CSC/ECE 517 Fall 2011/ch4 4c dm

Modules

Modules provide a structure to collect Ruby classes, methods, and constants into a single, separately named and defined unit. This is useful so that one can avoid clashes with existing classes, methods, and constants, and also so that you can add (mix in) the functionality of modules into your classes. The definition of a module is very similar to a class. Also, modules and classes are closely related. The Module class is the immediate ancestor of the Class class. Just like a class, a module can contain constants, methods and classes.

In Ruby programming language , a module is defined in the following way

 module <module name>
   statement1
   statement2
   ...........
 end

Consider for example ,a module called MyModule , which defines the happy and the sad times.

 module MyModule 
  GOODMOOD = "happy"
  BADMOOD = "sad" 
  def greet 
  return "I'm #{GOODMOOD}. How are you?"
  end
  def MyModule.greet
  return "I'm #{BADMOOD}. How are you?" 
  end
 end 

The above represents a module MyModule with a constant GOODMOOD and an “instance method” greet. In addition to instance methods a module may also have module methods. Just as class methods are prefixed with the name of the class, module methods are prefixed with the name of the module as shown above in MyModule.greet.

In spite of their similarities, there are two major features which classes possess but which modules do not: instances and inheritance. Classes can have instances (objects), superclasses (parents) and subclasses (children); modules can have none of these. Inspite of inability of the modules to be initialized and inherited., They provide a namespace and prevent name clashes and they implement the mixin facility.

Modules as Namespaces

Modules can be considered as a named ‘wrapper’ around a set of methods, constants and classes. The various bits of code inside the module share the same ‘namespace’ so they are all visible to each other, but not visible to code outside the module.

Consider the example of the MyModule described above

We can access the module constants just as we would access class constants using the :: scope resolution operator like this:

 puts(MyModule::GOODMOOD)   #->  happy
 puts(MyModule::BADMOOD)   #->  sad

We can access module methods using dot notation – that is, specifying the module name followed by a period and the method name For example

 puts( MyModule.greet )     # ->  I’m sad. How are you?

Since modules define a closed space, we cannot access the instance method “greet” from outside the module.

 puts greet   # -> NameError: undefined local variable or method `greet' for main:Object

In case of classes , we could have created instances of the class which would all have access to the instance methods of the class. However modules cannot be instantiated. This is where mixins come into picture.

Mixins

A mixin is a class that is mixed with a module or a set of modules. In other words the implementation of the class and module are intertwined and combined. The real usage of a mixin is exploited when the code in the mixin starts to interact with code in the class that uses it.In order to mix the modules into the class we make use of the “include” method. As it is included, the instance methods in the modules can be used just as though it were a normal instance method within the current scope.

The process of including a module is also called ‘mixing in’ – which explains why included modules are often called ‘mixins’. When you mix modules into a class definition, any objects created from that class will be able to use the instance methods of the mixed-in module just as though they were defined in the class itself.

 class MyClass
  include MyModule 
  def sayHi
  puts( greet )
  end 
 end 

Not only can the methods of this class access the greet method from MyModule, but so too can any objects created from the class:

 ob = MyClass.new 
 ob.sayHi     # -> I'm happy. How are you?
 puts(ob.greet)   # -> I'm happy. How are you?

Require / Load

As programs get bigger and bigger, the amount of reusable code also increases. It would best to break this code into separate files, so that these files can be shared across different programs. Typically the code organizes these files as class or module libraries. In order to incorporate the reusable code into new programs Ruby provides two statements.

 load "<filename.rb>”
 require “<filename>”

The load method includes the named Ruby source file every time the method is executed, whereas require loads any given file only once

Consider the following ruby module written in the file Week.rb.

 module Week
  FIRST_DAY = "Sunday"
  def Week.weeks_in_month
     puts "You have four weeks in a month"
  end
  def Week.weeks_in_year
     puts "You have 52 weeks in a year"
  end
 end

In order to include the module Week into a class in another file, we need to load the Week.rb file first and then include it in the class.

 class Decade
  require "Week"
  include Week
  no_of_yrs=10
  def no_of_months
     puts Week::FIRST_DAY
     number=10*12
     puts number
  end
 end
 d1=Decade.new
 puts Week::FIRST_DAY  # -> Sunday
 puts Week.weeks_in_month   #-> You have four weeks in a month
 puts Week.weeks_in_year    # -> You have 52 weeks in a year
 puts d1.no_of_months      # -> Sunday    120

The important things to consider is:

include makes features available, but does not execute the code.

require loads and executes the code one time (like a C #include).

load loads and executes the code every time it is encountered.

However, in order to allow mixins we need to use include.

Comparable and Enumerable

Comparable

The Comparable is a built in mixin module that provides the neat ability to define one’s own comparison ‘operators’ such as <, <=, ==, >= by mixing the module into your class and defining the <=> method. It is then possible to specify the criteria for comparing some value from the current object with some other value. The  compares the receiver against another object, returning -1, 0, or +1 depending on whether the receiver is less than, equal to, or greater than the other object respectively.

