* mu4e: allow for custom header fields (see `mu4e-header-info-custom')

mu4e/mu4e-headers.el

@@ -411,6 +411,16 @@ date. The formats used for date and time are
     (propertize (mu4e-get-mailing-list-shortname list) 'help-echo list)
     ""))
 
+(defun mu4e~headers-custom-field (msg field)
+  "Show some custom header field, or raise an error if it is not
+found."
+  (let* ((item (or (assoc field mu4e-header-info-custom)
+		 (mu4e-error "field %S not found" field)))
+	  (func (or (plist-get (cdr-safe item) :function)
+		  (mu4e-error "no :function defined for field %S %S" field (cdr item)))))
+    (funcall func msg)))
+
+
 ;; note: this function is very performance-sensitive
 (defun mu4e~headers-header-handler (msg &optional point)
     "Create a one line description of MSG in this buffer, at POINT,
@@ -441,7 +451,7 @@ if provided, or at the end of the buffer otherwise."
 			'help-echo (format "%S" val)))
 	      (:tags (propertize (mapconcat 'identity val ", ")))
 	      (:size (mu4e-display-size val))
-	      (t (mu4e-error "Unsupported header field (%S)" field))))
+	      (t (mu4e~headers-custom-field msg field))))
 	  (when str
 	    (setq line
 	      (concat line
@@ -677,7 +687,8 @@ after the end of the search results."
     (mapcar
       (lambda (item)
 	(let* ((field (car item)) (width (cdr item))
-		(info (cdr (assoc field mu4e-header-info)))
+		(info (cdr (assoc field
+			     (append mu4e-header-info mu4e-header-info-custom))))
 		(sortable (plist-get info :sortable))
 		(help (plist-get info :help))
 		(uparrow   (if mu4e-use-fancy-chars " ▲" " ^"))

mu4e/mu4e-vars.el

@@ -505,7 +505,6 @@ I.e. a message with the draft flag set."
   :group 'mu4e-faces)
 
 
-
 ;; headers info
 (defconst mu4e-header-info
   '( (:attachments .
@@ -608,7 +607,26 @@ symbol for /another/ field. For example, the `:human-date' field
 uses `:date' for that.
 
 Note, `:sortable' does not work for custom header fields.")
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+
+(defvar mu4e-header-info-custom
+  "A list of custom (user-defined) headerr. The format is similar
+to `mu4e-header-info', but addds a :function property, which should
+point to a function that takes a message p-list as argument, and
+returns a string. See the default value of `mu4e-header-info-custom
+for an example."
+  '( (:recipnum .
+       ( :name "Number of recipients"
+	 :shortname "Recip#"
+	 :help "Number of recipients for this message"
+	 :function
+	 (lambda (msg)
+	   (format "%d"
+	     (+ (length (mu4e-message-field msg :to))
+	       (length (mu4e-message-field msg :cc)))))))))
+
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

mu4e/mu4e-view.el

@@ -149,6 +149,16 @@ messages - for example, `mu4e-org'."
   ;; need an extra policy...
   (mu4e~proc-view msgid mu4e-view-show-images mu4e-decryption-policy))
 
+(defun mu4e~view-custom-field (msg field)
+  "Show some custom header field, or raise an error if it is not
+found."
+  (let* ((item (or (assoc field mu4e-header-info-custom)
+		 (mu4e-error "field %S not found" field)))
+	  (func (or (plist-get (cdr-safe item) :function)
+		  (mu4e-error "no :function defined for field %S %S"
+		    field (cdr item)))))
+    (funcall func msg)))
+
 
 (defun mu4e-view-message-text (msg)
   "Return the message to display (as a string), based on the MSG plist."
@@ -193,7 +203,8 @@ messages - for example, `mu4e-org'."
 	    (:attachments (mu4e~view-construct-attachments-header msg))
 	    ;; pgp-signatures
 	    (:signature   (mu4e~view-construct-signature-header msg))
-	    (t (mu4e-error "Unsupported field: %S" field)))))
+	    (t (mu4e~view-construct-header field
+		 (mu4e~view-custom-field msg field)))))) 
       mu4e-view-fields "")
     "\n"
     (mu4e-message-body-text msg)))
@@ -255,7 +266,8 @@ marking if it still had that."
   "Return header field FIELD (as in `mu4e-header-info') with value
 VAL if VAL is non-nil. If DONT-PROPERTIZE-VAL is non-nil, do not
 add text-properties to VAL."
-  (let* ((info (cdr (assoc field mu4e-header-info)))
+  (let* ((info (cdr (assoc field
+		      (append mu4e-header-info mu4e-header-info-custom))))
 	  (key (plist-get info :name))
 	  (help (plist-get info :help)))
     (if (and val (> (length val) 0))

mu4e/mu4e.texi

@@ -720,7 +720,9 @@ Some notes to explain what you see in the example:
 @itemize
 @item The fields shown in the headers view can be influenced by customizing
 the variable @code{mu4e-headers-fields}; see @code{mu4e-header-info} for the
-list of available fields.
+list of built-in fields. Apart from the built-in fields, you can also create
+custom fields using @code{mu4e-header-info-custom}; see its documentation for
+the details.
 @item By default, the date is shown with the @t{:human-date} field, which
 shows the @emph{time} for today's messages, and the @emph{date} for older
 messages. If you want to distinguish between 'today' and 'older', you can use
@@ -967,7 +969,10 @@ An example message view:
 
 Some notes:
 @itemize
-@item The variable @code{mu4e-view-fields} determines the header fields to be shown.
+@item The variable @code{mu4e-view-fields} determines the header fields to be
+shown; see @code{mu4e-header-info} for a list of built-in fields. Apart from
+the built-in fields, you can also create custom fields using
+@code{mu4e-header-info-custom}; see its documentation for the details.
 @item You can set the date format with the variable
 @code{mu4e-date-format-long}.
 @item By default, only the names of contacts in address fields are visible