RFC: Uncovered mutation example for blog post by @solnic

book.rb

class Book
  attr_reader :sections
  
  def initialize
    @sections = []
    @index    = {}
  end
  
  def section(number)
    @index.fetch(number) do
      raise "Book does not have a section with number: #{number}"
    end
  end
  
  def <<(section)
    @sections << section
    @index[section.number] = section # Statement deletion will delete this statement and side effects are uncovered from append_spec.rb
    self
  end
end

spec_book_append.rb

require 'spec_helper'

describe Book, '#<<' do
  let(:object) { Book.new }
  let(:section) { Section.new(1) }
  
  subject { book << section }
  
  it 'should return self' do
    should be(object)
  end
  
  it 'should add section to book' do
    expect { subject }.to change { object.sections.include?(section) }.from(false).to(true)
  end
end

spec_book_section.rb

require 'spec_helper'

describe Book, '#section' do
  let(:object) { described_class.new }
  let(:section) { Section.new(1) }
  
  subject { object.section(number) }
  
  before do
    object << section
  end

  context 'when section exists' do
    let(:number) { 1 }
    
    it 'should return section' do
      should be(section)
    end
  end

  context 'when section does not exist' do
    let(:number) { 2 }

    it 'should raise error' do
      expect { subject }.to raise_error(RuntimeError, "Book does not have section with number: 2")
    end
  end
end