Example Programs

Overview of example Leo programs demonstrating key language features

Overview

Leo includes several complete example programs that demonstrate real-world use cases and language features. These examples are actively maintained and tested in the Leo continuous integration pipeline.

Available Examples

Token

Transparent and shielded custom token with public and private transfers

Tic-Tac-Toe

Two-player game demonstrating structs and conditional logic

Lottery

Random winner selection using ChaCha RNG and block height checks

Example Categories

Financial Applications

The Token example demonstrates how to build financial primitives:

• Public and private balance tracking
• Minting and burning tokens
• Transfer functionality
• Mapping for on-chain state
• Record-based private state

Use Cases:

• Cryptocurrencies
• Stablecoins
• Reward tokens
• NFT implementations

Game Logic

The Tic-Tac-Toe example shows how to implement game rules:

• State representation with structs
• Turn-based logic
• Win condition checking
• Input validation
• Early termination

Use Cases:

• On-chain games
• Voting systems
• State machines
• Multi-party protocols

Randomness and Time-based Logic

The Lottery example demonstrates:

• Cryptographically secure randomness
• Block height-based expiration
• Winner selection
• Finalizer constraints

Use Cases:

• Lotteries
• Random selection
• Time-locked contracts
• Auctions

Key Language Features Demonstrated

Records

Private data structures used in Token and Lottery:

record Token {
    owner: address,
    amount: u64,
}

Learn more: Token Example

Structs

Composite data types used in Tic-Tac-Toe:

struct Board {
    r1: Row,
    r2: Row,
    r3: Row,
}

Learn more: Tic-Tac-Toe Example

Mappings

On-chain storage used in Token:

mapping account: address => u64;

Learn more: Token Example

Finalizers

On-chain computation used in all examples:

fn transfer_public(public receiver: address, public amount: u64) -> Final {
    return final { finalize_transfer(self.caller, receiver, amount); };
}

Learn more: Token Example

Assertions

Input validation and constraints:

assert(player == 1u8 || player == 2u8);
assert(block.height <= 1000u32);

Learn more: Lottery Example

Randomness

Cryptographically secure random number generation:

let is_winner: bool = ChaCha::rand_bool();

Learn more: Lottery Example

Running the Examples

All examples are located in the Leo repository at .circleci/:

# Clone the Leo repository
git clone https://github.com/ProvableHQ/leo
cd leo/.circleci/token

# Run the token example
leo build
leo run mint_public

Using the Leo CLI

Each example includes a run.sh script for easy execution:

./run.sh

This script demonstrates typical usage patterns and function calls.

Testing Examples

Examples are tested automatically in CI:

# Run example tests
cd leo
./.circleci/test-examples.sh

Example Structure

Each example follows a consistent structure:

example/
├── src/
│   └── main.leo          # Main program source
├── inputs/
│   └── example.in        # Input files for testing
├── README.md             # Example documentation
├── run.sh                # Demo script
└── program.json          # Project manifest

Building Your Own Application

Use these examples as starting points:

1. Choose a similar example: Pick the example closest to your use case
2. Copy the structure: Use leo new to create a new project
3. Adapt the code: Modify the example to fit your requirements
4. Test thoroughly: Write comprehensive tests
5. Deploy: Use leo deploy when ready

Example Complexity

Example	Lines of Code	Difficulty	Key Features
Lottery	~30	Beginner	Randomness, block height
Tic-Tac-Toe	~110	Intermediate	Structs, conditionals, helpers
Token	~130	Advanced	Records, mappings, finalizers

Learning Path

We recommend studying the examples in this order:

1. Lottery: Start here to learn basic finalizers and randomness
2. Tic-Tac-Toe: Progress to structs and complex logic
3. Token: Master records, mappings, and complete applications

Additional Resources

Leo Tutorial

Step-by-step guide to Leo programming

Language Reference

Complete syntax reference

Built-in Types

Type system documentation

Standard Library

Built-in functions and utilities

Community Examples

For more examples from the community:

• Aleo Developer Hub
• Leo GitHub Discussions
• Aleo Discord Community

Contributing Examples

Want to contribute an example? See our Contributing Guide for guidelines on submitting examples to the Leo repository.