Tell the constructor new settings in your own DSL. `WhateverService` is a toplevel module whose `configure` method returns `WhateverService::Client` when a block passed. This config block is going to be `instance_eval`-ed in `WhateverService::Client`'s constructor. You can also pass a hash to the `WhateverService::Client`'s constructor.

instance_config_in_dsl.rb

$: << '.'
require 'whatever_service'

wsc1 = WhateverService.configure do 
  user_id  'foo'
  password 'bar_baz'
end

wsc2 = WhateverService::Client.new(:user_id => 'baz', :password => 'foo_bar')

[wsc1, wsc2].each do |w|
  WhateverService::Client::CONFIG_NAMES.each do |name|
    puts "#{name} = #{w.config[name]}"
  end
end
# output:
#   user_id = foo
#   password = bar_baz
#   user_id = baz
#   password = foo_bar

whatever_service.rb

module WhateverService
  # WS_DIR = File.expand_path(File.dirname(__FILE__))
  # 
  # %w(client).each do |sub|
  #   require File.join(WS_DIR, 'whatever_service', sub)
  # end

  def self.configure(&blk)
    WhateverService::Client.new &blk
  end
end

# the following should be in whatever_service/client.rb
# but is here for Gist restriction

module WhateverService
  class Client
    CONFIG_NAMES = [:user_id, :password]
    CONFIG_NAMES.each do |name|
      define_method(name) { |val| @config[name] = val }
    end

    attr_reader :config

    def initialize(options = {}, &blk)
      @config = options
      instance_eval &blk unless blk.nil?
    end
  end
end