Included hook in Ruby - Mini Project: Build & Apply

Using the Included Hook in Ruby Modules

📖 Scenario: Imagine you are building a simple system where different classes can share a greeting message. You want to use a Ruby module to add this greeting feature to any class that includes it. To make it more interesting, you want to print a message automatically when the module is included in a class.

🎯 Goal: You will create a Ruby module with an included hook that prints a message when included. Then, you will include this module in a class and use the shared greeting method.

📋 What You'll Learn

Create a module called Greeter with a method greet that returns the string "Hello!".

Add an included hook inside the Greeter module that prints "Greeter module included!" when the module is included in a class.

Create a class called Person that includes the Greeter module.

Create an instance of Person and call the greet method, then print the result.

💡 Why This Matters

🌍 Real World

Modules with included hooks are useful to add shared behavior and setup code automatically when modules are included in classes, common in Ruby libraries and frameworks.

💼 Career

Understanding modules and hooks is important for Ruby developers working on code reuse, gems, and Rails applications.

1

Create the Greeter module with a greet method

Create a module called Greeter with a method greet that returns the string "Hello!".

Ruby

module Greeter
  # Your code here
end

Hint

Define a module with module Greeter and add a method greet that returns the greeting string.

2

Add the included hook to the Greeter module

Inside the Greeter module, add an included hook method that takes a parameter base and prints "Greeter module included!".

Ruby

module Greeter
  def greet
    "Hello!"
  end

  # Your code here
end

Hint

The included hook is a class method inside the module. Use def self.included(base) to define it.

3

Create the Person class and include the Greeter module

Create a class called Person and include the Greeter module inside it.

Ruby

module Greeter
  def greet
    "Hello!"
  end

  def self.included(base)
    puts "Greeter module included!"
  end
end

# Your code here

Hint

Define the class with class Person and include the module using include Greeter.

4

Create a Person instance and print the greeting

Create an instance of Person called person. Then call person.greet and print the result.

Ruby

module Greeter
  def greet
    "Hello!"
  end

  def self.included(base)
    puts "Greeter module included!"
  end
end

class Person
  include Greeter
end

# Your code here

Hint

Create the object with person = Person.new and print the greeting with puts person.greet.

Practice

1.

What does the included hook do in a Ruby module?

easy

A. It runs only when the module is extended, not included.

B. It runs code automatically when the module is included in a class.

C. It prevents the module from being included in any class.

D. It defines a method that must be called manually.

Solution

1. Step 1: Understand the purpose of the included hook

   The included hook is a special method in Ruby modules that runs automatically when the module is included in a class.

2. Step 2: Compare options with the definition

   Only It runs code automatically when the module is included in a class. correctly describes this automatic behavior. Other options describe manual calls, prevention, or extension, which are incorrect.

3. Final Answer:

   It runs code automatically when the module is included in a class. -> Option B

4. Quick Check:

   Included hook = automatic run on include [OK]

Hint: Remember: included runs automatically on module inclusion [OK]

Common Mistakes:

• Thinking included runs on extend instead of include
• Believing included requires manual method calls
• Confusing included with preventing inclusion

2.

Which of the following is the correct syntax to define an included hook inside a Ruby module?

module Example
  # What goes here?
end

easy

A. def self.included(base)
puts "Included in #{base}"
end

B. def included(base)
puts "Included in #{base}"
end

C. def included
puts "Included"
end

D. def self.include(base)
puts "Included in #{base}"
end

Solution

1. Step 1: Recall how to define class methods in modules

   The included hook must be a module method, so it needs to be defined with self.included.

2. Step 2: Check the method signature

   The method receives the including class as a parameter, so it must accept one argument (base).

3. Final Answer:

   def self.included(base) puts "Included in #{base}" end -> Option A

4. Quick Check:

   included hook = def self.included(base) [OK]

Hint: Use def self.included(base) for included hook syntax [OK]

Common Mistakes:

• Defining included without self (instance method instead of module method)
• Missing the base parameter
• Using def self.include instead of included

3.

What will be the output of this Ruby code?

module Greetings
  def self.included(base)
    puts "Greetings included in #{base}"
  end
end

class Person
  include Greetings
end

medium

A. SyntaxError

B. No output

C. Greetings included in Person

D. Greetings included in Greetings

Solution

1. Step 1: Identify when included hook runs

   The included hook runs automatically when the module is included in a class.

2. Step 2: Check the output inside the included method

   The method prints "Greetings included in #{base}", where base is the class Person.

3. Final Answer:

   Greetings included in Person -> Option C

4. Quick Check:

   included hook prints class name on include [OK]

Hint: included hook prints message with class name on include [OK]

Common Mistakes:

• Expecting no output because included is a hook
• Confusing base with module name
• Thinking included runs on class definition, not include

4.

Find the error in this Ruby code using the included hook:

module Logger
  def included(base)
    base.extend(ClassMethods)
  end

  module ClassMethods
    def log(msg)
      puts "Log: #{msg}"
    end
  end
end

class App
  include Logger
end

App.log("Start")

medium

A. The include statement should be extend.

B. The ClassMethods module should be included, not extended.

C. The log method should be an instance method, not class method.

D. The included method should be defined as self.included, not instance method.

Solution

1. Step 1: Check how included hook is defined

   The included hook must be a module method, defined as self.included, not an instance method.

2. Step 2: Understand the effect of incorrect definition

   Defining included without self means it won't run automatically on include, so base.extend won't happen.

3. Final Answer:

   The included method should be defined as self.included, not instance method. -> Option D

4. Quick Check:

   included hook must be self.included [OK]

Hint: Define included as self.included to run on module include [OK]

Common Mistakes:

• Defining included without self
• Confusing extend and include in hook
• Trying to call class method on instance

5.

You want to add a class method greet to any class that includes your module Friendly. Which is the best way to do this using the included hook?

module Friendly
  # Fill in the included hook
end

class Dog
  include Friendly
end

puts Dog.greet

hard

A. Define self.included(base) and inside it call base.extend with a module containing greet.

B. Define an instance method greet inside Friendly module.

C. Define a class method greet directly inside Friendly module without included hook.

D. Call include Friendly inside Dog.greet method.

Solution

1. Step 1: Understand how to add class methods via included hook

   To add class methods, define self.included(base) and call base.extend with a module containing those methods.

2. Step 2: Why other options fail

   Instance methods won't be class methods; defining class methods directly in module won't add them to including class; calling include inside method is incorrect.

3. Final Answer:

   Define self.included(base) and inside it call base.extend with a module containing greet. -> Option A

4. Quick Check:

   Use included hook + base.extend for class methods [OK]

Hint: Use included hook with base.extend to add class methods [OK]

Common Mistakes:

• Defining greet as instance method instead of class method
• Not using included hook to extend class
• Trying to call include inside a method