foldable.hs

import qualified Data.Foldable as F

data Tree a = Empty | Node a (Tree a) (Tree a)
  deriving (Show, Read, Eq)

instance F.Foldable Tree where
  foldMap f Empty = mempty
  foldMap f (Node x l r) = F.foldMap f l `mappend`
                           f x           `mappend`
                           F.foldMap f r

aTree :: Tree Int
aTree = Node 5
          (Node 3
            (Node 1 Empty Empty)
            (Node 6 Empty Empty)
          )
          (Node 9
            (Node 8 Empty Empty)
            (Node 10 Empty Empty)
          )