
 
-   Can be added to message-send-hook and will set the FCC header
-   based on the values of notmuch-fcc-directories (see the
-   variable customization there for examples). It uses the first
-   entry as default fallback if no From address matches."
-  ;; only do something if notmuch-fcc-dirs is set
-  (when notmuch-fcc-dirs
-    (let (subdir)
-      (if (stringp notmuch-fcc-dirs)
-          ;; notmuch-fcc-dirs is a string, just use it as subdir
-          (setq subdir notmuch-fcc-dirs)
-        ;; else: it's a list of alists (("sent") ("name1" . "sent1"))
-        (setq subdir (cdr (assoc-string (message-fetch-field "from") notmuch-fcc-dirs t)))
-         ;; if we found no hit, use the first entry as default fallback
-         (unless subdir (setq subdir (car (car notmuch-fcc-dirs)))))
+Can be added to `message-send-hook' and will set the Fcc header
+based on the values of `notmuch-fcc-dirs'. An existing Fcc header
+will NOT be removed or replaced."
 
-  ;; if there is no fcc header yet, add ours
-  (unless (message-fetch-field "fcc")
-    (message-add-header (concat "Fcc: "
-				(if (= (elt subdir 0) ?/)
-				    subdir
-				  (concat (notmuch-database-path) "/" subdir)))))
+  (let ((subdir
+	 (cond
+	  ((or (not notmuch-fcc-dirs)
+	       (message-fetch-field "Fcc"))
+	   ;; Nothing set or an existing header.
+	   nil)
 
-  ;; finally test if fcc points to a valid maildir
-  (let ((fcc-header (message-fetch-field "fcc")))
-    (unless (notmuch-maildir-fcc-dir-is-maildir-p fcc-header)
-      (cond ((not (file-writable-p fcc-header))
-             (error (format "%s is not a maildir, but you don't have permission to create one." fcc-header)))
-            ((y-or-n-p (format "%s is not a maildir. Create it? "
-                               fcc-header))
-             (notmuch-maildir-fcc-create-maildir fcc-header))
-            (t
-             (error "Not sending message."))))))))
+	  ((stringp notmuch-fcc-dirs)
+	   notmuch-fcc-dirs)
+
+	  ((and (listp notmuch-fcc-dirs)
+		(= 1 (length (car notmuch-fcc-dirs))))
+	   ;; Old style - no longer works.
+	   (error "Invalid `notmuch-fcc-dirs' setting (old style)"))
+
+	  ((listp notmuch-fcc-dirs)
+	   (let* ((from (message-fetch-field "From"))
+		  (match
+		   (catch 'first-match
+		     (dolist (re-folder notmuch-fcc-dirs)
+		       (when (string-match-p (car re-folder) from)
+			 (throw 'first-match re-folder))))))
+	     (if match
+		 (cdr match)
+	       (message "No Fcc header added.")
+	       nil)))
+
+	  (t
+	   (error "Invalid `notmuch-fcc-dirs' setting (neither string nor list)")))))
+
+    (when subdir
+      (message-add-header
+       (concat "Fcc: "
+	       ;; If the resulting directory is not an absolute path,
+	       ;; prepend the standard notmuch database path.
+	       (if (= (elt subdir 0) ?/)
+		   subdir
+		 (concat (notmuch-database-path) "/" subdir))))
+      
+      ;; finally test if fcc points to a valid maildir
+      (let ((fcc-header (message-fetch-field "Fcc")))
+	(unless (notmuch-maildir-fcc-dir-is-maildir-p fcc-header)
+	  (cond ((not (file-writable-p fcc-header))
+		 (error (format "No permission to create %s, which does not exist"
+				fcc-header)))
+		((y-or-n-p (format "%s is not a maildir. Create it? "
+				   fcc-header))
+		 (notmuch-maildir-fcc-create-maildir fcc-header))
+		(t
+		 (error "Message not sent"))))))))
  
 (defun notmuch-maildir-fcc-host-fixer (hostname)
   (replace-regexp-in-string "/\\|:"