dsl.rb

class FancyDSL
  def initialize(&block)
    instance_eval(&block)
  end

  def setup(value)
    @setup = value
  end
end

f = FancyDSL.new do
  setup :wadus
end

puts f.inspect

class Client
  def initialize(value)
    @value = value
  end

  def do_something
    f = FancyDSL.new do
      setup @value
    end

    puts f.inspect
  end
end

c = Client.new(:wadus)

c.do_something