foldMap, fold to monoid

foldMap.hs

data Tree a = Leaf | Branch a !(Tree a) !(Tree a)

inOrder g Leaf = mempty
inOrder g (Branch a l r) = g a <> inOrder g l <> inOrder g r

instance Foldable Tree where
  foldMap g = inOrder g

treeSum :: Tree Integer -> Integer
treeSum = getSum . foldMap Sum