Prepend for method chain insertion in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using prepend?

Consider the following Ruby code that uses prepend to insert a module into a class method chain. What will be printed when MyClass.new.greet is called?

Ruby

module M
  def greet
    "Hello from M, " + super
  end
end

class MyClass
  def greet
    "Hello from MyClass"
  end
end

MyClass.prepend(M)
puts MyClass.new.greet

A "Hello from M, Hello from MyClass"

B "Hello from MyClass"

C NoMethodError

D "Hello from M"

💡 Hint

Remember that prepend inserts the module before the class in the method lookup chain.

Using prepend, the module M is placed before MyClass in the method lookup chain. So when greet is called, Ruby first calls M#greet, which calls super to invoke MyClass#greet. The outputs concatenate, resulting in "Hello from M, Hello from MyClass".

❓ Predict Output

intermediate

What does this code output when using prepend with multiple modules?

Given the following Ruby code, what will be the output of obj.call?

Ruby

module A
  def call
    "A" + super
  end
end

module B
  def call
    "B" + super
  end
end

class Base
  def call
    "Base"
  end
end

class MyClass < Base
  prepend A
  prepend B
end

obj = MyClass.new
puts obj.call

A "ABBase"

B "BaseAB"

C "BABase"

D "BaseBA"

💡 Hint

Modules prepended later are placed before earlier ones in the lookup chain.

The last prepended module (B) is checked first, then A, then Base. So the call chain is B#call → A#call → Base#call. Each adds its letter before calling super, resulting in "BABase".

🔧 Debug

advanced

Why does this prepend example cause a SystemStackError?

Examine the code below. When MyClass.new.greet is called, it causes a SystemStackError (stack level too deep). Why does this happen?

Ruby

module M
  def greet
    "Hi " + greet
  end
end

class MyClass
  prepend M
  def greet
    "there"
  end
end

MyClass.new.greet

A The class method <code>greet</code> is missing a <code>super</code> call causing infinite recursion.

B The <code>greet</code> method in module <code>M</code> calls itself recursively without calling <code>super</code>.

C The module <code>M</code> is not properly prepended before the class.

D The <code>greet</code> method in <code>MyClass</code> is private and cannot be called.

💡 Hint

Look at how greet in M calls greet again instead of super.

The greet method in module M calls itself recursively by calling greet instead of super. This causes infinite recursion and eventually a SystemStackError.

📝 Syntax

advanced

Which option correctly prepends a module to insert behavior before a method?

You want to prepend module M to class MyClass so that M#hello runs before MyClass#hello. Which code snippet correctly does this?

A

class MyClass
  prepend M
  def hello
    "Hello from MyClass"
  end
end

B

class MyClass
  def hello
    "Hello from MyClass"
  end
  prepend M
end

C

module M
  def hello
    "Hello from M"
  end
end
class MyClass
  def hello
    "Hello from MyClass"
  end
end
MyClass.include(M)

D

class MyClass
  def hello
    "Hello from MyClass"
  end
end
M.prepend(MyClass)

💡 Hint

The prepend call must be inside the class body before method definitions or after, but not after class end.

Option A correctly prepends module M inside the class body before defining hello. This ensures M#hello is called before MyClass#hello. Option A is valid syntax in Ruby and will work correctly; prepend can appear after method definitions inside the class body. Option A tries to prepend a class to a module, which is invalid. Option A uses include instead of prepend, so M#hello would be called after MyClass#hello.

🚀 Application

expert

How to use prepend to log method calls without changing original class?

You have a class Calculator with a method add(a, b). You want to log every call to add without modifying Calculator directly. Which module and prepend usage correctly logs the call and then calls the original method?

Ruby

class Calculator
  def add(a, b)
    a + b
  end
end

A

module Logger
  def add(a, b)
    puts "Adding #{a} and #{b}"
  end
end
Calculator.prepend(Logger)

B

module Logger
  def add(a, b)
    puts "Adding #{a} and #{b}"
    add(a, b)
  end
end
Calculator.prepend(Logger)

C

module Logger
  def add(a, b)
    puts "Adding #{a} and #{b}"
    Calculator.new.add(a, b)
  end
end
Calculator.prepend(Logger)

D

module Logger
  def add(a, b)
    puts "Adding #{a} and #{b}"
    super
  end
end
Calculator.prepend(Logger)

💡 Hint

Use super to call the original method after logging.

Option D correctly defines Logger#add that logs the arguments and then calls super to invoke the original Calculator#add. This way, the original method is called without modifying Calculator. Option D causes infinite recursion by calling add inside add. Option D creates a new instance and calls add on it, which is inefficient and breaks method chain. Option D logs but does not call the original method, so it changes behavior.

