todo_list.rb

class TodoList
  attr_accessor :file
  
  def load(file = 'todo.txt')
    @file = file
    @list = []
    
    # Check that file exists
    if using_valid_file?    
      # read the file, create a list, create items, add them
      File.readlines(@file, "\n").each do |item|
        add(item)
      end
    end
  end

  def initialize(file)
    @list = []
    load(file)
  end

  def add(item)
    @list << TodoItem.new(item)
  end

  def write
    # raise "File(#{@file}) is not writeable." unless using_valid_file?
    
    # write the file, only write the undone items
    File.open(@file, "w") do |f| 
      f.write @list.reject(&:done?).join("\n")
    end
  end

  def [](id)
    @list[id]
  end
  
  def to_s
    ret_val = ""
    @list.each_index do |i|
      ret_val << "Item #{i}: '#{@list[i]}' is #{@list[i].done? ? 'done' : 'not done'}.\n"
    end
    return ret_val #I know return isn't needed... but I hate not having it, lol.
  end
  
  private
  
  def using_valid_file?
    File.writable?(@file)
  end
end

class TodoItem
# provide reader and setter for name and state
attr_accessor :name, :state
alias_method :done?, :state

  def initialize(name)
    # store name
    @name = name
    # set state to undone
    @state = false
  end
  
  def to_s
    @name
  end
end

# ---
# the library will be used like this:
# list = TodoList.load("todo.td")
# list[0].done = true
# list.add "another cool item"
# list.write
#