draw.rb

module Routedraw
  def self.define_api_routes(routes: Rails.application.routes,
                             version: :v1,
                             &block)
    routes.draw do
      namespace :api, defaults: { format: :json } do
        namespace version do
          # yield self to pass the current context to the block
          yield self
        end
      end
    end
  end
end

routes.rb

require 'routedraw'

Rails.application.routes.draw do
  resources :posts

  Routedraw.define_api_routes do |r|
    r.resources :comments
  end
end