RB tree insert only (debug)

{-# LANGUAGE DeriveGeneric #-}

module Tree where

import Debug.Trace
import Control.Monad (foldM)
import GHC.Generics (Generic)
import Control.DeepSeq (NFData, force)

data Color = R | B
  deriving (Show, Eq, Generic)
data Tree a = E | T Color (Tree a) a (Tree a)
  deriving (Show, Eq, Generic)

instance NFData Color
instance (NFData a) => NFData (Tree a)

insert :: (NFData a, Show a, Ord a) => a -> Tree a -> Tree a
insert x t =
  let T _ a y b = ins 0 t
  in T B (force a) y (force b)
  where
    ins i u
      | trace (indent i ++ "[ins " ++ show x ++ " " ++ desc u ++ "]") False = undefined
    ins _ E = T R E x E
    ins i s@(T col a y b)
      | x < y =
        let bt = balance (i + 1) col (ins (i + 1) a) y b
        in trace (indent (i + 1) ++ "   => " ++ desc bt) bt
      | x > y =
        let bt = balance (i + 1) col a y (ins (i + 1) b)
        in trace (indent (i + 1) ++ "   => " ++ desc bt) bt
      | otherwise = s

desc :: Show a => Tree a -> String
desc E = "E"
desc t = "(" ++ show t ++ ")"

indent :: Int -> String
indent i = concat $ replicate i "  "

balance :: Show a => Int -> Color -> Tree a -> a -> Tree a -> Tree a
balance i c t1 x t2 | trace (indent i ++ "[balance " ++ show c ++ " " ++ desc t1 ++ " " ++ show x ++ " " ++ desc t2 ++ "]") False = undefined
balance _ B (T R (T R a x b) y c ) z d = T R (T B a x b) y (T B c z d)
balance _ B (T R a x (T R b y c)) z d  = T R (T B a x b) y (T B c z d)
balance _ B a x (T R (T R b y c) z d ) = T R (T B a x b) y (T B c z d)
balance _ B a x (T R b y (T R c z d))  = T R (T B a x b) y (T B c z d)
balance _ col a x b = T col a x b

test :: IO ()
test = do
  let xs = [1..7] :: [Integer]
  ts <- foldM doStuff E xs :: IO (Tree Integer)
  putStrLn $ ">>> final = " ++ show ts
  where
    doStuff t x = do
      let t' = insert x t
      putStrLn $ ">>> insert x t = " ++ show t'
      return t'