Types and functions: a unit converter

Every value in Rust has a type. The common ones:

• i32: whole numbers, such as 42 or -7
• f64: numbers with a decimal point, such as 9.5
• bool: true or false
• &str and String: text

A function lists the type of each input, and -> shows the type it gives back. The last line, written without a semicolon, is the answer:

fn add(a: i32, b: i32) -> i32 {
    a + b
}

fn main() {
    println!("{}", add(2, 3));
}

Here is a converter, as engineers use every day. {:.2} shows 2 decimal places:

fn mm_to_inches(mm: f64) -> f64 {
    mm / 25.4
}

fn main() {
    println!("{:.2}", mm_to_inches(100.0));
}

Rust will not quietly mix types. Adding a whole number to a decimal is refused until you convert one of them, which stops a whole class of rounding bugs:

fn main() {
    let a: i32 = 5;
    let b: f64 = 2.5;
    println!("{}", a + b);
}

Rust refuses to build this.