Why Leo?

Leo is a Rust-inspired, statically-typed language designed for zero-knowledge (ZK) smart contracts on Aleo. It combines:

• Privacy by default (zk-SNARKs built-in)

• High performance (off-chain execution)

• Developer-friendly syntax (no need to manually write circuits)

If you're serious about ZK development, Leo is a must-learn. Here’s how to master it.

How to install Leo and standard project layout you can find in the previous article:

Now let’s go further.

This article is a guide to programming in the Leo language. For what? For creating private smart contracts using zk-SNARKs.

Here I explain the setup of the development environment, the basics of syntax (data types, structures, functions). Also I provide practical code examples, including a voting system and a balance check.

We will talk abt finding and correcting common errors, such as type inconsistencies and incorrect statements too. The article concludes with tips on code optimization and testing, as well as useful resources for further study.

Let’s go.

Core Leo Concepts (With Code)

Data Types & Structures

Leo supports:

• Primitives: u8, u32, bool, field, group

• Structs: For complex data

• Arrays: Fixed-size collections

Example: A Private Voting System

// Define a voter
struct Voter {
    address: address,
    voted: bool,
    weight: u32  // Voting power
}

// Store votes
mapping votes: address => u32;

// Cast a private vote
transition cast_vote(
    private voter: Voter,
    private candidate_id: u32
) -> bool {
    // Check voter hasn't voted yet
    assert(!voter.voted);

    // Update vote count
    votes[candidate_id] += voter.weight;

    // Mark as voted
    let updated_voter = Voter {
        voted: true,
        ..voter
    };

    return true;
}

Functions & ZK Logic

Key Rules:

1. Use private for sensitive data.

2. Public functions act as interfaces.

Example: Private Balance Check

function private check_balance(
    private balance: u64,
    private amount: u64
) -> bool {
    return balance >= amount;
}

// Public wrapper
function can_withdraw(
    public user: address,
    private balance: u64,
    private amount: u64
) -> bool {
    return check_balance(balance, amount);
}

Debugging Leo Programs

Common Bugs & Fixes

Bug #1: Type Mismatch

let x: u32 = 10;
let y: field = 0x123field;
let z = x + y;  // ERROR: Can't mix u32 and field

Fix: Explicitly convert types:

let z = x as field + y;

Bug #2: Unconstrained Assertions

assert(input > 0);  // Danger! No ZK proof if false

Fix: Handle errors gracefully:

if (input <= 0) {
    return false;
}

Bug #3: Infinite Loops

Leo doesn’t support recursion. This won’t compile:

function factorial(n: u32) -> u32 {
    if (n == 0) { return 1; }
    return n * factorial(n - 1);  // ERROR
}

Fix: Use iterative loops:

function factorial(n: u32) -> u32 {
    let result = 1u32;
    for i in 1u32..n {
        result *= i;
    }
    return result;
}

Advanced: Testing & Proving

Step 1: Write Tests

Add test cases in inputs/main.in:

[{
  "balance": 100u64,
  "amount": 50u64,
  "expected": true
}]

Step 2: Run Tests

leo run check_balance

Step 3: Generate Proofs

leo execute can_withdraw \
    --balance 100u64 \
    --amount 50u64

Optimization Tips

1. Minimize private computations (expensive in ZK).

2. Use inline functions for small, frequent ops.

3. Batch proofs where possible (e.g., process 10 votes at once).

So, Leo is the future of private smart contracts. To master it:

1. Code daily – Try Aleo’s challenges.

2. Join the community – Discord.

3. Audit your code ;)

Now go build something groundbreaking. 🚀

Need more? Check out:

• Leo Docs

• Example Contracts

  Links:

  Website ~ https://www.aleo.org/

  Twitter ~ https://twitter.com/AleoHQ

  Community Twitter ~ https://twitter.com/aleocommunity

  GitHub ~ https://github.com/AleoHQ

  Community Forum — https://community.aleo.org/

  Community Calendar ~ https://www.aleo.org/community/calendar

  YouTube — https://www.youtube.com/channel/UCS_HKT2heOC_q88YQLiJt0g

  Developer Documentation ~ https://developer.aleo.org/

  Leo Playground ~ https://play.leo-lang.org/

  Aleo Block Explorer ~ https://www.aleo.network/

  Community Blog ~ https://medium.com/@AleoHQ

  Announcements Blog ~ https://www.aleo.org/blog