Practice

1. What does the prepend keyword do in Ruby when used with a module and a class?

easy

A. It inserts the module before the class in the method lookup chain.

B. It appends the module after the class in the method lookup chain.

C. It replaces the class methods with the module methods permanently.

D. It creates a copy of the module inside the class without affecting method lookup.

Solution

1. Step 1: Understand method lookup order with prepend

   Using prepend places the module before the class in the method lookup chain, so module methods are checked first.

2. Step 2: Compare prepend with other module inclusion methods

   Unlike include which adds the module after the class, prepend inserts it before, changing method resolution order.

3. Final Answer:

   It inserts the module before the class in the method lookup chain. -> Option A

4. Quick Check:

   Prepend = module methods run first [OK]

Hint: Prepend means module methods run before class methods [OK]

Common Mistakes:

• Confusing prepend with include
• Thinking prepend replaces class methods permanently
• Assuming prepend appends module after class

2. Which of the following is the correct syntax to prepend a module Mod to a class MyClass?

easy

A. class MyClass; extend Mod; end

B. class MyClass; include Mod; end

C. class MyClass; prepend Mod; end

D. class MyClass; insert Mod; end

Solution

1. Step 1: Recall Ruby syntax for prepend

   The correct syntax to prepend a module is prepend ModuleName inside the class definition.

2. Step 2: Eliminate other options

   include adds module after class, extend adds methods as class methods, and insert is invalid syntax.

3. Final Answer:

   class MyClass; prepend Mod; end -> Option C

4. Quick Check:

   Prepend syntax = prepend ModuleName [OK]

Hint: Use 'prepend ModuleName' inside class to insert module first [OK]

Common Mistakes:

• Using include instead of prepend
• Using extend which affects class methods
• Typing invalid keywords like insert

3. What is the output of this Ruby code?

module M
  def greet
    "Hello from M, " + super
  end
end

class C
  prepend M
  def greet
    "Hello from C"
  end
end

puts C.new.greet

medium

A. "Hello from C"

B. "Hello from M, Hello from C"

C. NoMethodError

D. "Hello from M"

Solution

1. Step 1: Understand method lookup with prepend

   Because M is prepended, its greet method is called first.

2. Step 2: Follow the super call in module method

   The super inside M#greet calls the original greet in C, returning "Hello from C".

3. Step 3: Combine returned strings

   The module method returns "Hello from M, " + "Hello from C" resulting in "Hello from M, Hello from C".

4. Final Answer:

   "Hello from M, Hello from C" -> Option B

5. Quick Check:

   Prepend + super calls original method [OK]

Hint: Prepend runs module method first; super calls original method [OK]

Common Mistakes:

• Ignoring super call and expecting only module output
• Thinking prepend replaces original method without super
• Expecting NoMethodError due to super

4. Identify the error in this Ruby code using prepend:

module M
  def hello
    super + " world"
  end
end

class C
  prepend M
end

puts C.new.hello

medium

A. TypeError because prepend is used incorrectly

B. No error; outputs " world"

C. SyntaxError due to missing method in class

D. NoMethodError because super has no method to call

Solution

1. Step 1: Check method definitions

   The module M defines hello calling super, but class C has no hello method.

2. Step 2: Understand super call behavior

   Since C has no hello, super has no method to call, causing a NoMethodError.

3. Final Answer:

   NoMethodError because super has no method to call -> Option D

4. Quick Check:

   Super needs original method; missing causes NoMethodError [OK]

Hint: Super must call existing method; else NoMethodError [OK]

Common Mistakes:

• Assuming super works without original method
• Thinking prepend fixes missing methods automatically
• Confusing syntax error with runtime error

5. Given this code, what will be the output?

module A
  def info
    "A" + super
  end
end

module B
  def info
    "B" + super
  end
end

class Base
  def info
    "Base"
  end
end

class MyClass < Base
  prepend A
  prepend B
end

puts MyClass.new.info

hard

A. "BABase"

B. "BaseAB"

C. "ABBase"

D. "BASuperBase"

Solution

1. Step 1: Understand multiple prepends order

   Modules prepended last are checked first, so B is before A in lookup.

2. Step 2: Trace method calls

   B#info runs first, returns "B" + super, which calls A#info, returning "A" + super, which calls Base#info returning "Base".

3. Step 3: Combine results

   Concatenate strings: "B" + "A" + "Base" = "BABase".

4. Final Answer:

   "BABase" -> Option A

5. Quick Check:

   Last prepend runs first; super chains down [OK]

Hint: Last prepended module runs first in method chain [OK]

Common Mistakes:

• Assuming first prepended module runs first
• Ignoring method lookup order with multiple prepends
• Confusing prepend with include order