02_functions.md

Functions

Declared using fn and arguments, return type by ->.

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

Functions can be used before they are declared:

fn add_two(x: i32) -> i32 {
    add_one(add_one(x))
}

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

Return

Like in regular blocks, the last expression in a function block is the return value:

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

// is the same as
fn add_one_return(x: i32) -> i32 {
    return x + 1;
}

Main

Function main in src/main.rs is the application entrypoint.

It returns either ():

// src/main.rs
fn main() {
    println!("kek");
}

Or Result<(), E>:

use std::num::ParseIntError;

fn main() -> Result<(), ParseIntError> {
    let number_str = "10";
    let number = match number_str.parse::<i32>() {
        Ok(number) => number,
        Err(e) => return Err(e),
    };
    println!("{}", number);
    Ok(())
}

Pointers

Functions can be passed around as pointers:

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

fn do_twice(f: fn(i32) -> i32, arg: i32) -> i32 {
    f(arg) + f(arg)
}

fn add_two(x: i32) -> i32 {
    do_twice(add_one, x)
}

Const functions

Functions that only operate with arguments and operations performable at compile-time can be marked as const fn and used in such contexts:

const X: i32 = 1337;
const X_TWO: i32 = double(X); // 2674

const fn double(x: i32) -> i32 {
    x * 2
}