Writing Simplicity Programs with SimplicityHL

Simplicity is a programming language crafted specifically for blockchain smart contracts, characterized by its minimal design. Despite its straightforward core semantics, which are simple enough to be summarized on a T-shirt, Simplicity is not easy to use due to its functional and low-level nature. This aspect makes programming in Simplicity feel more akin to assembly language than to higher-level languages like Java or Python. However, its power and capability for formal verification make it an attractive option for certain blockchain applications. Recently, Simplicity was launched on the Liquid mainnet, enabling developers to write and deploy contracts in production environments using this language.

To bridge the gap between Simplicity's low-level operations and a more accessible development experience, SimplicityHL has been introduced. SimplicityHL is a high-level language designed for writing smart contracts on Elements and Liquid platforms, offering a syntax and user experience similar to Rust. Just as Rust compiles down to machine code, SimplicityHL compiles down to Simplicity bytecode, allowing developers to write in a more familiar and higher-level language while executing in Simplicity. Although currently not under active development, SimplicityHL presents an opportunity to make Simplicity-based development more approachable.

A practical example provided demonstrates the utility of SimplicityHL in addressing a common issue in Bitcoin transactions: fee estimation. Traditional methods like RBF (Replace-By-Fee), CPFP (Child Pays For Parent), and other solutions require active coordination from the sender or a third party. However, the example showcases how embedding fee bumping logic directly into a contract's script can autonomously adjust transaction fees based on the time elapsed since broadcast, without needing external intervention. This approach leverages SimplicityHL's ability to express spend conditions as functions over transaction data, illustrating the potential for creating complex and self-adjusting smart contracts.

The given code snippet enforces a base fee with an incremental increase over time, demonstrating the application of SimplicityHL in creating a non-interactive fee bumping mechanism. This method provides full control over transaction authorization logic, showcasing how SimplicityHL can simplify the creation of secure and auditable smart contracts. While the focus is purely on the programming model, it acknowledges the broader implications such as miner incentives and integration across different components of the blockchain ecosystem without delving into those aspects.

For further exploration, resources such as SimplicityHL and rust-simplicity are available, offering tools and examples for developers interested in working with these technologies.