Implement building discussion message tree from DB

src/Data/Tree/Local.hs

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+{- This file is part of Vervis.
+ -
+ - Written in 2016 by fr33domlover <fr33domlover@riseup.net>.
+ -
+ - ♡ Copying is an act of love. Please copy, reuse and share.
+ -
+ - The author(s) have dedicated all copyright and related and neighboring
+ - rights to this software to the public domain worldwide. This software is
+ - distributed without any warranty.
+ -
+ - You should have received a copy of the CC0 Public Domain Dedication along
+ - with this software. If not, see
+ - <http://creativecommons.org/publicdomain/zero/1.0/>.
+ -}
+
+module Data.Tree.Local
+    ( sortForestOn
+    )
+where
+
+import Prelude
+
+import Data.List (sortOn)
+import Data.Tree
+
+sortForestOn :: Ord b => (a -> b) -> Forest a -> Forest a
+sortForestOn f =
+    sortOn (f . rootLabel) . map (\ (Node r s) -> Node r $ sortForestOn f s)