external.config

application.rb

c = Configuration.new
c.instance_eval(File.read('config_file.rb'))

config_file.rb

sql_connection do
  host "127.0.0.1"
end

configuration.rb

class Configuration
  def sql_connection
    @sql_connection = SqlConnection.new
    @sql_connection.instance_exec(Proc.new) if block_given?
  end
end

sql_connection.rb

class SqlConnection
  def host(value=nil)
    @host = value if value
    @host
  end
end