Binary and library crate
- Rust course
- Intermediate
- Lesson 77/115
- 4 min
Learn the difference between binary and library crates.
Binary and library crates
A crate is the smallest unit the compiler compiles at a time. There are two kinds: a binary crate (src/main.rs, an executable program with a main function) and a library crate (src/lib.rs, code for others to use). A package is a unit defined by a Cargo.toml, containing at most one library and any number of binaries:
fatura/
├── Cargo.toml
└── src/
├── lib.rs // library: calculations
├── main.rs // main program: use fatura::kdv_ekle;
└── bin/
└── rapor.rs // extra binary: cargo run --bin raporUsing the library
A binary in the same package reaches the library by the package name:
// src/lib.rs
pub fn kdv_ekle(tutar: f64) -> f64 {
tutar * 1.20
}
// src/main.rs
use fatura::kdv_ekle;
fn main() {
println!("{:.2}", kdv_ekle(100.0));
}Since the exercise has no second file, we'll imitate the library with a mod.
Practice Library and program
Inside mod fatura, write pub fn kdv_ekle(tutar: f64) -> f64 (adds 20 percent). In main, bring it in with use fatura::kdv_ekle; and print the result of kdv_ekle(100.0) with {:.2}.
Starter code
fn main() {
// buraya yaz
}Sample solution
mod fatura {
pub fn kdv_ekle(tutar: f64) -> f64 {
tutar * 1.20
}
}
use fatura::kdv_ekle;
fn main() {
println!("{:.2}", kdv_ekle(100.0));
}Expected output
120.00
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What's the benefit of putting logic in <code>lib.rs</code> and input/output in <code>main.rs</code>?
- The logic becomes testable and reusable in other programs Correct answer
- Compilation always gets faster
- main.rs isn't required
- No difference
Explanation: Integration tests can only reach the library crate; main.rs should stay a thin shell.
2 Library root
fatura/
└── src/
├── ____ // library crate
└── main.rs // binary cratefatura/
└── src/
├── lib.rs // library crate
└── main.rs // binary crateExplanation: The library crate's root file is src/lib.rs.
3 File and role
| Term | Meaning |
|---|---|
src/main.rs |
Main binary |
src/lib.rs |
Library |
src/bin/rapor.rs |
Extra binary |
Cargo.toml |
Package definition |