Functions

A function is a named piece of code you can run again and again. You make one with fn.

Parameters

Inputs go in the round brackets. Every parameter needs a name and a type.

1fn show(n: i32) {2    println!("n = {n}");3}

When you call show(4), you must pass exactly one i32.

Return types

-> T after the brackets says what the function gives back.

1fn add(a: i32, b: i32) -> i32 {2    a + b3}

let x = add(2, 3); sets x to 5.

Expressions

In Rust, a block { } has a value: its last expression. That is why add needs no return.

1let y = {2    let x = 3;3    x + 14};  // y is 4

You can still use return to leave a function early: return x; gives back x right away.

The semicolon trap

A semicolon turns an expression into a statement. A statement has no value, so the block gives back () instead.

1fn add(a: i32, b: i32) -> i32 {2    a + b;   // error E03083}

The function promised an i32 but its body now gives (). Remove the semicolon on the last line to fix it.

The unit type

() is called the unit type. It means "nothing useful to give back". A function with no -> returns (). So fn hello() is the same as fn hello() -> ().

Remember

• Every parameter needs a type.
• -> T says what comes back.
• The last expression, with no semicolon, is the return value.
• An extra semicolon on that last line is error E0308.
• No -> means the function returns ().