Defining modules

Define modules with mod.

Defining modules

Modules split code into named sections and form a tree. The root of the tree is the crate (main.rs or lib.rs):

mod matematik {
    pub fn kare(x: i32) -> i32 {
        x * x
    }

    pub mod istatistik {
        pub fn ortalama(v: &[f64]) -> f64 {
            v.iter().sum::<f64>() / v.len() as f64
        }
    }
}

// crate
//  └── matematik
//       ├── kare
//       └── istatistik
//            └── ortalama

Items are reached by path: matematik::kare(7), matematik::istatistik::ortalama(&v).

Why modules?

Modules do three jobs: they keep related code together, prevent name clashes (daire::alan and kare::alan can coexist) and draw a privacy boundary: only the API marked pub is visible outside, and internal details can change freely.

mod sayac {
    fn ic_yardimci() {}          // module only
    pub fn artir(n: u32) -> u32 { n + 1 }
}

Exercise First module

Define pub fn kare(x: i32) -> i32 inside mod matematik and print the result of matematik::kare(7) in main.

Show example solution

mod matematik {
    pub fn kare(x: i32) -> i32 {
        x * x
    }
}

fn main() {
    println!("{}", matematik::kare(7));
}

Test yourself

What happens if <code>main</code> accesses a non-<code>pub</code> function inside a module?
- It warns
- Runtime error
- E0603: the function is private
- It works
Show answer

E0603: the function is private — Privacy is the default; a parent can't see its child's private items. A child module can see its ancestors' items.

Open a module

____ matematik {
    pub fn kare(x: i32) -> i32 { x * x }
}

Show answer

mod matematik {
    pub fn kare(x: i32) -> i32 { x * x }
}

Path and target

matematik::kare

Function in the module
matematik::istatistik

Inner module
crate

Root of the tree
fn ic_yardimci (no pub)

Module only

Next lesson: use

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