Code Execution

In this chapter we will answer the question, when does Ruby execute code as it encounters the code?

At the Top Level

Open the editor and print 'hi' to the standard output.

puts 'hi'

This prints:

hi

Ruby encountered the puts() method and it executed the instruction. You see the output in the terminal.

Inside a Class

Define a class and print something to the standard output within the class.

class Rabbit
  puts "I am within Rabbit class"
end

Running this program prints:

I am within Rabbit class

This dynamic nature of Ruby surprises developers who are familiar with other languages.

Inside a Module

Define a module and print something to the standard output within the module.

module Rabbit
  puts "I am within Rabbit module"
end

Running this program prints:

I am within Rabbit module

Inside a Method in a Class

Let's now add a method to the Rabbit class:

class Rabbit
  def speak
    puts "My name is Bugs Bunny" 
  end
end

Running this program does not print anything to the standard output.

Invoking the Instance Method

Why? Because, we need an instance of Rabbit to send the speak() message to it.

class Rabbit
  def speak
    puts "My name is Bugs Bunny" 
  end
end

bugs = Rabbit.new
bugs.speak

Running this program prints:

My name is Bugs Bunny

Inside a Method in a Module

What happens when we define a method in a module?

module Rabbit
  def speak
    puts "My name is Bugs Bunny"
  end
end

Running this program prints nothing to the standard output.

Mixin the Module

We can mixin the Rabbit module to the top level and invoke the speak() method.

module Rabbit
  def speak
    puts "My name is Bugs Bunny"
  end
end

include Rabbit
speak

Running this program prints:

My name is Bugs Bunny

Summary

In this chapter, we learned that Ruby executes code as it encounters code:

1. At the top level.
2. Inside a class.
3. Inside a module.

But, it does not execute the code inside the instance method defined in a class or a method defined inside a module as it encounters it. We need an object to execute an instance method defined in a class or mixin the method defined in a module and then call the method.