Defining modules in Rust - Step-by-Step Execution

Concept Flow - Defining modules

Start main.rs

↓

Declare module with mod

↓

Rust compiler looks for module file or inline code

↓

Module code is compiled

↓

Use module items with use or fully qualified path

↓

Program runs using module's functions/variables

This flow shows how Rust finds and compiles modules declared with mod, then how the main code uses them.

Execution Sample

Rust

mod greetings {
    pub fn hello() {
        println!("Hello from module!");
    }
}

fn main() {
    greetings::hello();
}

Defines a module greetings with a public function hello, then calls it from main.

Execution Table

Step	Action	Code Evaluated	Result/Output
1	Declare module greetings	mod greetings { ... }	Module greetings is registered
2	Define public function hello inside greetings	pub fn hello() { println!(...) }	Function hello is available publicly
3	Start main function	fn main() { ... }	Program entry point
4	Call greetings::hello()	greetings::hello();	Prints: Hello from module!
5	Program ends	}	Program terminates normally

💡 Program ends after main finishes execution

Variable Tracker

Variable	Start	After greetings::hello() call	Final
No variables	N/A	N/A	N/A

Key Moments - 2 Insights

Why do we need to use pub before the function hello?

Without pub, the function hello is private to the module and cannot be called from main. See execution_table step 2 and 4 where hello is declared public and then called.

What does mod greetings mean in the code?

It tells Rust to create a module named greetings. The compiler looks for code inside the braces or in greetings.rs file. See execution_table step 1.

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 4?

A The program prints 'Hello from module!'

B The module greetings is declared

C The program ends

D The function hello is defined

💡 Hint

Check the 'Result/Output' column at step 4 in execution_table

At which step does the program start running main()?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Action' column for when main() starts in execution_table

If we remove pub from the function hello, what will happen when calling greetings::hello()?

A It will print the message anyway

B Runtime error when calling hello

C Compilation error because hello is private

D The program will ignore the call

💡 Hint

Refer to key_moments about pub keyword and execution_table step 4

Concept Snapshot

Defining modules in Rust:
- Use mod keyword to declare a module
- Module code can be inline or in separate file
- Use pub to make functions accessible outside module
- Call module items with module_name::item_name
- Modules help organize code into namespaces

Full Transcript

This example shows how to define a module in Rust using the mod keyword. The module greetings contains a public function hello. The main function calls greetings::hello() which prints a message. The pub keyword is important to allow access to the function from outside the module. The execution steps show module declaration, function definition, main start, function call, and program end. Variables are not used here. Common confusions include why pub is needed and what mod means. The quiz tests understanding of these steps and concepts.