CSC/ECE 517 Fall 2007/wiki1b 1 as

Background

Ruby does not implement true multiple inheritance, but provides Modules as a way to reuse chunks of codes in many classes.

Modules, unlike like classes in OO languages such as Java, cannot be instantiated or sub-classed. Modules are included in class definitions by using the ‘include’ method which will mix that module’s methods into the calling class. The module’s methods will then become instance methods.

A class can include several modules within the class definition. However, a problem exists when a class includes multiple modules that contain a method of the same name. Since the class will have access to both of these methods, unexpected behavior may occur when the names of the methods conflict.

Let’s look at a few examples!

Examples

1. Method Name Conflict

In the following example, we have the class Zap. This class includes the module Foo and the module Test. Both modules contain a method named bar.

  module Foo
     def bar
        "hello"
     end
  end
  module Test
     def bar
        "goodbye"
     end
  end   
  class Zap
   include Foo
   include Test
  end

If a call is made to Zap's bar method, the caller is unsure whether the result will be "hello" or "goodbye".

  z = Zap.new
  puts z.bar

The result of this call is:

  goodbye

This is because Ruby will first search the last module included, and continue in a descending order. The method that is invoked may not be the method that the caller expected to run. Therefore, precaution to be taken within the class to eliminate ambiguity.

2. Using Namespaces to Avoid Conflict

 module Grouchy
  def Grouchy.say_hello(string='somebody')
   puts "#{string} says: Don't tell me what to do!" 
  end
 end

Grouchy.say_hello is the class method of the module Grouchy We have a class Person which includes the module Grouchy

class Person

 require "grouchy" 

 attr_accessor :name

 def initialize(name='somebody')
   @name = name
 end

end person = Person.new('Charlie') Grouchy.say_hello(person.name)

It products:

Charlie says: Don't tell me what to do!

When facing the name conflicts problem, we can use namespace to tell the different of two methods.

module Debug

 def Debug.who_am_i
   "Debug"
 end

end

module Burp

 def Burp.who_am_i
   "Burp"
 end

end

class EightTrack

 include Debug
 include Burp
 def who_am_i
   puts Burp.who_am_i
   puts Debug.who_am_i    
 end

end

et = EightTrack.new

et.who_am_i

It products:

Burp Debug

Another way is using the alias method, we still have two modules have the same name method who_am_i. module Debug

 def who_am_i
   "Debug"
 end

end

module Burp

 def who_am_i
   "Burp"
 end

end

class RubyTest

 include Burp  
 alias :Burp_who_am_i :who_am_i   
 include Debug
 alias :Debug_who_am_i :who_am_i   

end

rt = RubyTest.new puts rt.Debug_who_am_i puts rt.Burp_who_am_i

Conclusion

As illustrated in the examples, there are at least two ways to ensure that your class runs as expected even when using modules with method name conflicts.

The two approaches are:

1. using namespaces
2. using an alias

In our opinion, using namespaces is the better approach. Inheriting from the module and using the qualified names to avoid conflicts is a more efficient OO design. It is also easier to read and maintain because in viewing the call, a reader will immediately gather the expected behavior of the call, as opposed to finding the alias definition and trying to make the connection.

With the alias approach, the inheritance is limited and the programmer will need to constantly update the list of aliases for methods needed as they arise.

References

1. Modules, Mixins, and Inheritance
2. Ruby User's Guide