[PATCH] emacs: notmuch-show-header-line: allow format strings and functions

[jao <jao@gnu.org>]

If a string value is assigned to notmuch-show-header-line, it's used
as a format string to be passed passed to format-spec with `%s`
substituted by the message's subject.  If a function is given, it's
called with the subject as argument, and its return value used as
header line.

As before, t means displaying the subject and nil not using any header
line.

Signed-off-by: jao <jao@gnu.org>
---
 emacs/notmuch-show.el | 42 +++++++++++++++++++++++++++++++++---------
 1 file changed, 33 insertions(+), 9 deletions(-)

diff --git a/emacs/notmuch-show.el b/emacs/notmuch-show.el
index 69f6c845..266afbaa 100644
--- a/emacs/notmuch-show.el
+++ b/emacs/notmuch-show.el
@@ -85,8 +85,26 @@ visible for any given message."
   :group 'notmuch-show)
 
 (defcustom notmuch-show-header-line t
-  "Show a header line with the current message's subject."
-  :type 'boolean
+  "Show a header line in notmuch show buffers.
+
+If t (the default), the header line will contain the current
+message's subject.
+
+If a string, this value is interpreted as a format string to be
+passed to `format-spec` with `%s` as the substitution variable
+for the message's subject.  E.g., to display the subject trimmed
+to a maximum of 80 columns, you could use \"%>-80s\" as format.
+
+If you assign to this variable a function, the function will be
+called with the subject as argument and its return value used as
+the header line.
+
+Finally, if this variabale is set to nil, no header is
+displayed."
+  :type '(choice (const :tag "No header" ni)
+                 (const :tag "Subject" t)
+                 (string :tag "Format")
+		 (function :tag "Function"))
   :group 'notmuch-show)
 
 (defcustom notmuch-show-relative-dates t
@@ -1313,6 +1331,18 @@ fallback if the prior matches no messages."
       (push (list thread "and (" context ")") queries))
     queries))
 
+(defun notmuch-show--header-line-format ()
+  "Compute the header line format of a notmuch-show buffer."
+  (when notmuch-show-header-line
+    (let* ((s (notmuch-sanitize
+	       (notmuch-show-strip-re (notmuch-show-get-subject))))
+	   (subject (replace-regexp-in-string "%" "%%" s)))
+      (cond ((stringp notmuch-show-header-line)
+             (format-spec notmuch-show-header-line `((?s . ,subject))))
+	    ((functionp notmuch-show-header-line)
+	     (funcall notmuch-show-header-line subject))
+	    (notmuch-show-header-line subject)))))
+
 (defun notmuch-show--build-buffer (&optional state)
   "Display messages matching the current buffer context.
 
@@ -1343,13 +1373,7 @@ If no messages match the query return NIL."
       ;; display changes.
       (notmuch-show-mapc
        (lambda () (notmuch-show-set-prop :orig-tags (notmuch-show-get-tags))))
-      ;; Set the header line to the subject of the first message.
-      (when notmuch-show-header-line
-	(setq header-line-format
-	      (replace-regexp-in-string "%" "%%"
-					(notmuch-sanitize
-					 (notmuch-show-strip-re
-					  (notmuch-show-get-subject))))))
+      (setq header-line-format (notmuch-show--header-line-format))
       (run-hooks 'notmuch-show-hook)
       (if state
 	  (notmuch-show-apply-state state)
-- 
2.36.1