Return value

Specify the return type with the -> arrow.

Return values

The return type is written with ->. The body's last expression (without a semicolon) is the return value; there's no need to write return:

fn kare(n: i32) -> i32 {
    n * n          // no semicolon → value returned
}

fn mutlak(n: i32) -> i32 {
    if n < 0 { -n } else { n }
}

Accidentally putting ; on the last line turns it into a statement and the function returns (); the compiler reports "mismatched types".

if is an expression

In Rust, if, match and blocks produce values; that's why no ternary operator (?:) is needed:

let yas = 20;
let durum = if yas >= 18 { "yetişkin" } else { "çocuk" };
let kdvli = {
    let fiyat = 100.0;
    fiyat * 1.2
};
println!("{durum} {kdvli}");

The if-else arms must have the same type.

Exercise Absolute value

Write the fn mutlak(n: i32) -> i32 function with an if expression and without writing return. Print mutlak(-7) and mutlak(4) on one line.

Show example solution

fn mutlak(n: i32) -> i32 {
    if n < 0 { -n } else { n }
}

fn main() {
    println!("{} {}", mutlak(-7), mutlak(4));
}

Test yourself

Why doesn't <code>fn f() -> i32 { 5; }</code> compile?
- 5 isn't an i32
- return is missing and required
- It compiles
- 5; is a statement; the function returns () but i32 is expected
Show answer

5; is a statement; the function returns () but i32 is expected — The semicolon turns the expression into a statement and discards its value; writing just 5 (or return 5;) compiles.

Return type

fn ortalama(a: f64, b: f64) ____ f64 {
    (a + b) / 2.0
}

Show answer

fn ortalama(a: f64, b: f64) -> f64 {
    (a + b) / 2.0
}

Body and return

{ n * n }

Returns n * n
{ n * n; }

Returns ()
if a { 1 } else { 2 }

Expression
return x;

Explicit return

Next lesson: Expressions and statements

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