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A Flying Tour of Leo

The best way to get to know Leo is by writing some code. We will fly through a high level overview of a Leo file. To gain a deeper understanding of the Leo language, continue to the next page.

Square Example: Let's prove that we know the square of a number.


function main(a: u32, b: u32) -> bool {
return square(a, b);
function square(a: u32, b: u32) -> bool {
return a * a == b;
function test_square() {
let a: u32 = 5;
let b: u32 = 25;
let result = square(a, b);
console.assert(result == true);


// The program input for /src/main.leo
a: u32 = 5;
b: u32 = 25;
r: u32 = false;

1. Data Types#

Leo supports boolean, unsigned integer, signed integer, field, group, and address data types. Data types in Leo must be explicitly stated let a: u32 = 5; Collections of data types can be created in the form of static arrays and tuples.

2 Functions#

The main function is the entrypoint of a Leo program. leo run will provide private inputs directly to the function for proving and store the program result in an output file.

The square function is called by main with private inputs a and b which are both unsigned u32 integers.

3. Testing#

A naive way to test square would be to execute leo run several times on different inputs and check the output of the program each time.

Luckily, we can write unit tests in Leo using the @test syntax and run them using leo test. In test_square we can sanity check our code without having to load in private inputs from a file every time. Want to upgrade your unit test into an integration test? In Leo you can provide arguments that load different sets of private inputs to make your test suite even more robust.

The last line of test_square uses the console function console.assert. This function along with console.log, console.debug, and console.error provide developers with tools that are run without affecting the underlying constraint system.

4. Circuits#

Circuits Example


circuit Point {
x: u32,
y: u32,
function new() -> Self {
return Self {
x: 0,
y: 0,
function add(self) -> u32 {
return self.x + self.y;
function main() {
let p = Point::new();
p.x = 4u32;
p.y = 6u32;
let sum = p.add();
console.log("The sum is {}", sum);

Circuits in leo are similar to structures in other object-oriented languages. They provide a composite data type that can store any value and provide functions for instantiation and computation.

The new function is static and can be called without instantiating the circuit.

Leo introduces Self and self keywords to access circuit member variables and functions.

5. Imports#

Imports fetch other circuits and functions and bring them into the current file scope. Leo supports imports for dependencies that are declared locally or in an imported package. Importing packages can be accomplished using the leo add command in the CLI.

This completes the flying tour!

The Leo Language#

To learn more about how to use the Leo Language, start here

The Leo CLI#

To learn more about how to use the Leo CLI, start here