What is cardano-sl?

Cardano SL (Settlement Layer) is the original source code for the Cardano cryptocurrency, developed by IOHK, a blockchain research and engineering company. It represents the first full implementation of the Cardano blockchain, written in Haskell, a programming language known for its emphasis on mathematical correctness and formal verification. The Settlement Layer is the base layer of the Cardano network: it handles recording and transferring ownership of ADA, the native coin of the Cardano blockchain. The name "Settlement Layer" comes from the original Cardano architecture, which separated the currency ledger (settlement) from the computation and smart contract layer (a separate planned component at the time). The key technical feature this codebase implements is the Ouroboros proof-of-stake protocol. In most early blockchains, new blocks were added by computers competing to solve computational puzzles (proof of work), which consumes large amounts of energy. Ouroboros instead selects block producers based on how much of the currency they hold and have staked, which achieves the same network security properties with far less energy use. Ouroboros was notable at the time of release because it came with a formal academic security proof, published in peer-reviewed research. The README for this repository is sparse and points to external documentation for installation instructions, building from source, and contributing guidelines. This codebase is historical in nature: Cardano has since undergone multiple major upgrades (Shelley, Goguen, Basho, Voltaire) and the active development has moved to successor repositories.