Functions - The Rust Programming Language

## Functions

Functions are prevalent in Rust code. You’ve already seen one of the most important functions in the language: the `main` function, which is the entry point of many programs. You’ve also seen the `fn` keyword, which allows you to declare new functions.

Rust code uses snake case as the conventional style for function and variable names, in which all letters are lowercase and underscores separate words. Here’s a program that contains an example function definition:

Filename: src/main.rs

    fn main() {
        println!("Hello, world!");

        another_function();
    }

    fn another_function() {
        println!("Another function.");
    }

We define a function in Rust by entering `fn` followed by a function name and a set of parentheses. The curly brackets tell the compiler where the function body begins and ends.

We can call any function we’ve defined by entering its name followed by a set of parentheses. Because `another_function` is defined in the program, it can be called from inside the `main` function. Note that we defined `another_function` after the `main` function in the source code; we could have defined it before as well. Rust doesn’t care where you define your functions, only that they’re defined somewhere in a scope that can be seen by the caller.

Let’s start a new binary project named functions to explore functions further. Place the `another_function` example in src/main.rs and run it. You should see the following output:

    $ cargo run
       Compiling functions v0.1.0 (file:///projects/functions)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.28s
         Running `target/debug/functions`
    Hello, world!
    Another function.

The lines execute in the order in which they appear in the `main` function. First the “Hello, world!” message prints, and then `another_function` is called and its message is printed.

### Parameters

We can define functions to have parameters, which are special variables that are part of a function’s signature. When a function has parameters, you can provide it with concrete values for those parameters. Technically, the concrete values are called arguments, but in casual conversation, people tend to use the words parameter and argument interchangeably for either the variables in a function’s definition or the concrete values passed in when you call a function.

In this version of `another_function` we add a parameter:

Filename: src/main.rs

    fn main() {
        another_function(5);
    }

    fn another_function(x: i32) {
        println!("The value of x is: {x}");
    }

Try running this program; you should get the following output:

    $ cargo run
       Compiling functions v0.1.0 (file:///projects/functions)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.21s
         Running `target/debug/functions`
    The value of x is: 5

The declaration of `another_function` has one parameter named `x`. The type of `x` is specified as `i32`. When we pass `5` in to `another_function`, the `println!` macro puts `5` where the pair of curly brackets containing `x` was in the format string.

In function signatures, you must declare the type of each parameter. This is a deliberate decision in Rust’s design: requiring type annotations in function definitions means the compiler almost never needs you to use them elsewhere in the code to figure out what type you mean. The compiler is also able to give more helpful error messages if it knows what types the function expects.

When defining multiple parameters, separate the parameter declarations with commas, like this:

Filename: src/main.rs

    fn main() {
        print_labeled_measurement(5, 'h');
    }

    fn print_labeled_measurement(value: i32, unit_label: char) {
        println!("The measurement is: {value}{unit_label}");
    }

This example creates a function named `print_labeled_measurement` with two parameters. The first parameter is named `value` and is an `i32`. The second is named `unit_label` and is type `char`. The function then prints text containing both the `value` and the `unit_label`.

Let’s try running this code. Replace the program currently in your functions project’s src/main.rs file with the preceding example and run it using `cargo run`:

    $ cargo run
       Compiling functions v0.1.0 (file:///projects/functions)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.31s
         Running `target/debug/functions`
    The measurement is: 5h

Because we called the function with `5` as the value for `value` and `'h'` as the value for `unit_label`, the program output contains those values.

### Statements and Expressions

Function bodies are made up of a series of statements optionally ending in an expression. So far, the functions we’ve covered haven’t included an ending expression, but you have seen an expression as part of a statement. Because Rust is an expression-based language, this is an important distinction to understand. Other languages don’t have the same distinctions, so let’s look at what statements and expressions are and how their differences affect the bodies of functions.

- Statements are instructions that perform some action and do not return a value.
- Expressions evaluate to a resultant value. Let’s look at some examples.

We’ve actually already used statements and expressions. Creating a variable and assigning a value to it with the `let` keyword is a statement. In Listing 3-1, `let y = 6;` is a statement.

Filename: src/main.rs

    fn main() {
        let y = 6;
    }

Listing 3-1: A `main` function declaration containing one statement

Function definitions are also statements; the entire preceding example is a statement in itself. (As we will see below, calling a function is not a statement.)

