mu4e: let-bind context vars in mu4e-get-*-folder

When we determine the drafts/refile/trash/sent folder, see if we can
find a context matching the particular message. If we find one, let-bind
the vars for that context before determining the folder.

This allows e.g for moving message to context-specific folder, rather
than the one for the current context - useful when dealing with headers
buffers with message belonging to different contexts.

mu4e/mu4e-context.el

@@ -22,16 +22,20 @@
 
 ;;; Commentary:
 
-;; A mu4e 'context' is a a set of variable-settings and hooks, which can be used e.g. to switch
-;; between accounts.
+;; A mu4e 'context' is a a set of variable-settings and functions, which can be
+;; used e.g. to switch between accounts.
 
+(eval-when-compile (byte-compile-disable-warning 'cl-functions))
 (require 'cl)
+
 (require 'mu4e-utils)
 
-(defvar mu4e-contexts nil "The list of context objects.")
+(defvar mu4e-contexts nil "The list of `mu4e-context' objects
+describing mu4e's contexts.")
 
 (defvar mu4e~context-current nil
-  "The current context. Use `mu4e-context-switch' to change it.")
+  "The current context; for internal use. Use
+  `mu4e-context-switch' to change it.")
 
 (defun mu4e-context-current ()
   "Get the currently active context, or nil if there is none."
@@ -47,11 +51,15 @@
 (defstruct mu4e-context
   "A mu4e context object with the following members:
 - `name': the name of the context, eg. \"Work\" or \"Private\".'
-- `enter-func': a parameterless function invoked when entering this context, or nil
-- `leave-func':a parameterless fuction invoked when leaving this context, or nil
-- `match-func': a function called when comnposing a new messages, and takes a message plist
-for the message replied to or forwarded, and nil otherwise. Before composing a new message,
-`mu4e' switches to the first context for which `match-func' return t."
+- `enter-func': a parameterless function invoked when entering
+  this context, or nil
+- `leave-func':a parameterless fuction invoked when leaving this
+  context, or nil
+- `match-func': a function called when comnposing a new messages,
+  and takes a message plist
+for the message replied to or forwarded, and nil
+otherwise. Before composing a new message, `mu4e' switches to the
+first context for which `match-func' return t."
   name                      ;; name of the context, e.g. "work"
   (enter-func nil)          ;; function invoked when entering the context
   (leave-func nil)          ;; function invoked when leaving the context
@@ -62,7 +70,8 @@ for the message replied to or forwarded, and nil otherwise. Before composing a n
 (defun mu4e~context-ask-user (prompt)
   "Let user choose some context based on its name."
   (when mu4e-contexts
-    (let* ((names (map 'list (lambda (context) (cons (mu4e-context-name context) context))
+    (let* ((names (map 'list (lambda (context)
+			       (cons (mu4e-context-name context) context))
 		    mu4e-contexts))
 	    (context (mu4e-read-option prompt names)))
       (or context (mu4e-error "No such context")))))
@@ -87,7 +96,8 @@ non-nil."
     (unless context (mu4e-error "No such context"))
     ;; if new context is same as old one one switch with FORCE is set.
     (when (or force (not (eq context (mu4e-context-current))))
-      (when (and (mu4e-context-current) (mu4e-context-leave-func mu4e~context-current))
+      (when (and (mu4e-context-current)
+	      (mu4e-context-leave-func mu4e~context-current))
 	(funcall (mu4e-context-leave-func mu4e~context-current)))
       ;; enter the new context
       (when (mu4e-context-enter-func context)
@@ -124,13 +134,14 @@ match, POLICY determines what to do:
 
