test.hs

import System.Environment
import Data.List
import Control.Applicative

data Field = Field (Int, Int) (Int, Int)

size :: Field -> (Int, Int)
size (Field (w, h) _) = (w, h)

pos :: Field -> (Int, Int)
pos (Field _ (x, y)) = (x, y)

moveTo :: Field -> (Int, Int) -> Field
moveTo (Field (w, h) (x, y)) (nx, ny)
  | nx < 0 || nx >= w = (Field (w, h) (x, y))
  | ny < 0 || ny >= h = (Field (w, h) (x, y))
  | otherwise = (Field (w, h) (nx, ny))

showField :: Field -> String
showField (Field (w, h) (x, y)) =
  concat $ replRow $ replicate h $ (replicate w '.') ++ "\n"
  where
    replCell row = map (\(c, i) -> if i == x then '@' else c) $ zip row [0..w]
    replRow lst = map (\(r, i) -> if i == y then (replCell r) else r) $ zip lst [0..h - 1]

move :: Field -> String -> Field
move field "j" = moveTo field (fst $ pos field, (snd $ pos field) + 1)
move field "l" = moveTo field ((fst $ pos field) + 1, snd $ pos field)
move field "h" = moveTo field ((fst $ pos field) - 1, snd $ pos field) 
move field "k" = moveTo field (fst $ pos field, (snd $ pos field) - 1)
move field _ = field

-- main = forever $ interact $ showField . (move (Field (20, 20) (0, 0)))

mainLoop field = do
  putStrLn $ showField field
  newField <- move field <$> getLine
  mainLoop newField

main = mainLoop $ Field (20, 20) (0, 0)