Statements do not return values. Therefore, you can’t assign a `let` statement to another variable, as the following code tries to do; you’ll get an error:

Filename: src/main.rs

    fn main() {
        let x = (let y = 6);
    }

When you run this program, the error you’ll get looks like this:

    $ cargo run
       Compiling functions v0.1.0 (file:///projects/functions)
    error: expected expression, found `let` statement
     --> src/main.rs:2:14
      |
    2 |     let x = (let y = 6);
      |              ^^^
      |
      = note: only supported directly in conditions of `if` and `while` expressions

    warning: unnecessary parentheses around assigned value
     --> src/main.rs:2:13
      |
    2 |     let x = (let y = 6);
      |             ^         ^
      |
      = note: `#[warn(unused_parens)]` on by default
    help: remove these parentheses
      |
    2 -     let x = (let y = 6);
    2 +     let x = let y = 6;
      |

    warning: `functions` (bin "functions") generated 1 warning
    error: could not compile `functions` (bin "functions") due to 1 previous error; 1 warning emitted

The `let y = 6` statement does not return a value, so there isn’t anything for `x` to bind to. This is different from what happens in other languages, such as C and Ruby, where the assignment returns the value of the assignment. In those languages, you can write `x = y = 6` and have both `x` and `y` have the value `6`; that is not the case in Rust.

Expressions evaluate to a value and make up most of the rest of the code that you’ll write in Rust. Consider a math operation, such as `5 + 6`, which is an expression that evaluates to the value `11`. Expressions can be part of statements: in Listing 3-1, the `6` in the statement `let y = 6;` is an expression that evaluates to the value `6`. Calling a function is an expression. Calling a macro is an expression. A new scope block created with curly brackets is an expression, for example:

Filename: src/main.rs

    fn main() {
        let y = {
            let x = 3;
            x + 1
        };

        println!("The value of y is: {y}");
    }

This expression:

    {
        let x = 3;
        x + 1
    }

is a block that, in this case, evaluates to `4`. That value gets bound to `y` as part of the `let` statement. Note that the `x + 1` line doesn’t have a semicolon at the end, which is unlike most of the lines you’ve seen so far. Expressions do not include ending semicolons. If you add a semicolon to the end of an expression, you turn it into a statement, and it will then not return a value. Keep this in mind as you explore function return values and expressions next.

### Functions with Return Values

Functions can return values to the code that calls them. We don’t name return values, but we must declare their type after an arrow (`->`). In Rust, the return value of the function is synonymous with the value of the final expression in the block of the body of a function. You can return early from a function by using the `return` keyword and specifying a value, but most functions return the last expression implicitly. Here’s an example of a function that returns a value:

Filename: src/main.rs

    fn five() -> i32 {
        5
    }

    fn main() {
        let x = five();

        println!("The value of x is: {x}");
    }

There are no function calls, macros, or even `let` statements in the `five` function—just the number `5` by itself. That’s a perfectly valid function in Rust. Note that the function’s return type is specified too, as `-> i32`. Try running this code; the output should look like this:

    $ cargo run
       Compiling functions v0.1.0 (file:///projects/functions)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.30s
         Running `target/debug/functions`
    The value of x is: 5

The `5` in `five` is the function’s return value, which is why the return type is `i32`. Let’s examine this in more detail. There are two important bits: first, the line `let x = five();` shows that we’re using the return value of a function to initialize a variable. Because the function `five` returns a `5`, that line is the same as the following:

    #![allow(unused)]
    fn main() {
    let x = 5;
    }

Second, the `five` function has no parameters and defines the type of the return value, but the body of the function is a lonely `5` with no semicolon because it’s an expression whose value we want to return.

Let’s look at another example:

Filename: src/main.rs

    fn main() {
        let x = plus_one(5);

        println!("The value of x is: {x}");
    }

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

Running this code will print `The value of x is: 6`. But if we place a semicolon at the end of the line containing `x + 1`, changing it from an expression to a statement, we’ll get an error:

Filename: src/main.rs

    fn main() {
        let x = plus_one(5);

        println!("The value of x is: {x}");
    }

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

Compiling this code produces an error, as follows:

    $ cargo run
       Compiling functions v0.1.0 (file:///projects/functions)
    error[E0308]: mismatched types
     --> src/main.rs:7:24
      |
    7 | fn plus_one(x: i32) -> i32 {
      |    --------            ^^^ expected `i32`, found `()`
      |    |
      |    implicitly returns `()` as its body has no tail or `return` expression
    8 |     x + 1;
      |          - help: remove this semicolon to return this value

    For more information about this error, try `rustc --explain E0308`.
    error: could not compile `functions` (bin "functions") due to 1 previous error

The main error message, `mismatched types`, reveals the core issue with this code. The definition of the function `plus_one` says that it will return an `i32`, but statements don’t evaluate to a value, which is expressed by `()`, the unit type. Therefore, nothing is returned, which contradicts the function definition and results in an error. In this output, Rust provides a message to possibly help rectify this issue: it suggests removing the semicolon, which would fix the error.