Method lookup chain in Ruby - Mini Project: Build & Apply

Understanding Method Lookup Chain in Ruby

📖 Scenario: Imagine you are creating a simple program to understand how Ruby finds methods when you call them on an object. This is like asking who answers a question first in a group of friends. Ruby looks for the method in a specific order called the method lookup chain.

🎯 Goal: You will build a small Ruby program with a class and a module. You will see how Ruby finds methods by calling the same method name from different places. This will help you understand the method lookup chain.

📋 What You'll Learn

Create a class called Animal with a method sound that returns the string "Generic sound".

Create a module called Walkable with a method sound that returns the string "Walking sound".

Include the module Walkable in the class Animal.

Create an instance of Animal called dog.

Call the sound method on dog and print the result.

💡 Why This Matters

🌍 Real World

Understanding method lookup helps when you use modules and inheritance in Ruby programs, which is common in web development and scripting.

💼 Career

Knowing method lookup chains is important for debugging and designing clean, reusable Ruby code in jobs like backend development and automation.

1

Create the Animal class with a sound method

Create a class called Animal with a method sound that returns the string "Generic sound".

Ruby

# Your code here

Hint

Use class Animal to start the class and def sound to define the method.

2

Create the Walkable module with a sound method

Create a module called Walkable with a method sound that returns the string "Walking sound".

Ruby

class Animal
  def sound
    "Generic sound"
  end
end

# Your code here

Hint

Use module Walkable to start the module and def sound to define the method.

3

Include the Walkable module in the Animal class

Add the line include Walkable inside the Animal class, below the sound method.

Ruby

class Animal
  def sound
    "Generic sound"
  end
  # Your code here
end

module Walkable
  def sound
    "Walking sound"
  end
end

Hint

Write include Walkable inside the Animal class to add the module.

4

Create an instance and call the sound method

Create an instance of Animal called dog. Then call dog.sound and print the result using puts.

Ruby

class Animal
  def sound
    "Generic sound"
  end
  include Walkable
end

module Walkable
  def sound
    "Walking sound"
  end
end

# Your code here

Hint

Create dog = Animal.new and then puts dog.sound to see which method Ruby uses.

Practice

1.

What does the method lookup chain in Ruby represent?

easy

A. The list of all variables in a class

B. The order Ruby searches to find a method to execute

C. The sequence of method calls inside a method

D. The order Ruby compiles the code

Solution

1. Step 1: Understand method lookup chain meaning

   The method lookup chain is the path Ruby follows to find a method when called on an object.

2. Step 2: Compare options with definition

   Only The order Ruby searches to find a method to execute correctly describes this as the order Ruby searches for a method to execute.

3. Final Answer:

   The order Ruby searches to find a method to execute -> Option B

4. Quick Check:

   Method lookup chain = method search order [OK]

Hint: Think 'where Ruby looks for methods' [OK]

Common Mistakes:

• Confusing method lookup with variable list
• Thinking it shows method call sequence
• Mixing lookup with code compilation order

2.

Which Ruby code correctly shows the method owner for to_s called on a string?

"hello".method(:to_s).owner

easy

A. String

B. Object

C. BasicObject

D. Kernel

Solution

1. Step 1: Recall where to_s is defined for String

   The to_s method for String objects is actually defined in the Kernel module, which is included in Object, and thus available to String.

2. Step 2: Check method owner for to_s on a String object

   Using "hello".method(:to_s).owner returns Kernel because String does not override to_s.

3. Final Answer:

   Kernel -> Option D

4. Quick Check:

   to_s owner for String = Kernel [OK]

Hint: String inherits to_s from Kernel [OK]

Common Mistakes:

• Assuming method is owned by String
• Confusing Object and BasicObject
• Forgetting Kernel defines to_s

3.

Given this Ruby code, what is the output?

module M1
  def greet; "Hello from M1"; end
end

class C
  include M1
  def greet; "Hello from C"; end
end

obj = C.new
puts obj.greet

medium

A. Hello from C

B. Hello from M1

C. NoMethodError

D. Hello from Object

Solution

1. Step 1: Understand method lookup order with include and override

   Class C includes module M1, but also defines its own greet method, which overrides the module's method.

2. Step 2: Determine which greet method is called

   Ruby looks first in C, finds greet there, so it calls "Hello from C".

3. Final Answer:

   Hello from C -> Option A

4. Quick Check:

   Method override in class wins over included module [OK]

Hint: Class methods override included module methods [OK]

Common Mistakes:

• Thinking included module method runs first
• Expecting NoMethodError due to confusion
• Assuming Object's method is called

4.

What is the error in this Ruby code snippet?

module M
  def hello; "Hi from M"; end
end

class A
  prepend M
  def hello; "Hi from A"; end
end

obj = A.new
puts obj.hello

medium

A. No error, output is "Hi from M"

B. No error, output is "Hi from A"

C. SyntaxError due to prepend usage

D. NoMethodError for hello

Solution

1. Step 1: Understand prepend effect on method lookup

   Prepending module M places it before class A in the lookup chain, so M's methods override A's.

2. Step 2: Determine which hello method runs

   Calling hello on obj calls M's method, outputting "Hi from M".

3. Final Answer:

   No error, output is "Hi from M" -> Option A

4. Quick Check:

   Prepend puts module before class in lookup [OK]

Hint: Prepend modules override class methods [OK]

Common Mistakes:

• Expecting class method to run instead
• Thinking prepend causes syntax error
• Confusing prepend with include

5.

Consider this Ruby code:

module M1
  def info; "M1"; end
end

module M2
  def info; "M2"; end
end

class Base
  def info; "Base"; end
end

class Derived < Base
  include M1
  prepend M2
end

obj = Derived.new
puts obj.info

What will be printed and why?

hard

A. M1, because include puts M1 before Base

B. Base, because class method overrides modules

C. M2, because prepend puts M2 before Derived in lookup

D. NoMethodError, info not found

Solution

1. Step 1: Analyze method lookup order with prepend and include

   Prepend M2 places it before Derived, then Derived, then included M1, then Base.

2. Step 2: Identify which info method is called

   Since M2 is first in lookup, its info method runs, printing "M2".

3. Final Answer:

   M2, because prepend puts M2 before Derived in lookup -> Option C

4. Quick Check:

   Prepend > class > include > superclass [OK]

Hint: Prepend modules come before class, include after [OK]

Common Mistakes:

• Thinking include module method runs first
• Assuming class method overrides prepend module
• Ignoring method lookup chain order