Summary - Modules

Combine mod, use and pub to set up a multi-file crate.

Modules summary

mod builds a module tree; items are private by default and opened with pub (struct fields individually). use shortens paths, as renames, pub use re-exports. Paths can start with crate::, super:: or self::. Modules can go in separate files (ag.rs, ag/istemci.rs). Fine-grained visibility: pub(crate), pub(super).

A private inner module

Library authors put details into private inner modules and expose a small surface:

mod kutuphane {
    mod ic {
        pub fn gizli_hesap(x: i32) -> i32 {
            x * 3
        }
    }

    pub fn hesapla(x: i32) -> i32 {
        ic::gizli_hesap(x) + 1
    }
}

// kutuphane::ic::gizli_hesap(4);   // error: ic is private
println!("{}", kutuphane::hesapla(4));   // 13

Exercise Small surface

Write the kutuphane module above: gizli_hesap in the private ic module, and pub fn hesapla outside. Print the result of kutuphane::hesapla(4).

Show example solution

mod kutuphane {
    mod ic {
        pub fn gizli_hesap(x: i32) -> i32 {
            x * 3
        }
    }

    pub fn hesapla(x: i32) -> i32 {
        ic::gizli_hesap(x) + 1
    }
}

fn main() {
    println!("{}", kutuphane::hesapla(4));
}

Test yourself

How do you fully hide an inner module and expose just one function?
- It isn't possible
- Leave the module non-pub and wrap it with a pub fn above, or pub use it
- Make the module pub
- Use unsafe
Show answer

Leave the module non-pub and wrap it with a pub fn above, or pub use it — The pub items of a private module can only be used by code that can see that module; exposing them needs a door from the parent.

Crate-wide visibility

____(crate) fn yardimci() {}

Show answer

pub(crate) fn yardimci() {}

Group summary

mod x { }

Module
pub

Exposes
use a::b

Shortcut
pub use

Re-export

Next lesson: Binary and library crate

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