File modules

Learn the file-based module hierarchy.

File modules

In large projects modules are put in separate files. A mod ag; declaration (without a body) tells the compiler to read the content from a file:

src/
├── main.rs        // mod ag;
├── ag.rs          // pub mod istemci;
└── ag/
    └── istemci.rs // pub fn baglan() { .. }

The older layout ag/mod.rs is still valid; the two can't be used at the same time. The file system structure mirrors the module tree exactly.

Re-exporting with pub use

Even when the internal structure is deep, users can be offered short paths. pub use re-exports an item in a parent module:

// src/ag.rs
pub mod istemci;
pub use istemci::baglan;   // also reachable as ag::baglan

// main.rs
mod ag;
fn main() {
    ag::baglan();
    ag::istemci::baglan();   // both work
}

Libraries use this to keep the public API stable even when the internal file layout changes.

Exercise Re-exporting

Imitate the file layout with inline modules: pub mod istemci inside mod ag, and inside it pub fn baglan() -> &'static str returning "bağlandı". Write pub use istemci::baglan; inside ag. Print the results of ag::baglan() and ag::istemci::baglan() separated by a space.

Show example solution

mod ag {
    pub mod istemci {
        pub fn baglan() -> &'static str {
            "bağlandı"
        }
    }
    pub use istemci::baglan;
}

fn main() {
    println!("{} {}", ag::baglan(), ag::istemci::baglan());
}

Test yourself

If <code>src/ag.rs</code> exists but <code>main.rs</code> doesn't declare <code>mod ag;</code>, what happens?
- It's included automatically
- Compile error
- It warns
- The file isn't compiled at all; it isn't part of the module tree
Show answer

The file isn't compiled at all; it isn't part of the module tree — In Rust files aren't found automatically; every module must be declared with mod in its parent.

Re-export

pub mod istemci;
____ istemci::baglan;

Show answer

pub mod istemci;
pub use istemci::baglan;

File and module path

src/main.rs

crate
src/ag.rs

crate::ag
src/ag/istemci.rs

crate::ag::istemci
src/ag/mod.rs

crate::ag (old layout)

Next lesson: crate and super

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