list.rb

require_relative "task"

class List
  attr_reader :title, :tasks

  def initialize(title, tasks = [])
    @title = title
    @tasks = tasks
  end

  def add_task(task)
    tasks << task
  end

  def complete_task(index)
    return false unless tasks[index]
    if tasks[index].complete!
      return true
    end
  end

  def delete_task(index)
    tasks.delete_at(index)
  end

  def completed_tasks
    tasks.select { |task| task.complete? }
  end

  def incomplete_tasks
    tasks.select { |task| !task.complete? }
  end
end

list_spec.rb

require "rspec"

require_relative "list"
require_relative "task"

describe List do
  let(:title) { "Silly List" }
  let(:task1) { Task.new("jiggle") }
  let(:task2) { Task.new("jiggle") }
  let(:list) { List.new(title, [task1, task2]) }

  describe "#initialize" do
    it "takes a title for its first argument" do
      List.new("Little List").should be_an_instance_of List
    end

    it "requires one argument" do
      expect { List.new }.to raise_error(ArgumentError)
    end
  end

  describe "#add_task" do
    it "adds task to collection of task" do
      expect {
        list.add_task("eat")
        }.to change{ list.tasks.count }.by(1)
    end
  end

  describe "#complete_task" do
    it "will be false of task index doesn't exist" do
      expect(list.complete_task(200)).to be_false
    end

    it "will be complete task if index exists" do
      expect(list.complete_task(0)).to be_true
    end
  end

  describe "#delete_task" do
    it "will delete task from list" do
      list.delete_task(0)
      expect(list.tasks).not_to include [task1]
    end
  end

  describe "#completed_tasks" do
    it "will return completed tasks" do
      list.complete_task(0)
      expect(list.completed_tasks).to eql [task1]
    end
  end

  describe "#incomplete_tasks" do
    it "will return incomplete tasks" do
      list.complete_task(0)
      expect(list.incomplete_tasks).to eql [task2]
    end
  end

end

task.rb

class Task
  attr_reader :description

  def initialize(description)
    @description = description
    @complete = false
  end

  def complete?
    @complete
  end

  def complete!
    @complete = true
  end
end

task_spec.rb

require "rspec"

require_relative "task"

describe Task do

  let(:description) { "Walk the giraffe" }
  let(:task)        { Task.new(description) }

  describe "#initialize" do
    it "takes a description for its first argument" do
      Task.new("Feed the parakeet").should be_an_instance_of Task
    end

    it "requires one argument" do
      expect { Task.new }.to raise_error(ArgumentError)
    end
  end

  describe "#description" do
    it "returns the correct description for the task" do
      task.description.should eq description
    end
  end

  describe "#complete?" do
    it "returns false for incomplete tasks" do
      task.complete?.should be_false
    end

    it "returns true for completed tasks" do
      task.complete!
      task.complete?.should be_true
    end
  end

  describe "#complete!" do
    it "changes the task from incomplete to completed" do
      #
      # Note the use of the `be_complete` method here.
      # This is some RSpec goodness which expects a
      # method `complete?` to be defined which returns
      # true or false.
      #
      task.should_not be_complete
      task.complete!
      task.should be_complete
    end
  end
end