From: John Smirnoff <jsmirn@...>
Date: 2009-09-13T07:26:01+09:00
Subject: choosing a module name at runtime

My application chooses at runtime the input format of some data, for
example :xml or :mysql. Then, the app instantiates a DataManager. The
DataManager class should 'include' the appropriate Module (either
DataIOXML or DataIOMySQL).
I tried to use include in the initialize method of DataManager, but that
doesn't work.
Can I include a module in the initialize method of the class? If yes,
how?
If no: Any other ways how I can choose the correct module to include at
runtime?

Important: All the information about Modules and file names is stored in
the DataManager class. My app only knows the DataManger class itself,
not the names of the modules (so I can't subclass separate IO classes
from DataManager, and instantiate them directly from my app).

Here's the example code for the app, the DataManger class, and the :xml
Module:



#! /usr/bin/ruby -w
# main app
#
# here, only the name 'DataManager' and the IO format (:xml or :mysql)
is known,
# not the name of the format-specific module.
require "DataManager.rb"
dm = DataManager.new(:xml)
dm.say_something



# DataManager.rb
class DataManager

  # theDataManager class knows all possible IO formats, and the
  # corresponding Module names:
  @@io_mixins = {
    :xml => 'DataIOXML',
    :mysql => 'DataIOMySQL',
  }

  def initialize(format=:xml)

    unless @@io_mixins.include? format
      raise NotImplementedError, "IO Format #{format} unknown"
    end

    require @@io_mixins[format]+".rb"
    # the next line causes an error
    include DataIOXML

  end
end



# DataIOXML.rb
# XML-specific module
module DataIOXML
  def say_something
    puts "I'm DataIOXML"
  end
end
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