 - pick-first: pick the first of the contexts available
 - ask: ask the user
-- otherwise, return nil. Effectively, this leaves the current context in place."
+- otherwise, return nil. Effectively, this leaves the current context as it is."
   (when mu4e-contexts
     (if (eq policy 'always-ask)
       (mu4e~context-ask-user "Select context: ")
       (or (find-if (lambda (context)
 		     (and (mu4e-context-match-func context)
-		       (funcall (mu4e-context-match-func context) msg))) mu4e-contexts)
+		       (funcall (mu4e-context-match-func context) msg)))
+	    mu4e-contexts)
 	;; no context found
 	(case policy
 	  (pick-first (car mu4e-contexts))

mu4e/mu4e-utils.el

@@ -46,8 +46,9 @@
 (declare-function mu4e~proc-mkdir     "mu4e-proc")
 (declare-function mu4e~proc-running-p "mu4e-proc")
 
-(declare-function mu4e~context-autoswitch  "mu4e-context")
-
+(declare-function mu4e~context-autoswitch "mu4e-context")
+(declare-function mu4e-context-determine  "mu4e-context")
+(declare-function mu4e-context-vars       "mu4e-context")
 (declare-function show-all "org")
 
 
@@ -90,26 +91,36 @@ User's addresses are set in `mu4e-user-mail-address-list')."
     t))
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-
+(defmacro with~mu4e-context-vars (context &rest body)
+  "Evaluate BODY, with variables let-bound for CONTEXT (if any).
+`funcall'."
+  (declare (indent 2))
+  `(let* ((vars (and ,context (mu4e-context-vars ,context))))
+     (progv ;; XXX: perhaps use eval's lexical environment instead of progv?
+      (mapcar (lambda(cell) (car cell)) vars)
+      (mapcar (lambda(cell) (cdr cell)) vars)
+      (eval ,@body))))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; the standard folders can be functions too
 (defun mu4e~get-folder (foldervar msg)
-  "Get message folder FOLDERVAR.
+  "Within the mu-context of MSG, get message folder FOLDERVAR.
 If FOLDER is a string, return it, if it is a function, evaluate
 this function with MSG as parameter (which may be `nil'), and
 return the result."
-  (unless (member foldervar '(mu4e-sent-folder mu4e-drafts-folder
-			       mu4e-trash-folder mu4e-refile-folder))
-    (mu4e-error "Folder must be either mu4e-sent-folder,
-    mu4e-drafts-folder or mu4e-trash-folder (not %S)" foldervar))
-  (let* ((folder (symbol-value foldervar))
-	  (val
-	    (cond
-	      ((stringp   folder) folder)
-	      ((functionp folder) (funcall folder msg))
-	      (t (mu4e-error "unsupported type for %S" folder)))))
-    (or val (mu4e-error "%S evaluates to nil" foldervar))))
+  (unless (member foldervar
+	    '(mu4e-sent-folder mu4e-drafts-folder
+	       mu4e-trash-folder mu4e-refile-folder))
+    (mu4e-error "Folder must be one of mu4e-(sent|drafts|trash|refile)-folder"))
+  ;; get the value with the vars for the relevants context let-bound
+  (with~mu4e-context-vars (mu4e-context-determine msg nil)
+    (let* ((folder (symbol-value foldervar))
+	    (val
+	      (cond
+		((stringp   folder) folder)
+		((functionp folder) (funcall folder msg))
+		(t (mu4e-error "unsupported type for %S" folder)))))
+      (or val (mu4e-error "%S evaluates to nil" foldervar)))))
 
 (defun mu4e-get-drafts-folder (&optional msg)
   "Get the sent folder. See `mu4e-drafts-folder'."

mu4e/mu4e.texi

@@ -439,11 +439,13 @@ with @t{MU4E-PATH} replaced with the actual path.
 @section Folders
 
 The next step is to tell @t{mu4e} where it can find your Maildir, and
-some special folders. So, for example@footnote{Note that the folders
-(@t{mu4e-sent-folder}, @t{mu4e-drafts-folder}, @t{mu4e-trash-folder} and
+some special folders. 
+
+So, for example@footnote{Note that the folders (@t{mu4e-sent-folder},
+@t{mu4e-drafts-folder}, @t{mu4e-trash-folder} and
 @t{mu4e-refile-folder}) can also be @emph{functions} that are evaluated
 at runtime. This allows for dynamically changing them depending on the
-context. See @ref{Dynamic folders} for details.}:
+situation. See @ref{Dynamic folders} for details.}:
 @lisp
   ;; these are actually the defaults
   (setq
@@ -458,6 +460,9 @@ Note, @code{mu4e-maildir} takes an actual filesystem-path, the other
 folder names are all relative to @code{mu4e-maildir}. Also note that
 this must @emph{not} be a symbolic link.
 
