mu4e: allow mu4e-get-mail-command to be a function

For making it easier to influence the behavior.

mu4e/mu4e-helpers.el

@@ -559,7 +559,12 @@ BUFNAME is the name of the buffer, and DATA is lisp-data, if any."
   "Set VAR to VAL if VAR is nil."
   `(unless ,var (setq ,var ,val)))
 
+
 ;;; Misc
+(defun mu4e--fun-val (val)
+  "If var is a function, return (funcall VAL), otherwise return VAL."
+  (if (functionp val) (funcall val) val))
+
 (defun mu4e-join-paths (directory &rest components)
   "Append COMPONENTS to DIRECTORY and return the resulting string.
 

mu4e/mu4e-update.el

@@ -33,15 +33,20 @@
 ;;; Customization
 
 (defcustom mu4e-get-mail-command "true"
-  "Shell command for retrieving new mail.
+  "Shell command for retrieving new mail or a function.
+
 Common values are \"offlineimap\", \"fetchmail\" or \"mbsync\", but
-arbitrary shell-commands can be used.
+arbitrary shell-commands can be used. If it is a function, it should
+return a string specifying the same.
 
 When set to the literal string \"true\" (the default), the
 command simply finishes successfully (running the \"true\"
 command) without retrieving any mail. This can be useful when
 mail is already retrieved in another way, such as a local MDA."
   :type 'string
+  :type '(choice
+          (string :tag "Shell command")
+          (function :tag "Function that returns a string (shell command)"))
   :group 'mu4e
   :safe 'stringp)
 
@@ -105,8 +110,8 @@ running."
 
 (defvar mu4e-update-pre-hook nil
   "Hook run just *before* the mail-retrieval / database updating process starts.
-You can use this hook for example to `mu4e-get-mail-command' with
-some specific setting.")
+You can use this hook for example to update
+`mu4e-get-mail-command' with some specific setting.")
 
 (defcustom mu4e-hide-index-messages nil
   "Whether to hide the \"Indexing...\" and contacts messages."
@@ -271,9 +276,9 @@ If
 RUN-IN-BACKGROUND is non-nil (or called with prefix-argument),
 run in the background; otherwise, pop up a window."
   (let* ((process-connection-type t)
+         (cmd (mu4e--fun-val mu4e-get-mail-command))
          (proc (start-process-shell-command
-                mu4e--update-name mu4e--update-name
-                mu4e-get-mail-command))
+                mu4e--update-name mu4e--update-name cmd))
          (buf (process-buffer proc))
          (win (or run-in-background
                   (mu4e--temp-window buf mu4e--update-buffer-height))))

mu4e/mu4e.texi

@@ -521,11 +521,12 @@ Emacs/@t{mu4e}. However, you can also do it from within
 @subsection Basics
 
 To set up mail-retrieval from within @t{mu4e}, set the variable
-@code{mu4e-get-mail-command} to the program or shell command you want to
-use for retrieving mail. You can then get your e-mail using @kbd{M-x
-mu4e-update-mail-and-index}, or @kbd{C-S-u} in all @t{mu4e}-views;
-alternatively, you can use @kbd{C-c C-u}, which may be more convenient
-if you use emacs in a terminal.
+@code{mu4e-get-mail-command} to a shell command you want to use for retrieving
+mail. It can also be a function which returns such a shell-command.
+
+You can then get your e-mail using @kbd{M-x mu4e-update-mail-and-index}, or
+@kbd{C-S-u} in all @t{mu4e}-views; alternatively, you can use @kbd{C-c C-u},
+which may be more convenient if you use emacs in a terminal.
 
 You can kill the (foreground) update process with @kbd{q}.
 
@@ -608,10 +609,9 @@ A simple setup could look something like:
   mu4e-update-interval 300)             ;; update every 5 minutes
 @end lisp
 
-A hook @code{mu4e-update-pre-hook} is available which is run right
-before starting the process. That can be useful, for example, to
-influence, @code{mu4e-get-mail-command} based on the the current
-situation (location, time of day, ...).
+To influence the details, a hook @code{mu4e-update-pre-hook} is available, which
+is called right before starting the process. It is also possible to set
+@code{mu4e-get-mail-command} to a function which returns a command.
 
 It is possible to get notifications when the indexing process does any
 updates --- for example when receiving new mail. See