Changelog

Version history and changes to the Leo programming language

Overview

This document tracks the version history of Leo, including new features, bug fixes, and breaking changes.

Leo follows semantic versioning. The current stable version is 3.4.0.

Version 3.4.0 (Current)

Highlights

• Rowan-based parser for improved error recovery and IDE support
• Enhanced type checking for complex expressions
• Improved code generation optimizations
• Better error messages with more context

Parser Improvements

Rowan Integration

• Migrated to rowan-based parser for better error handling
• Improved syntax error recovery
• Better support for IDE features (autocomplete, go-to-definition)
• Preserved whitespace and comments in parse tree

Error Messages

• More descriptive parser errors with context
• Better suggestions for common mistakes
• Improved error spans pointing to exact locations

Language Features

Type System

• Enhanced type inference in complex expressions
• Better handling of composite types
• Improved const evaluation

Standard Library

• Complete ChaCha random number generation support
• Mapping operations in finalizers
• Block height access in finalizers

Compiler

Optimizations

• Dead code elimination improvements
• Better constant folding
• Reduced circuit complexity for common patterns
• Improved register allocation

Code Generation

• More efficient Aleo instruction generation
• Reduced redundant operations
• Better handling of complex control flow

Tooling

Leo CLI

• Improved leo build performance
• Better leo run output formatting
• Enhanced leo test framework
• leo fmt improvements

Developer Experience

• Faster compilation times
• Better error reporting
• Improved documentation
• More example programs

Bug Fixes

• Fixed type checking edge cases in tuple destructuring
• Resolved issues with nested struct access
• Corrected mapping operation behavior in finalizers
• Fixed parser issues with complex expressions
• Resolved integer literal parsing edge cases

Breaking Changes

None in this release.

Version 3.3.x

Version 3.3.0

Language Features

• Introduction of finalizer functions
• Mapping support for on-chain storage
• Enhanced record types

Compiler

• Loop unrolling improvements
• Better type checking for finalizers
• Improved error messages

Breaking Changes

• Finalizer syntax changes
• Record syntax updates

Version 3.2.x

Version 3.2.0

Language Features

• Vector type support
• Optional type introduction
• Future type for deferred computation

Compiler

• Enhanced optimization passes
• Better dead code elimination
• Improved static analysis

Tooling

• leo disassemble command
• Better test framework
• Improved CLI error handling

Version 3.1.x

Version 3.1.0

Language Features

• Tuple destructuring
• Enhanced array operations
• Improved string handling

Compiler

• Faster compilation
• Better error recovery
• Improved type inference

Bug Fixes

• Fixed array indexing edge cases
• Resolved struct field access issues
• Corrected type casting behavior

Version 3.0.x

Version 3.0.0

Major Release

This was a major rewrite with significant breaking changes.

Language Features

• Complete syntax overhaul
• Records for private state
• Mappings for public state
• Finalizers for on-chain execution
• New type system

Compiler

• Complete rewrite in Rust
• Multi-pass architecture
• Better optimization
• Improved error messages

Breaking Changes

• Syntax changes throughout
• New program structure
• Record and mapping introduction
• Function signature changes

Version 2.x (Legacy)

Version 2.0.0

Historical Release

Early version of Leo with basic functionality.

Features

• Basic type system
• Simple function definitions
• Circuit compilation

Version 2.x is no longer supported. Please upgrade to 3.x.

Migration Guides

Migrating to 3.4.x from 3.3.x

No breaking changes. Simply update:

leo update

Or:

cargo install leo-lang --force

Migrating to 3.x from 2.x

Major syntax changes required. Key updates:

Program Structure

// 2.x
circuit Token {
    owner: address,
    amount: u64,
}

// 3.x
program token.aleo {
    record Token {
        owner: address,
        amount: u64,
    }
}

Function Definitions

// 2.x
function transfer(amount: u64) -> u64 {
    return amount;
}

// 3.x
fn transfer(amount: u64) -> u64 {
    return amount;
}

On-Chain State

// 2.x
// No direct equivalent

// 3.x
mapping balances: address => u64;

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

final fn finalize_transfer(receiver: address, amount: u64) {
    Mapping::set(balances, receiver, amount);
}

Deprecation Notices

Planned for Future Versions

The following features are under consideration for deprecation:

• Legacy syntax compatibility mode (if added)
• Certain error codes may be consolidated

No immediate deprecations are planned for 3.x.

Upcoming Features

Roadmap

Language

• Enhanced generic support
• Trait system
• More built-in functions
• Improved string handling

Compiler

• Incremental compilation
• Better optimization passes
• Parallel compilation
• Enhanced error recovery

Tooling

• Integrated debugger
• Profiler for circuit complexity
• Package manager improvements
• Better IDE integration

Standard Library

• Hash functions (Poseidon, SHA-256)
• Signature verification utilities
• Merkle tree operations
• More cryptographic primitives

Roadmap items are subject to change based on community feedback and priorities.

Release Schedule

Leo follows a regular release schedule:

• Major versions (X.0.0): As needed for breaking changes
• Minor versions (3.X.0): Monthly with new features
• Patch versions (3.4.X): As needed for bug fixes

Version Support

Version	Status	Support Until
3.4.x	Current	Ongoing
3.3.x	Maintenance	6 months after 3.4.0
3.2.x	End of Life	No longer supported
3.1.x	End of Life	No longer supported
3.0.x	End of Life	No longer supported
2.x	End of Life	No longer supported

How to Update

Using Leo CLI

leo update

Using Cargo

cargo install leo-lang --force

Verify Version

leo --version

Should output:

leo 3.4.0

Staying Informed

Release Announcements

Stay up to date with Leo releases:

• GitHub Releases: https://github.com/ProvableHQ/leo/releases
• Discord: Aleo Discord #announcements
• Twitter: @ProvableHQ

Security Updates

Critical security updates are announced via:

• GitHub Security Advisories
• Discord security channel
• Direct email to maintainers

Always update to the latest patch version for security fixes.

Contributing to Releases

Help shape future Leo versions:

1. Feature Requests: Open issues on GitHub
2. Bug Reports: File detailed bug reports
3. Pull Requests: Contribute code improvements
4. Testing: Test beta releases and provide feedback

See the Contributing Guide for more information.

Release Notes Archive

For detailed release notes of all versions, see:

• GitHub Releases: https://github.com/ProvableHQ/leo/releases
• CHANGELOG.md: In the Leo repository

Version History Summary

v3.4.0 - 2026-03 - Current release
v3.3.0 - 2025-12 - Finalizers and mappings
v3.2.0 - 2025-09 - Vector and Optional types
v3.1.0 - 2025-06 - Tuple destructuring
v3.0.0 - 2025-03 - Major rewrite
v2.0.0 - 2024-01 - Legacy version

Version dates are illustrative. Check GitHub releases for actual release dates.

Related Resources

GitHub Releases

Official release notes

Contributing

Help build Leo

Troubleshooting

Common issues and solutions

Installation

Install the latest version