Hello, The Data.Tree API seems rather poor. Some research on hackage shows some additional functions being defined in very unrelated packages: http://hackage.haskell.org/package/debian-3.81/docs/Debian-Apt-Dependencies.... http://hackage.haskell.org/package/hledger-lib-0.22.1/docs/Hledger-Utils.htm... I propose the addition of the following functions, that seem rather straigh forward to me: -- | get the sub-tree rooted at the first (left-most, depth-first) occurrence -- of the specified node value lookupTree :: Eq a => a -> Tree a -> Maybe (Tree a) -- | get the sub-tree rooted at the first (left-most, depth-first) value that -- matches the provided condition findTree :: (a -> Bool) -> Tree a -> Maybe (Tree a) -- | keep only the elements that match the provided condition filter :: (a -> Bool) -> Tree a -> Tree a The 'Tree' is appended in the name, to distinguish from the Foldable instances, that return the value, and not a sub-tree. Additionally, the following two functions might also be useful (even if the implementation is very simple in the second case): -- | get the sub-tree for the specified node value in the first tree in -- forest in which it occurs. lookupTreeInForest :: Eq a => a -> [Tree a] -> Maybe (Tree a) -- | Length of the tree length :: Tree a -> Int There are probably more useful functions to add, so suggestions are welcomed. Previous discussion: https://github.com/haskell/containers/issues/39#issuecomment-35799628 Discussion period: 2 weeks Cheers, João