File modules
- Rust course
- Intermediate
- Lesson 74/115
- 4 min
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.
Practice 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.
Starter code
fn main() {
// buraya yaz
}Sample solution
mod ag {
pub mod istemci {
pub fn baglan() -> &'static str {
"bağlandı"
}
}
pub use istemci::baglan;
}
fn main() {
println!("{} {}", ag::baglan(), ag::istemci::baglan());
}Expected output
bağlandı bağlandı
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 Correct answer
Explanation: In Rust files aren't found automatically; every module must be declared with mod in its parent.
2 Re-export
pub mod istemci;
____ istemci::baglan;pub mod istemci;
pub use istemci::baglan;Explanation: pub use adds the item to the API of the module it's in.
3 File and module path
| Term | Meaning |
|---|---|
src/main.rs |
crate |
src/ag.rs |
crate::ag |
src/ag/istemci.rs |
crate::ag::istemci |
src/ag/mod.rs |
crate::ag (old layout) |