Prepend for method chain insertion in Ruby - Step-by-Step Execution

Concept Flow - Prepend for method chain insertion

Define original class

↓

Define module with new method

↓

Prepend module to class

↓

Create instance

↓

Call method

↓

Module method runs first

↓

Call super to original method

↓

Original method runs

↓

Combined output

This flow shows how prepending a module inserts its methods before the original class methods, allowing method chaining with super.

Execution Sample

Ruby

class Greeter
  def greet
    "Hello"
  end
end

module Enthusiastic
  def greet
    super + "!!!"
  end
end

Greeter.prepend(Enthusiastic)
puts Greeter.new.greet

This code prepends a module to add excitement to the greet method by chaining with super.

Execution Table

Step	Action	Method Called	Result	Output
1	Create Greeter class with greet method	N/A	greet returns "Hello"	N/A
2	Define Enthusiastic module with greet method	N/A	greet calls super + "!!!"	N/A
3	Prepend Enthusiastic to Greeter	N/A	Enthusiastic methods inserted before Greeter	N/A
4	Create Greeter instance	N/A	Instance ready	N/A
5	Call greet on instance	Enthusiastic#greet	Calls super	N/A
6	Inside Enthusiastic#greet calls super	Greeter#greet	Returns "Hello"	N/A
7	Enthusiastic#greet adds "!!!"	N/A	Returns "Hello!!!"	N/A
8	Print output	N/A	Output is "Hello!!!"	Hello!!!

💡 Method chain completes after Enthusiastic#greet returns combined string.

Variable Tracker

Variable	Start	After Step 4	After Step 7	Final
instance	nil	Greeter instance	Greeter instance	Greeter instance
method_call	none	none	super called inside greet	greet returns "Hello!!!"
output	none	none	none	"Hello!!!"

Key Moments - 3 Insights

Why does the module's greet method run before the class's greet method?

Because prepending inserts the module before the class in the method lookup chain, so the module's method is found first (see execution_table step 5).

What does the super keyword do inside the module's greet method?

It calls the original greet method from the class that was prepended, allowing method chaining (see execution_table step 6).

What would happen if we didn't call super in the module's greet method?

The original greet method would not run, so the output would only be what the module returns, missing the original "Hello" (not shown in table but implied).

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the original Greeter#greet method called?

A Step 7

B Step 5

C Step 6

D Step 8

💡 Hint

Check the 'Method Called' column for when Greeter#greet runs.

According to variable_tracker, what is the output after step 7?

A nil

B No output yet

C "Hello"

D "Hello!!!"

💡 Hint

Look at the 'output' variable value after step 7.

If we remove the super call in Enthusiastic#greet, what changes in the output?

A Output becomes "!!!"

B Output becomes "Hello"

C Output becomes "Hello!!!"

D Output is empty

💡 Hint

Recall that super calls the original method; without it, only the module's addition remains.

Concept Snapshot

Prepend inserts a module before a class in method lookup.
Module methods run first, can call super to chain.
Use prepend to add behavior before original methods.
Example: module with greet calls super + "!!!".
Prepending modifies method chain order.

Full Transcript

This visual trace shows how Ruby's prepend works to insert a module's methods before a class's methods. First, the Greeter class defines greet returning "Hello". Then, the Enthusiastic module defines greet calling super plus "!!!". Prepending Enthusiastic to Greeter changes the method lookup order so Enthusiastic#greet runs first. When greet is called on a Greeter instance, Enthusiastic#greet runs, calls super which runs Greeter#greet returning "Hello", then adds "!!!" to produce "Hello!!!". The output is printed. Variables track the instance creation, method calls, and final output. Key moments clarify why the module method runs first and the role of super. Quizzes test understanding of method call order and output changes. The snapshot summarizes prepend usage for method chain insertion.