-- A Tree is either a Branch with subtrees or a Leaf.
-- Both carry a String as a tag.
data Tree = Branch String [Tree] | Leaf String
  deriving (Show)

findNode :: String -> Tree -> Maybe Tree
-- A function can happily walk a tree via pattern
-- matching.
findNode q t@(Leaf tag)
  | tag == q    = Just t
  | otherwise   = Nothing
findNode q t@(Branch tag subtrees)
  | tag == q    = Just t
  | otherwise   = foldl findInBranch Nothing subtrees
    where 
      findInBranch acc subtree = case acc of
        Just _  -> acc
        Nothing -> findNode q subtree


allLeavesTags :: Tree -> [String]
allLeavesTags (Leaf tag) = [tag]
allLeavesTags (Branch _ subtrees) = foldl allInBranch [] subtrees
    where 
      allInBranch acc subtree = acc ++ (allLeavesTags subtree)

t = Branch "a" [Branch "b" [Branch "c" [], Leaf "d"], Leaf "j"]

main = do  
  putStrLn . show $ findNode "c" t
  putStrLn . show $ allLeavesTags t

Just (Branch "c" [])
["d","j"]