Functions

Functions are prevalent in Cairo 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.

Cairo 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:

use debug::PrintTrait;

fn another_function() {
    'Another function.'.print();
}

fn main() {
    'Hello, world!'.print();
    another_function();
}

We define a function in Cairo 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 before the main function in the source code; we could have defined it after as well. Cairo 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 project with Scarb named functions to explore functions further. Place the another_function example in src/lib.cairo and run it. You should see the following output:

$ cairo-run src/lib.cairo
[DEBUG] Hello, world!                (raw: 5735816763073854953388147237921)
[DEBUG] Another function.            (raw: 22265147635379277118623944509513687592494)

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:

use debug::PrintTrait;

fn main() {
    another_function(5);
}

fn another_function(x: felt252) {
    x.print();
}

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

$ cairo-run src/lib.cairo
[DEBUG]                                 (raw: 5)

The declaration of another_function has one parameter named x. The type of x is specified as felt252. When we pass 5 in to another_function, the .print() function outputs 5 in the console.

In function signatures, you must declare the type of each parameter. This is a deliberate decision in Cairo’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:

use debug::PrintTrait;

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

fn another_function(x: felt252, y:felt252) {
    x.print();
    y.print();
}

This example creates a function named another_function with two parameters. The first parameter is named x and is an felt252. The second is named y and is type felt252 too. The function then prints the content of the felt x and then the content of the felt y.

Let’s try running this code. Replace the program currently in your functions project’s src/lib.cairo file with the preceding example and run it using cairo-run src/lib.cairo:

$ cairo-run src/lib.cairo
[DEBUG]                                 (raw: 5)
[DEBUG]                                 (raw: 6)

Because we called the function with 5 as the value for x and 6 as the value for y, 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 Cairo 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.

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.

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:

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

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

$ cairo-run src/lib.cairo
error: Missing token TerminalRParen.
 --> src/lib.cairo:2:14
    let x = (let y = 6);
             ^

error: Missing token TerminalSemicolon.
 --> src/lib.cairo:2:14
    let x = (let y = 6);
             ^

error: Missing token TerminalSemicolon.
 --> src/lib.cairo:2:14
    let x = (let y = 6);
                      ^

error: Skipped tokens. Expected: statement.
 --> src/lib.cairo:2:14
    let x = (let y = 6);

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 Cairo.

Expressions evaluate to a value and make up most of the rest of the code that you’ll write in Cairo. 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. A new scope block created with curly brackets is an expression, for example:

use debug::PrintTrait;
fn main() {
    let y = {
        let x = 3;
        x + 1
    };

    y.print();
}

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 Cairo, 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:

use debug::PrintTrait;

fn five() -> u32 {
    5_u32
}

fn main() {
    let x = five();
    x.print();
}

There are no function calls, or even let statements in the five function—just the number 5 by itself. That’s a perfectly valid function in Cairo. Note that the function’s return type is specified too, as -> u32. Try running this code; the output should look like this:

$ cairo-run src/lib.cairo
[DEBUG]                                 (raw: 5)

The 5 in five is the function’s return value, which is why the return type is u32. 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:

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:

use debug::PrintTrait;

fn main() {
    let x = plus_one(5_u32);

    x.print();
}

fn plus_one(x: u32) -> u32 {
    x + 1_u32
}

Running this code will print [DEBUG] (raw: 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:

use debug::PrintTrait;

fn main() {
    let x = plus_one(5_u32);

    x.print();
}

fn plus_one(x: u32) -> u32 {
    x + 1_u32;
}

Compiling this code produces an error, as follows:

error: Unexpected return type. Expected: "core::integer::u32", found: "()".

The main error message, Unexpected return type, reveals the core issue with this code. The definition of the function plus_one says that it will return an u32, 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.