From 8cd528869e787ddcbec776ea8bb32ae8ef9fc2be Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophile=20Choutri=20de=20Tarl=C3=A9?=
 <theophile@choutri.eu>
Date: Mon, 6 Jul 2026 13:39:28 +0200
Subject: [PATCH] Add filterTree to Data.Tree

Sponsored-By: Scrive AB
---
 containers/src/Data/Tree.hs | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/containers/src/Data/Tree.hs b/containers/src/Data/Tree.hs
index c9c2bb5a4..31158b801 100644
--- a/containers/src/Data/Tree.hs
+++ b/containers/src/Data/Tree.hs
@@ -43,6 +43,7 @@ module Data.Tree(
     -- * Elimination
     , foldTree
     , flatten
+    , filterTree
     , levels
     , leaves
     , edges
@@ -61,6 +62,7 @@ import Data.Bits ((.&.))
 import Data.Foldable (toList)
 import qualified Data.Foldable as Foldable
 import Data.List.NonEmpty (NonEmpty(..))
+import Data.Maybe (mapMaybe)
 import Data.Traversable (foldMapDefault)
 import Control.Monad (liftM)
 import Control.Monad.Fix (MonadFix (..), fix)
@@ -390,6 +392,41 @@ draw (Node x ts0) = lines x ++ drawSubTrees ts0
 flatten :: Tree a -> [a]
 flatten = toList
 
+-- | Apply a predicate on a tree and prune off branches 
+-- when a node does not satisfy the predicate.
+--
+-- ==== __Examples__
+--
+-- >>> filterTree (< 3) (Node 1 [Node 2 [], Node 3 [Node 4 [], Node 1 [Node 2 []]]])
+-- Just (Node 1 [Node 2 []])
+--
+-- @
+-- 1                     1
+-- |                     |
+-- +- 2                  `- 2
+-- |
+-- `- 3         => 
+--    |
+--    +- 4
+--    |
+--    `- 1
+--       |
+--       `- 2
+-- @
+--
+-- @since FIXME
+
+filterTree
+  :: (a -> Bool)
+  -- ^ Predicate
+  -> Tree a
+  -> Maybe (Tree a)
+filterTree predicate (Node root forest)
+  | predicate root = Just (Node root (pruneForest forest))
+  | otherwise = Nothing
+  where
+    pruneForest = mapMaybe (filterTree predicate)
+
 -- | Returns the list of nodes at each level of the tree.
 --
 -- @