Assume we have a square class.

 class Square
  attr_reader :x
  def initialize(x)
  @x = x
  end
 end

In order to compare the areas of two Squares , we first need to include the Comparable mixin.

 class square
  include Comparable
  .
  .
  .
  def area
  x*x
  end
  def <=>(other)
  self.area<=>other.area
  end
 end

The <=> function uses Comparable mixin to compare the area of two Squares. We can call the Comparable methods on Square objects.

 s1 = Square.new(3)
 s2 = Square.new(4)
  if s1 < s2
  puts "The area of Square 1 is smaller than Square 2"   # -> This is the output printed.
  else if s1 > s2
  puts "The area of Square 1 is larger than Square 2"
  else
  puts "The area of Square 1 equals to Square 2"    
  end
 end

Composing modules

Enumerable is a standard mixin, which can be included in any class. It is a built in mix-in module for enumeration which provides collection classes with several traversal and searching methods, and with the ability to sort. The Enumerable method is already included in the Array class and provides arrays with a number of useful methods such as include? which returns true if a specific value is found in an array, min which returns the smallest value, max which returns the largest and collect which creates a new array made up of values returned from a block.

 arr = [1,2,3,4,5]
  y = arr.collect{ |i| i } #=> y = [1, 2, 3, 4]
  z = arr.collect{ |i| i * i } #=> z = [1, 4, 9, 16, 25] 
  arr.include?( 3 ) #=> true 
  arr.include?( 6 ) #=> false 
  arr.min #=> 1 
  arr.max #=> 5 

Another important method which is provided by the Enumerable module is the inject method. The inject method can be used to repeatedly apply an operation to adjacent elements in a collection.

Consider the example of summing the elements of an array.

 [1, 2, 3, 4].inject  { |result, element| result + element }   # => 10

The inject method takes an argument and a block. The block will be executed once for each element contained in the object that inject was called on ([1,2,3,4] in our example). The argument passed to inject will be yielded as the first argument to the block, the first time it's executed. The second argument yielded to the block will be the first element of the object that we called inject on. If a default value is not passed in as an argument when the block executes for the first time, the first argument will be set to the first element of the enumerable and the second argument will be set to the second element of the enumerable.

Since there is no default value passed in as an argument, the first time the block executes the first argument (result from the example) will be set to the first element of the array (1 from the example) and the second argument (element from the example) will be set to the second element of the enumerable (2 from the example).

The block will need to be executed 3 times, since the first execution will yield both the first and the second element. The first time the block executes it will add the result, 1, to the element, 2, and return a value of 3. The second time the block executes the result will be 3 and the element will also be 3 giving a return value of 6. The third and the final time the block executes, the result will be 6 and the element will be 4 , giving a return value of 10 which is the output.

The inject method and the other methods as mentioned above can be directly applied to built in classes such as Arrays , Range , Hash etc which contains the Enumerable module.

It is also possible to be able to apply Enumerable methods to classes which do not descend from existing classes which implement those methods. This can be done by including the Enumerable module in the class and then writing an iterator method called “each” . Consider a class Collection, which does not have the enumerable method in its ancestors.

 class Collection 
  include Enumerable
  def initialize( someItems ) 
  @items = someItems 
  end 
  def each 
  @items.each{ |i| 
  yield( i ) }
  end
 end

The Enumerable module is included in the Collection class. We can hence initialize a Collection object with an array, which will be stored in the instance variable, @items. When we call one of the methods provided by the Enumerable module (such as min, max or collect) this will, ‘behind the scenes’, call the each method in order to obtain each piece of data one at a time.

 things = Collection.new(['x','yz','defgh','ij','klmno']) 
  puts( things.min ) #=>  "defgh" 
  puts( things.max ) #=>  "yz"
  puts( things.collect{ |i| i.upcase } ) #=> ["X", "YZ", "DEFGH", "IJ", "KLMNO"]

Multiple Inheritance

Multiple Inheritance refers to a feature in some object oriented programming languages in which a class can inherit behaviors from more than one superclass. This contrasts with single inheritance in which a class can inherit behavior from at most one superclass. Though Multiple Inheritance provides its own advantages of improved modularity and ease of reuse of code in certain applications, it has its own set of disadvantages which sometimes outweigh the possible advantages.

One of the ambiguities associated with multiple inheritance is the “diamond problem”. Different languages have approach this problem in different ways.

Java makes use of the concept of Interfaces. Using interfaces, Java allows multiple inheritance of interfaces but implementation which consists of methods and variables maybe only singly inherited. This way , Java ensure that there is no confusion over which method to call of which instance variable to use.

Eiffel makes use of the renaming clause and ensures that the ancestor methods used in the descendants are explicitly specified. This allows the methods of the base class to be shared between all its descendants.

Ruby does not support multiple inheritance. Mixin provide a way which eliminates the need for multiple inheritance. Ruby has modules which are just like abstract classes in Java. Modules can have different methods implemented in them. They cannot be instantiated like abstract classes. They cannot inherit from other modules or classes. Ruby classes can inherit from only one superclass but can have unlimited number of modules in them to exploit the usage of predefined implementations in these modules. This provides functionality similar to multiple inheritance avoiding any ambiguities.

The below code in Ruby demonstrates two modules Horse and Bird, each of which contain the talk method defined in them. The Pegasus class includes these two modules and defines its own talk method. This example was chosen to exhibit on how Ruby resolves method calls at run time.

module Horse
     def talk
         puts "I can Neigh !!!!! "
     end
 end

 module Bird
     def talk
         puts "I can Chirp !!!!!"
     end
 end

 class Pegasus
     include Horse
     include Bird
        def talk
            puts "I am a whinnying horse who can fly and chirp ! "
        end
 end

 Aviary = Pegasus.new 
 Aviary.talk         

When the method talk is called on the Aviary object, Ruby makes sure that the latest method defined in the hierarchy is chosen. Here in this example, the talk method in the Pegasus class is defined the latest and hence “I am a whinnying horse who can fly and chirp !” is printed. If there was no method defined in the class, method from the Bird is printed since it is next in the hierarchy specified by the include statements. This way Ruby prevents any naming conflicts by always executing the most recently defined method.