[Haskell-beginners] A good data structure for representing a tic-tac-toe board?

Hi Folks, Currently I am representing a tic-tac-toe board as a string, with 'X' denoting player 1 and 'O' denoting player 2. For example, I represent this 2x2 game board: 'X' | ----------------------- | 'O' with this string: "X O" The nice thing about that representation is that it is each to identify which cells are filled or empty, and it is easy to mark a cell with an 'X' or 'O'. The problem with the representation is that it is difficult to determine when a player has won. Can you recommend a representation that makes it easy to: 1. determine when a player has won 2. identify cells that are filled or empty 3. mark an empty cell /Roger