Day 21: Modules

Modules help organize code into logical units with controlled visibility.

Defining Modules

mod garden {
    pub mod vegetables {
        pub fn plant() {
            println!("Planting vegetables!");
        }
    }
    
    fn private_helper() {
        // Only accessible within garden module
    }
}

Visibility Rules

Keyword	Visibility
(none)	Private to current module
`pub`	Public
`pub(crate)`	Public within crate
`pub(super)`	Public to parent module

In Rust, everything is private by default! You must explicitly mark items as pub to make them accessible.

Using Items

// Absolute path
crate::garden::vegetables::plant();

// Relative path
garden::vegetables::plant();

// With use statement
use garden::vegetables::plant;
plant();

The super Keyword

Access parent module:

mod parent {
    pub fn helper() -> i32 { 42 }
    
    mod child {
        pub fn use_parent() -> i32 {
            super::helper()  // Access parent's function
        }
    }
}

File-Based Modules

src/
├── main.rs
├── garden.rs           // mod garden
└── garden/
    └── vegetables.rs   // mod garden::vegetables

The Task

Fix the module visibility:

Make math::add public
Make math::advanced::multiply_and_add public
Use super::add in multiply_and_add

Requirements

math::add(5, 3) → 8
math::advanced::multiply_and_add(2, 3, 4) → 10 (2*3 + 4)

Hints

mod math {
    pub fn add(a: i32, b: i32) -> i32 {
        a + b
    }
    
    pub mod advanced {
        pub fn multiply_and_add(a: i32, b: i32, c: i32) -> i32 {
            super::add(a * b, c)
        }
    }
}