Defining modules in Rust - Mini Project: Build & Apply

Defining modules

📖 Scenario: You are organizing a small Rust program that manages book information. To keep your code clean and easy to understand, you want to split it into modules.

🎯 Goal: You will create a module named library that contains a function to display a book's title. Then you will call this function from the main program.

📋 What You'll Learn

Create a module named library

Inside library, define a public function print_book_title that prints the book title

Call library::print_book_title from the main function

💡 Why This Matters

🌍 Real World

Modules help organize large programs by grouping related code together, making it easier to maintain and understand.

💼 Career

Understanding modules is essential for writing clean, scalable Rust code in professional software development.

1

Create the library module

Create a module named library using mod library { } syntax.

Rust

# Your code here

Hint

Use mod library { } to define the module.

2

Add a public function print_book_title inside library

Inside the library module, define a public function named print_book_title that prints "Book title: The Rust Book".

Rust

mod library {
    // Your code here
}

fn main() {
}

Hint

Use pub fn print_book_title() { println!("Book title: The Rust Book"); } inside the module.

3

Call print_book_title from main

In the main function, call the function print_book_title from the library module using library::print_book_title().

Rust

mod library {
    pub fn print_book_title() {
        println!("Book title: The Rust Book");
    }
}

fn main() {
    // Your code here
}

Hint

Call the function with library::print_book_title(); inside main.

4

Print the output

Run the program and print the output from calling library::print_book_title().

Rust

mod library {
    pub fn print_book_title() {
        println!("Book title: The Rust Book");
    }
}

fn main() {
    library::print_book_title();
    // Your code here
}

Hint

The program should print exactly Book title: The Rust Book.

Practice

1. What keyword do you use in Rust to define a new module inside a file?

easy

A. module

B. use

C. mod

D. pub

Solution

1. Step 1: Understand Rust module declaration

   Rust uses the keyword mod to declare a module inside a file.

2. Step 2: Differentiate from other keywords

   use imports modules, pub makes items public, and module is not a Rust keyword.

3. Final Answer:

   mod -> Option C

4. Quick Check:

   Keyword for defining modules = mod [OK]

Hint: Remember: 'mod' means module in Rust [OK]

Common Mistakes:

• Using 'module' instead of 'mod'
• Confusing 'use' with 'mod'
• Forgetting 'mod' keyword

2. Which of the following is the correct syntax to declare a module named network in Rust?

easy

A. use network {}

B. module network {}

C. pub mod network() {}

D. mod network {}

Solution

1. Step 1: Recall correct module syntax

   The correct syntax to declare a module is mod name { } without parentheses.

2. Step 2: Check each option

   mod network {} uses mod network {} which is correct. module network {} uses 'module' which is invalid. pub mod network() {} adds parentheses which are not allowed. use network {} uses 'use' which is for importing.

3. Final Answer:

   mod network {} -> Option D

4. Quick Check:

   Module declaration = mod name {} [OK]

Hint: Modules use curly braces, no parentheses [OK]

Common Mistakes:

• Adding parentheses after module name
• Using 'module' instead of 'mod'
• Confusing 'use' with 'mod'

3. What will be the output of this Rust code?

mod math {
    pub fn add(a: i32, b: i32) -> i32 {
        a + b
    }
}

fn main() {
    println!("{}", math::add(3, 4));
}

medium

A. 7

B. Compilation error

C. math::add(3, 4)

D. 0

Solution

1. Step 1: Check module and function visibility

   The function add is marked pub, so it can be accessed outside the module.

2. Step 2: Evaluate the function call

   Calling math::add(3, 4) returns 3 + 4 = 7, which is printed.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   Public function call returns sum = 7 [OK]

Hint: Public functions in modules can be called with module::function [OK]

Common Mistakes:

• Forgetting 'pub' causes compilation error
• Expecting function name printed
• Confusing output with zero

4. Identify the error in this Rust code:

mod utils {
    fn helper() {
        println!("Helper function");
    }
}

fn main() {
    utils::helper();
}

medium

A. Function 'helper' is private and cannot be accessed outside the module

B. Missing semicolon after println!

C. Module 'utils' is not declared correctly

D. Cannot call functions inside modules

Solution

1. Step 1: Check function visibility

   The function helper is not marked pub, so it is private to the module.

2. Step 2: Understand access from main

   Trying to call utils::helper() from outside causes a compilation error due to privacy.

3. Final Answer:

   Function 'helper' is private and cannot be accessed outside the module -> Option A

4. Quick Check:

   Private functions can't be called outside module [OK]

Hint: Add 'pub' to functions to access them outside modules [OK]

Common Mistakes:

• Assuming all functions are public by default
• Ignoring privacy rules
• Thinking modules block all calls

5. You want to organize your Rust project by splitting code into two files: main.rs and network.rs. How do you correctly declare and use the network module in main.rs?

hard

A. Write pub mod network {} in main.rs and put code inside network.rs

B. Write mod network; in main.rs and make functions in network.rs public with pub

C. Write use network; in main.rs and declare mod network {} inside network.rs

D. Write extern crate network; in main.rs and import functions manually

Solution

1. Step 1: Understand module file linking

   To link an external file as a module, use mod network; in main.rs. This tells Rust to load network.rs.

2. Step 2: Make functions public in network.rs

   Functions inside network.rs must be marked pub to be accessible from main.rs.

3. Final Answer:

   Write mod network; in main.rs and make functions in network.rs public with pub -> Option B

4. Quick Check:

   Use mod filename; and pub functions for multi-file modules [OK]

Hint: Use 'mod filename;' in main and 'pub' for access [OK]

Common Mistakes:

• Using 'use' instead of 'mod' to declare modules
• Declaring mod inside the module file
• Using extern crate for local modules