haskell trees

tree-with-defined-fold.hs

import Data.Monoid
import Data.Foldable

data Tree a = Empty
            | Node (Maybe a)
                   (Tree a)
                   (Tree a)
            deriving (Show)

instance Foldable Tree where
  foldMap f Empty = mempty
  foldMap f (Node a l r) = foldMap f a
                        <> foldMap f l
                        <> foldMap f r

instance Functor Tree where
  fmap f Empty = Empty
  fmap f (Node a l r) = Node (fmap f a)
                             (fmap f l)
                             (fmap f r)

tree :: Tree Integer
tree = Node (Just 3)
            (Node (Just 2)
                  Empty
                  (Node (Just 5)
                        Empty
                        Empty
                  )
            )
            (Node Nothing Empty Empty)

main :: IO ()
main = do
  putStrLn $ "sum: "  <> show (sum tree)
  putStrLn $ "fmap: " <> show ((*2) <$> tree)

tree-with-derived-fold.hs

{-# LANGUAGE DeriveFoldable #-}
{-# LANGUAGE DeriveFunctor #-}

import Data.Monoid
import Data.Foldable

data Tree a = Empty
            | Node (Maybe a)
                   (Tree a)
                   (Tree a)
            deriving (Show, Functor, Foldable)

tree :: Tree Integer
tree = Node (Just 3)
            (Node (Just 2)
                  Empty
                  (Node (Just 5)
                        Empty
                        Empty
                  )
            )
            (Node Nothing Empty Empty)

main :: IO ()
main = do
  putStrLn $ "sum: "  <> show (sum tree)
  putStrLn $ "fmap: " <> show ((*2) <$> tree)