[PATCH 3/6] emacs: update defcustom for notmuch-search-result-format.

[David Bremner <david@tethera.net>]

It seems redundant to have the previous example, since the default
value is always show by describe variable.

Enforce more restrictions on the keys in the alist, since arbitrary
strings don't work as field names.

Document that functions can be used in lieu of field names.
---
 emacs/notmuch-tree.el | 29 ++++++++++++++++++++---------
 1 file changed, 20 insertions(+), 9 deletions(-)

diff --git a/emacs/notmuch-tree.el b/emacs/notmuch-tree.el
index 8b246a2e..68614623 100644
--- a/emacs/notmuch-tree.el
+++ b/emacs/notmuch-tree.el
@@ -97,6 +97,15 @@ different kind of arrow point."
   :type '(alist :key-type symbol :value-type string)
   :group 'notmuch-tree)
 
+(defconst notmuch-tree--field-names
+  '(choice :tag "Field"
+	   (const :tag "Date" "date")
+	   (const :tag "Authors" "authors")
+	   (const :tag "Subject" "subject")
+	   (const :tag "Tree" "tree")
+	   (const :tag "Tags" "tags")
+	   (function)))
+
 (defcustom notmuch-tree-result-format
   `(("date" . "%12s  ")
     ("authors" . "%-20s")
@@ -106,7 +115,11 @@ different kind of arrow point."
     ("tags" . "(%s)"))
   "Result formatting for tree view.
 
-Supported fields are: date, authors, subject, tree, tags.
+List of pairs of (field . format-string).  Supported field
+strings are: \"date\", \"authors\", \"subject\", \"tree\",
+\"tags\".  It is also supported to pass a function in place of a
+field-name. In this case the function is passed the thread
+object (plist) and format string.
 
 Tree means the thread tree box graphics. The field may
 also be a list in which case the formatting rules are
@@ -114,14 +127,12 @@ applied recursively and then the output of all the fields
 in the list is inserted according to format-string.
 
 Note that the author string should not contain whitespace
-\(put it in the neighbouring fields instead). For example:
-    (setq notmuch-tree-result-format
-          '((\"authors\" . \"%-40s\")
-            (\"subject\" . \"%s\")))"
-  :type '(alist :key-type (choice string
-				  (alist :key-type string
-					 :value-type string))
-		:value-type string)
+\(put it in the neighbouring fields instead)."
+
+  :type `(alist :key-type (choice ,notmuch-tree--field-names
+				  (alist :key-type ,notmuch-tree--field-names
+					 :value-type (string :tag "Format")))
+		:value-type (string :tag "Format"))
   :group 'notmuch-tree)
 
 (defcustom notmuch-unthreaded-result-format
-- 
2.35.1