rspec.rb

class Example
  attr_reader :context_name

  def initialize(context_name, &block)
    @context_name = context_name
    instance_eval &block
  end

  def expect(result)
    @result = result
    self
  end

  def to(expectation)
    self
  end

  def eq(expectation)
    Proc.new { |n| n.eql?(expectation) }
  end

  private
  attr_reader :result
end

describe NumberService do
  describe '#number' do
    it 'returns 12' do
      expect(NumberService.new.number).to eq(12)
    end
  end
end