FoldrMonoid2.hs

import Data.Monoid
import qualified Data.Foldable as F

data Tree a = EmptyTree | Node a (Tree a) (Tree a) deriving (Show)

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

testTree = Node 5
           (Node 3
               (Node 1 EmptyTree EmptyTree)
               (Node 6 EmptyTree EmptyTree)
           )
           (Node 9
               (Node 8 EmptyTree EmptyTree)
               (Node 10 EmptyTree EmptyTree)
           )

main = do
    print $ F.foldr (+) 0 testTree
    print $ F.foldl (*) 1 testTree
    print $ getAny $ F.foldMap ( \ x -> Any $ x == 3) testTree
    print $ getAny $ F.foldMap ( \ x -> Any $ x > 15) testTree
    print $ F.foldMap ( \ x -> [x]) testTree