Functions, parameters and returned values

A school might apply the same bonus rule to many marks. A function gives that operation a name, accepts inputs through parameters and returns a result. A call supplies arguments. Separating the rule from printing lets you test the calculation independently.

Rust function parameters need declared types. -> i32 declares the returned type. A final expression without a semicolon supplies the result; a statement with a semicolon normally evaluates to unit, written (). You can also use return for an early exit.

Connect this idea with references and read-only borrowing.

Save the complete source as main.rs in the Rust environment. No standard input is needed.

Rust 2024 · complete program

fn add_bonus(score: i32, bonus: i32) -> i32 {
  score + bonus
}
fn main() {
  let adjusted = add_bonus(72, 8);
  println!("{adjusted}");
}

Expected output

80

Follow the execution

The arguments 72 and 8 bind to score and bonus. Their sum is 80, which the final expression returns. Main stores the result and prints it. Nothing inside add_bonus prints or changes an external mark.

Try the complete program in the Rust compiler. For tests, Cargo commands and local files, use the specified local environment.

Your task: What happens if you add a semicolon after score + bonus without adding a return?

Rust 2024 · complete program

fn add_bonus(score: i32, bonus: i32) -> i32 {
  score + bonus
}
fn main() {
  let adjusted = add_bonus(72, 8);
  println!("{adjusted}");
}

Worked answer

The function body no longer supplies the required i32 result. Compilation fails because the declared return type and the body result disagree.

Rust does not provide ordinary default or named arguments for functions. A returned value and a side effect are different outcomes. Introduce a helper when it names a useful operation, rather than creating tiny functions that hide straightforward logic.

The function body no longer supplies the required i32 result. Compilation fails because the declared return type and the body result disagree. Rust does not provide ordinary default or named arguments for functions.

Further reading: the official Rust Book on functions, parameters and returned values.