+If you use @t{mu4e-context}, see @ref{Contexts and special folders} for
+what that means for these special folders.
+
 @node Retrieval and indexing
 @section Retrieval and indexing with mu4e
 
@@ -2274,6 +2279,7 @@ example:
 @menu
 * Defining a context::
 * Context policies::
+* Contexts and special folders::
 * Contexts example::
 * Some context tricks::
 @end menu
@@ -2345,6 +2351,31 @@ none of the contexts' match-functions returns @code{t}:
 @item @t{nil}: don't change the context
 @end itemize
 
+@node Contexts and special folders
+@section Contexts and special folders
+
+As we discussed in @ref{Folders} and @ref{Dynamic folders}, @t{mu4e}
+recognizes a number of special folders: @code{mu4e-sent-folder},
+@code{mu4e-drafts-folder}, @code{mu4e-trash-folder} and
+@code{mu4e-refile-folder}.
+
+When you have a headers-buffer with messages that belong to different
+contexts (say, a few different accounts), it is desirable for each of
+them to use the specific folders for their own context - so, for
+instance, if you trash a message, it needs to go to the trash-folder for
+the account it belongs to, which is not necessarily the current context.
+
+To make this easy to do, whenever @t{mu4e} needs to know the value for
+such a special folder for a given message, it tries to determine the
+appropriate context using @code{mu4e-context-determine}. If it finds
+one, it let-binds the @code{vars} for that account, and then determines
+the value for the folder. It does not, however, call the @t{enter-func},
+since we are not really switching context.
+
+In practice, this what this means that a long as each of the accounts
+has a good @t{match-func}, all message operations automatically find the
+appropriate folders.
+
 @node Contexts example
 @section Example
 
@@ -2362,7 +2393,8 @@ when starting; see the discussion in the previous section.
 	:enter-func (lambda () (mu4e-message "Switch to the Private context"))
         ;; leave-func not defined
 	:match-func (lambda (msg)
-		      (when msg (mu4e-message-contact-field-matches msg :to "aliced@@home.example.com")))
+		      (when msg (mu4e-message-contact-field-matches msg 
+                      :to "aliced@@home.example.com")))
 	:vars '(  ( mail-reply-to          . "aliced@@home.example.com" )
 		  ( user-mail-address      . "aliced@@home.example.com"  )
 		  ( user-full-name         . "Alice Derleth" )
@@ -2375,7 +2407,8 @@ when starting; see the discussion in the previous section.
 	:enter-func (lambda () (mu4e-message "Switch to the Work context"))
         ;; leave-fun not defined
 	:match-func (lambda (msg)
-		      (when msg (mu4e-message-contact-field-matches msg :to "aderleth@@miskatonic.example.com")))
+		      (when msg (mu4e-message-contact-field-matches msg 
+                      :to "aderleth@@miskatonic.example.com")))
 	:vars '(  ( mail-reply-to          . "aderleth@@miskatonic.example.com" )
 		  ( user-mail-address      . "aderleth@@miskatonic.example.com" )
 		  ( user-full-name         . "Alice Derleth" )
@@ -2446,6 +2479,9 @@ parameter, and returns the desired folder name. This chapter shows you how to
 do that. For a more general discussion of how to extend @t{mu4e} and writing
 your own functions, see @ref{Extending mu4e}.
 
+If you use @t{mu4e-context}, see @ref{Contexts and special folders} for
+what that means for these special folders.
+
 @menu
 * Smart refiling:: Automatically choose the target folder
 * Other dynamic folders:: Flexible folders for sent, trash, drafts