Simplicity
Simplicity is a typed, combinator-based functional blockchain language designed by Russell O'Connor as an alternative to Bitcoin Script and Tapscript. Programs are built from a small set of combinators without loops or recursion. The language is deliberately not Turing complete, yet it can express any finitary function. Formal denotational semantics in a proof assistant and an operational cost model called the Bit Machine support static resource bounds and machine-checkable reasoning about contract behavior.
Simplicity is intended eventually to appear as a versioned leaf alongside Script inside Taproot trees on Bitcoin. As of the Liquid Network deployment, it runs on that sidechain's consensus rules rather than on Bitcoin mainnet. Jets provide optimized implementations of recognized subexpressions such as cryptographic primitives. Higher-level languages such as SimplicityHL compile down to Simplicity. Mainnet Bitcoin would need a consensus definition of the leaf version, cost model, and jet set before transactions could execute Simplicity programs under Bitcoin consensus.