debug-CS4621-a2.hs

-- Custom implementation of 'show' for producing a more readable string
-- representation of BST's (convenient for debugging the 2nd CS4621 assignment).
-- The following data declaration is assumed:
--
--     data BST a = EmptyBST | Node (BST a) a (BST a)
--
-- *** I N S T R U C T I O N S ***
--
--   1. Copy and paste the following lines at the bottom of your file 'BST.hs'.
--   2. (Re)load 'BSTplus.hs' in GHCi.
--   3. Try running
--
--          insert 4 $ insert 5 $ insert 3 emptyBST
--
--      in GHCi ('insert' is defined in 'BSTplus.hs'). The output should be
--
--          Node 3
--             EmptyBST
--             Node 5
--                Node 4
--                   EmptyBST
--                   EmptyBST
--                EmptyBST

-- make 'BST a' showable, under the condition that 'a' be showable
instance (Show a) => Show (BST a) where
  show bst = pretty bst 0

-- pretty bst lvl : a string representation of BST 'bst',
--                  with indentation level 'lvl'
pretty :: Show a => BST a -> Int -> String
pretty bst lvl =
  let indent  = take (lvl * tabsize) $ repeat ' '
      tabsize = 3
  in  case bst of
        EmptyBST               -> indent ++ "EmptyBST\n"
        Node lsub rootval rsub -> indent ++ "Node " ++ show rootval ++ "\n" ++
                                  pretty lsub (lvl + 1)                     ++
                                  pretty rsub (lvl + 1)