Generate and prettyprint binary trees

-- small experiment with enumerating binary trees

data T = B T T | L deriving (Eq)

instance Show T where
  show L = "."
  show (B l r) = "(" ++ show l ++ show r ++ ")"

buildTs :: Int -> [T]
buildTs 0 = [L]
buildTs n = [B l r | i <- [0..n-1],
                     l <- buildTs i,
                     r <- buildTs (n-1-i) ]

randomT :: (MonadRandom m) => Int -> m T
randomT 0 = return L
randomT n = do
  i <- getRandomR (0, n-1)
  l <- randomT i
  r <- randomT (n-1-i)
  return $ B l r

main :: IO ()
main = do
  let gen = mkStdGen 42
  let t = evalRand (randomT 4) gen
  print t

-- prints ((..)((..).))