haskell functor examples

Functor.hs

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

instance Functor Tree where
    fmap g (Leaf v)   = Leaf (g v)
    fmap g (Node l r) = Node (fmap g l) (fmap g r)

sampleTree :: Tree String
sampleTree = Node (Node (Leaf "hello") (Leaf "foo")) (Leaf "baar")

treeToLength :: Tree [a] -> Tree Int
treeToLength t = fmap length t

treeExample :: IO ()
treeExample = do
    putStrLn $ show $ sampleTree
    putStrLn "-->"
    putStrLn $ show $ treeToLength sampleTree


maybeExample :: IO ()
maybeExample = do
    putStrLn $ show (Just "foobar")
    putStrLn "-->"
    putStrLn $ show $ fmap length (Just "foobar")

maybeExample' :: IO ()
maybeExample' = do
    putStrLn $ show (Nothing :: Maybe ())
    putStrLn "-->"
    putStrLn $ show $ fmap length (Nothing)

main = do
        let seperator = take 45 $ repeat '-'
        treeExample
        putStrLn seperator
        maybeExample
        putStrLn seperator
        maybeExample'
        putStrLn seperator