The in_n function is important which calculates the reachable squares reachable in n steps on the chessboard(quite inefficient though)

knight_pos.hs

import Data.List (nub)

class Monad m => MonadPlus m where
  mzero :: m a
  mplus :: m a -> m a -> m a


instance MonadPlus [] where
  mzero = []
  mplus = (++)


guard :: (MonadPlus m) => Bool -> m ()
guard True = return ()
guard False = mzero


type KnightPos = (Integer, Integer)


moveKnight :: KnightPos -> [KnightPos]
moveKnight (c, r) = do
  (c', r') <- [(c + dc, r + dr) | dc <- [-2..2], dr <- [-2..2], abs(dc) + abs(dr) == 3]
  guard (c' `elem` [1..8] && r' `elem` [1..8])
  return (c', r')


in_3 :: KnightPos -> [KnightPos]
in_3 start = do
  first <- moveKnight start
  second <- moveKnight first
  moveKnight second


in_n :: Int -> KnightPos -> [KnightPos]
in_n n pos = foldr (\f x -> f x) [pos] $ take n $ repeat $ nub . (>>= moveKnight)