An contrived example [dry-container, dry-configurable, dry-auto_inject]

example.rb

#!/usr/bin/env ruby

require 'bundler/inline'

gemfile(true) do
  source "https://rubygems.org"
  gem 'dry-system', '0.8.0'
  gem 'dry-configurable'
  gem 'dry-container'
  gem 'dry-auto_inject', '0.4.4'
  #gem 'dry-auto_inject'
end
puts "\n"

require 'dry-auto_inject'
require 'dry-configurable'
require 'dry-container'

class SystemContainer
  extend Dry::Container::Mixin
  register(:namespaces, -> { Dry::Container.new }, memoize: true, call: false)
end

# Autoloaded from something like system/boot maybe?
SystemContainer[:namespaces].register :persistence do

  Dry::Container::Namespace.new :persistence do
    new_container = Dry::Container.new
    new_container.register :verb_past_tense, -> { self.verb_past_tense }
    new_container.register :singular_noun, -> { self.singular_noun }
    new_container.register :place, -> { self.place }

    register :some_config, new_container

    register :some_instantiation do # Kinda like creating a ROM.container?
      config = self['persistence.some_config']
      'I created something that %s from %s onto a %s' % [
        config[:verb_past_tense], config[:place], config[:singular_noun]
      ]
    end
  end

end

ImportSystemContainer = Dry::AutoInject(SystemContainer)

class MadLib
  extend Dry::Configurable
  include Dry::Container::Mixin
  
  include ImportSystemContainer[:namespaces]

  def self.attrs
    %i| verb_past_tense singular_noun place |
  end

  attrs.each do |attr|
    setting attr
    attr_accessor attr
  end

  def initialize(args = {})
    super

    self.class.attrs.each do |attr|
      send("#{attr}=".to_sym, args[attr] || self.class.config[attr] || raise(ArgumentError, "'#{attr}' hasn't been configured"))
      #puts "#{attr}=" + send(attr).to_s
    end

    # In this example there's only one namespace so far.
    import namespaces[:persistence]

    register('persistence.madlib', -> { self['persistence.some_instantiation'] })
  end

  def madlib
    self['persistence.madlib']
  end

end


### No more infrastructure...


begin
  puts "'place' is deliberately unconfigured."
  client = MadLib.new verb_past_tense: 'throttled', singular_noun: 'firetruck'
  puts client.madlib
  puts ''

rescue ArgumentError => e
  puts e.message
  puts '*** ArgumentError caught. Oh well, moving on...'
  puts ''
end

MadLib.configure do |config|
  config.place = 'the local diner'
end

puts "'place' is configured as a default. "
client = MadLib.new verb_past_tense: 'throttled', singular_noun: 'firetruck'
puts client.madlib
puts ''

puts "'place' stays the same while the rest change"
new_client = MadLib.new verb_past_tense: 'dropped', singular_noun: 'volcano'
puts new_client.madlib
puts ''

puts "Here we explicitly set 'place'"
newer_client = MadLib.new place: 'the mayors boudoir', verb_past_tense: 'flooded', singular_noun: 'penthouse'
puts newer_client.madlib
puts ''