minimax :: Ord val => (pos -> val) -> (pos -> [pos]) -> Int -> Bool -> pos -> val
minimax val nexts = mm where
  mm d player1 p =
    if d == 0 then val p else
    case nexts p of
      [] -> val p
      ps -> let vs = map (mm (d-1) (not player1)) ps
            in if player1 then maximum vs else minimum vs