TcTree.hs

import Test.Hspec

main = hspec $ do
  describe "tree fold" $ 
    it "works!" $
      visit    (Branch 
--              /    \
      (Node "h")      (Branch 
--                  /         \
              (Branch       -- \
--             /    \           \
        (Node "e") (Node "l"))-- \
                             (Branch 
--                            /    \
                       (Node "l") (Node "o"))))
        `shouldBe` "hello"
  describe "tc tree fold" $ 
    it "works too!" $
      visit'   (Branch 
--              /    \
      (Node "h")      (Branch 
--                  /         \
              (Branch       -- \
--             /    \           \
        (Node "e") (Node "l"))-- \
                             (Branch 
--                            /    \
                       (Node "l") (Node "o"))))
        `shouldBe` "hello"

data Tree = Node String
          | Branch Tree Tree

visit :: Tree -> String
visit (Node s) = s
visit (Branch a b) = visit a ++ visit b

visit' :: Tree -> String
visit' t = treeFold [t] ""

treeFold :: [Tree] -> String -> String
treeFold [] acc     = acc  
treeFold (t:ts) acc = case t of
  Node s     -> treeFold ts (acc ++ s)
  Branch a b -> treeFold (a:b:ts) acc