Dear All, I have started learning Haskell recently, and I try to reimplement interesting algorithms that are challenging to do in other languages. I would like to ask you for your kind help with a problem that I am stuck with. A generic tree is given in the form of nested sequences (nested lists or nested tuples) and I would like to traverse it in (1) depth-first order, (2) lazily and (3) without copying the data into some other datastructure first. I test the algorithm by pretty printing the tree. My goal is learning, so Data.Tree is not an option (although I did look into its implementation). Here is my first attempt: data Tree a = Leaf a | Node [Tree a] tree = Node [Leaf 'a', Node [Leaf 'b', Leaf 'c', Node []], Leaf 'd'] toString :: (Show a) => Tree a -> String toString (Leaf leaf) = " " ++ show leaf ++ " " toString (Node nodes) = " [ " ++ concatMap toString nodes ++ " ] " main = print $ toString tree It (more or less) satisfies the above requirements. The pretty printing is not very pretty but the part that I am really not happy with is the lot of code duplication in giving the tree. I would like to enter it as: tree = ['a', ['b', 'c', []], 'd'] which of course won't work. Is there a (not too obscure) way to help the compiler so that it understands that 'a' means Leaf 'a', etc? I have also tried tree = ('a', ('b', 'c', ()), 'd') but then I do not know how to destructure the nested tuples. Any help is greatly appreciated. Ali