Functions

You've already seen one function so far, the main function:

fn main() {
}

This is the simplest possible function declaration. As we mentioned before, fn says "this is a function," followed by the name, some parentheses because this function takes no arguments, and then some curly braces to indicate the body. Here's a function named foo:

fn foo() {
}

So, what about taking arguments? Here's a function that prints a number:

fn print_number(x: i32) {
    println!("x is: {}", x);
}

Here's a complete program that uses print_number:

fn main() {
    print_number(5);
}

fn print_number(x: i32) {
    println!("x is: {}", x);
}

As you can see, function arguments work very similar to let declarations: you add a type to the argument name, after a colon.

Here's a complete program that adds two numbers together and prints them:

fn main() {
    print_sum(5, 6);
}

fn print_sum(x: i32, y: i32) {
    println!("sum is: {}", x + y);
}

You separate arguments with a comma, both when you call the function, as well as when you declare it.

Unlike let, you must declare the types of function arguments. This does not work:

fn print_sum(x, y) {
    println!("sum is: {}", x + y);
}

You get this error:

hello.rs:5:18: 5:19 expected one of `!`, `:`, or `@`, found `)`
hello.rs:5 fn print_number(x, y) {

This is a deliberate design decision. While full-program inference is possible, languages which have it, like Haskell, often suggest that documenting your types explicitly is a best-practice. We agree that forcing functions to declare types while allowing for inference inside of function bodies is a wonderful sweet spot between full inference and no inference.

What about returning a value? Here's a function that adds one to an integer:

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

Rust functions return exactly one value, and you declare the type after an "arrow," which is a dash (-) followed by a greater-than sign (>).

You'll note the lack of a semicolon here. If we added it in:

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

We would get an error:

error: not all control paths return a value
fn add_one(x: i32) -> i32 {
     x + 1;
}

help: consider removing this semicolon:
     x + 1;
          ^

Remember our earlier discussions about semicolons and ()? Our function claims to return an i32, but with a semicolon, it would return () instead. Rust realizes this probably isn't what we want, and suggests removing the semicolon.

This is very much like our if statement before: the result of the block ({}) is the value of the expression. Other expression-oriented languages, such as Ruby, work like this, but it's a bit unusual in the systems programming world. When people first learn about this, they usually assume that it introduces bugs. But because Rust's type system is so strong, and because unit is its own unique type, we have never seen an issue where adding or removing a semicolon in a return position would cause a bug.

But what about early returns? Rust does have a keyword for that, return:

fn foo(x: i32) -> i32 {
    if x < 5 { return x; }

    x + 1
}

Using a return as the last line of a function works, but is considered poor style:

fn foo(x: i32) -> i32 {
    if x < 5 { return x; }

    return x + 1;
}

The previous definition without return may look a bit strange if you haven't worked in an expression-based language before, but it becomes intuitive over time. If this were production code, we wouldn't write it in that way anyway, we'd write this:

fn foo(x: i32) -> i32 {
    if x < 5 {
        x
    } else {
        x + 1
    }
}

Because if is an expression, and it's the only expression in this function, the value will be the result of the if.

Diverging functions

Rust has some special syntax for 'diverging functions', which are functions that do not return:

fn diverges() -> ! {
    panic!("This function never returns!");
}

panic! is a macro, similar to println!() that we've already seen. Unlike println!(), panic!() causes the current thread of execution to crash with the given message.

Because this function will cause a crash, it will never return, and so it has the type '!', which is read "diverges." A diverging function can be used as any type:

let x: i32 = diverges();
let x: String = diverges();

We don't have a good use for diverging functions yet, because they're used in conjunction with other Rust features. But when you see -> ! later, you'll know what it's called.