Prepend for method chain insertion in Ruby

Introduction

Prepend lets you add a module's methods before others in a class. This helps change or add behavior without changing original code.

You want to add extra steps before existing methods run.

You need to fix or change behavior in a class without editing it directly.

You want to share common behavior across classes by inserting modules.

You want to keep original methods but add new features before them.

Syntax

Ruby

module YourModule
  def method_name
    # new behavior
    super  # calls original method
  end
end

class YourClass
  prepend YourModule
end

Use prepend inside the class to insert the module before the class in method lookup.

Calling super inside the module method runs the original method.

Examples

This example adds a greeting before the original hello method.

Ruby

module Greeting
  def hello
    puts "Hi!"  # new behavior
    super       # call original
  end
end

class Person
  def hello
    puts "Hello from Person"
  end
  prepend Greeting
end

Person.new.hello

Logger module prepends to add logging before saving.

Ruby

module Logger
  def save
    puts "Logging before save"
    super
  end
end

class Record
  def save
    puts "Saving record"
  end
  prepend Logger
end

Record.new.save

Sample Program

This program shows how prepend inserts Announcer before Speaker. The speak method in Announcer runs first, then calls the original speak.

Ruby

module Announcer
  def speak
    puts "Starting to speak..."
    super
  end
end

class Speaker
  def speak
    puts "Hello, world!"
  end
  prepend Announcer
end

Speaker.new.speak

Important Notes

Prepend changes the order Ruby looks for methods, putting the module first.

Use super in the module method to run the original method after your added code.

Prepend is useful for adding behavior without rewriting or copying code.

Summary

Prepend inserts a module before a class in method lookup.

It lets you add or change behavior by running module methods first.

Use super in the module to call the original method.

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