Binary and library crate

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 rapor

Using 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.

Exercise 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}.

Show example 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));
}

Test yourself

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
- Compilation always gets faster
- main.rs isn't required
- No difference
Show answer

The logic becomes testable and reusable in other programs — Integration tests can only reach the library crate; main.rs should stay a thin shell.

Library root

fatura/
└── src/
    ├── ____    // library crate
    └── main.rs   // binary crate

Show answer

fatura/
└── src/
    ├── lib.rs    // library crate
    └── main.rs   // binary crate

File and role

src/main.rs

Main binary
src/lib.rs

Library
src/bin/rapor.rs

Extra binary
Cargo.toml

Package definition

Next lesson: Cargo.toml

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