local function save(filename, data)
  local handle, err = fs.open(filename, 'w') -- w for write
  if handle then
    handle.write(textutils.serialize(data)) -- write the data, serialized.
    handle.close() -- closing the handle is what actually saves.
    -- very important to close filehandles.
  else
    -- Throw an error pointing to the calling function.
    error("Failed to save data: " .. tostring(err), 2)
  end
end

local function load(filename, default_value)
  local handle = fs.open(filename, 'r') -- r for read
  if handle then
    local data = handle.readAll()
    handle.close() -- again, important to close these.

    return textutils.unserialize(data) -- Unserialize the data and return it.
  end

  -- If we got here, the file did not exist.
  return default_value
end

-- Usage

local data = {x = 32, y = 64}

save("position.txt", data)

local loaded_data = load("position.txt", {x = 0, y = 0})
-- loaded_data will be {x = 32, y = 64}
-- Unless you delete `position.txt`, in which case it will be {x = 0, y = 0}