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How Functions Work

Functions are pervasive 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 snake case, 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.");
}

Function definitions in Rust start with fn and have a set of parentheses after the function name. The curly braces 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.

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)
     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.

Function Parameters

Functions can also be defined to have parameters, which are special variables that are part of a function's signature. When a function has parameters, we 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.

The following rewritten version of another_function shows what parameters look like in Rust:

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)
     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 5 is passed to another_function, the println! macro puts 5 where the pair of curly braces were 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 you mean.

When you want a function to have multiple parameters, separate the parameter declarations with commas, like this:

Filename: src/main.rs

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

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

This example creates a function with two parameters, both of which are i32 types. The function then prints out the values in both of its parameters. Note that function parameters don't all need to be the same type, they just happen to be in this example.

Let’s try running this code. Replace the program currently in your function 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)
     Running `target/debug/functions`
The value of x is: 5
The value of y is: 6

Because we called the function with 5 as the value for x and 6 is passed as the value for y, the two strings are printed with these values.

Function Bodies

Function bodies are made up of a series of statements optionally ending in an expression. So far, we’ve only covered functions without an ending expression, but we have seen expressions as parts of statements. 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 and Expressions

We’ve actually already used statements and expressions. Statements are instructions that perform some action and do not return a value. Expressions evaluate to a resulting value. Let’s look at some examples.

Creating a variable and assigning a value to it with the let keyword is a statement. In Listing 3-3, let y = 6; is a statement:

Filename: src/main.rs

fn main() {
    let y = 6;
}

Listing 3-3: A main function declaration containing one statement.

Function definitions are also statements; the entire preceding example is a statement in itself.

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

Filename: src/main.rs

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

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

$ cargo run
   Compiling functions v0.1.0 (file:///projects/functions)
error: expected expression, found statement (`let`)
 --> src/main.rs:2:14
  |
2 |     let x = (let y = 6);
  |              ^^^
  |
  = note: variable declaration using `let` is a statement

The let y = 6 statement does not return a value, so there isn’t anything for x to bind to. This is different than 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 something and make up most of the rest of the code that you’ll write in Rust. Consider a simple math operation, such as 5 + 6, which is an expression that evaluates to the value 11. Expressions can be part of statements: in Listing 3-3 that had the statement let y = 6;, 6 is an expression that evaluates to the value 6. Calling a function is an expression. Calling a macro is an expression. The block that we use to create new scopes, {}, is an expression, for example:

Filename: src/main.rs

fn main() {
    let x = 5;

    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 the line without a semicolon at the end, 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, which 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 do 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. 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)
     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_variables)]
#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. What happens if we place a semicolon at the end of the line containing x + 1, changing it from an expression to a statement?

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 produces an error, as follows:

error[E0308]: mismatched types
 --> src/main.rs:7:28
  |
7 |   fn plus_one(x: i32) -> i32 {
  |  ____________________________^
8 | |     x + 1;
9 | | }
  | |_^ expected i32, found ()
  |
  = note: expected type `i32`
             found type `()`
help: consider removing this semicolon:
 --> src/main.rs:8:10
  |
8 |     x + 1;
  |          ^

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 empty tuple. 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.