A Ruby DSL builder helper. By using a delegator object we can isolate the DSL methods, making them only available inside the `build` block.

dsl.rb

require "delegate"

module DSL
  def self.build(klass, &block)
    delegator_klass = Class.new(SimpleDelegator, &block)
    klass.define_singleton_method(:build) do |*args, &block|
      klass.new(*args).tap { |base| delegator_klass.new(base).instance_eval(&block) }
    end
  end
end

dsl_spec.rb

require "minitest/spec"
require "minitest/autorun"
require_relative "dsl"

describe DSL do

  class TestConfig
    attr_reader :file_paths

    def initialize
      @file_paths = []
    end

    def add_paths(*paths)
      @file_paths += paths.map { |p| File.expand_path(p) }
    end

    DSL.build(self) do
      def files(*paths)
        add_paths(*paths)
      end
    end
  end

  it "can add a build method with scoped DSL methods" do
    config = TestConfig.build do
      files "dsl.rb", "dsl_spec.rb"
    end
    config.file_paths.must_equal %w[dsl.rb dsl_spec.rb].map { |f| File.expand_path(f) }
  end
end