Another Haskell solution to the task given at Oslo Coding Dojo, April 2011

Tree.hs

{-# LANGUAGE TypeFamilies #-}
{-# OPTIONS_GHC
 -XFlexibleInstances -XOverlappingInstances -XUndecidableInstances
 -XFlexibleContexts -XScopedTypeVariables #-}

module Tree where


class Tree t where
    type Contents t
    fold :: (Contents t -> [a] -> a) -> t -> a

instance (Tree t, Show (Contents t)) => Show t where
    show = unlines . fold f
        where f x = (show x :) . map ("\t" ++) . concat


data ConcreteTree a = Node a [ConcreteTree a]

instance Tree (ConcreteTree a) where
    type Contents (ConcreteTree a) = a
    fold f (Node x rest) = f x $ map (fold f) rest

testTree =
    Node "parent" [
              Node "child1" [],
              Node "child2" [
                        Node "grandchild1" [],
                        Node "grandchild2" []]]