Flatten.hs

module Main where

import HList
import Data.Function (fix)

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

unwrap :: (Tree a -> [a]) -> (Tree a -> H a)
unwrap f = repH . f

wrap :: (Tree a -> H a) -> (Tree a -> [a])
wrap g = absH . g

flatten :: Tree a -> [a]
flatten (Leaf a)   = [a]
flatten (Node l r) = flatten l ++ flatten r

main :: IO ()
main = print (flatten (Node (Leaf 'h') (Leaf 'i')))