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Layout of a Leo Program

A Leo program contains declarations of Imports, Functions, Circuits, and Tests. Ordering is not enforced. However, it is best practice to declare imports at the top of the file and tests at the bottom. Declarations are locally accessible within a program file. If you need a declaration from another program file, you must import it.


Functions contain statements that can compute values. Functions must be in a program's current scope to be called.

function square(a: u32) -> u32 {
return a * a;


Circuits are similar to structs in object-oriented languages. They can contain members that store values or declare functions.

circuit Point {
x: u32,
y: u32,
function sum(self) -> u32 {
return self.x + self.y;


Imports fetch other circuits and functions and bring them into the current file scope. You can import dependencies that are declared locally in the src directory or downloaded to the imports directory.

import math.square; // Import the function `square` from a package `math`.
function main() {
let a: u32 = square(5u32);


Each test function generates new constraints for an isolated test circuit. The input to a test can be specified with the test annotation. Tests are executed with the Leo test command.

function square(a: u32) -> u32 {
return a * a;
function test_square() {
let expected: u32 = 25;
let actual = square(5u32);
console.assert(expected == actual);

Leo Binaries#

The main.leo file in a Leo project is similar to a binary executable created in languages such as Rust. Leo CLI commands use main.leo as a starting point to pass in witness input values and generate proofs to produce a result.