Lesson 05-function

한국어(Korean) Page

The basic usage of functions is similar to JavaScript, but in Rust, fn is used instead of function.

fn main() {}

1. Parameters

fn main() {
    another_function(5);
}

fn another_function(x: i32) {
    println!("The value of x is: {}", x);
}

In Rust, it is mandatory to define the type of parameters. Failure to do so will result in an error, a point that JavaScript developers should note carefully. Type annotations are crucial for Rust’s strong typing system.

2. Return Values

In Rust, returning values differs from JavaScript. The return keyword can be omitted, and the semicolon (;) should also be removed. This creates an expression-based return.

fn five() -> i32 {
    5
}

fn plus_one(x: i32) -> i32 {
    x + 1
}

fn main() {
    let x = five();

    println!("The value of x is: {}", x);
}

In fn five() -> i32, i32 indicates the return type of the function. Defining return types enables compile-time checks for type consistency, enhancing code reliability.

3. Attributes

• Conditional Execution

Using #[cfg(condition)] allows a function to be executed only when the specified condition is met.

The example below is executed only on Windows; otherwise, an error occurs.

fn main() {
    on_windows();
}

#[cfg(target_os = "windows")]
fn on_windows() {
    println!("This OS is windows.")
}

// Result - when the OS is Windows:
// This OS is windows.

// Result - when the OS is Linux:
// |
// 2 |  on_windows();
// |  ^^^^^^^^^^ not found in this scope

• Testing

Used with #[test], this attribute tests whether a function can be executed.

You can test with rustc or cargo as follows:

rustc --test [filename]

Or:

cargo test

We will cover the #[...] syntax in more detail later. Attributes provide powerful mechanisms for conditional compilation and testing in Rust.