% Functions

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

```{rust}
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`:

```{rust}
fn foo() {
}
```

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

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

Here's a complete program that uses `print_number`:

```{rust}
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:

```{rust}
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:

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

You get this error:

```text
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:

```{rust}
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:

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

We would get an error:

```text
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`:

```{rust}
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:

```{rust}
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:

```rust
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`.

There are some additional ways to define functions, but they involve features
that we haven't learned about yet, so let's just leave it at that for now.
