Plinth and Plutus Core

Understanding the roles and relationships between different languages is key to the effective and efficient development of smart contracts.

Plinth (formerly Plutus Tx)

Plinth, the primary focus of this user guide, is a high-level language for writing the validation logic of the contract, the logic that determines whether a transaction is allowed to perform things such as spending a UTXO, minting or burning assets, and more. Plinth is not a new language, but rather a subset of Haskell, and it is compiled into Untyped Plutus Core (UPLC).

Plinth was previously known as Plutus Tx. It was renamed to avoid confusion around the term "Plutus", which has been used ambiguously to refer to both "Plutus Tx" and "Plutus Core". This led to misunderstandings - particularly the incorrect assumption that Cardano uses a Haskell-based language for on-chain execution.

In reality, the on-chain language is Untyped Plutus Core, a low-level language based on lambda calculus. Plinth is one of several high-level languages that compile to it, alongside other alternatives. See Overview of Languages Compiling to UPLC for more details.

To minimize confusion, the term "Plutus" on its own should either be avoided or used exclusively to refer to Untyped Plutus Core.

Untyped Plutus Core

Untyped Plutus Core (UPLC), also known simply as Plutus Core or Plutus, is a low-level, Turing-complete language based on untyped lambda calculus, a simple and well-established computational model that dates back to the 1930s. Thanks to its simplicity and extensive academic research, the need for updates or modifications to the language over time is minimal, ensuring long-term stability. It also facilitates the creation of simple, formally verified evaluators.

Along with UPLC and its evaluator, we provide a compiler from Plinth, a subset of Haskell, to UPLC. However, UPLC can be an easy compilation target for any language that supports functional-style programming, in particular immutable data and higher-order functions, both of which are widely adopted today in programming languages, and are particularly suited to Cardano's UTXO ledger model, where UTXOs are immutable.

UPLC is the code that runs on-chain, i.e., by every node validating the transaction, using an interpreter known as the CEK machine. A UPLC program included in a Cardano transaction is often referred to as a Plutus script or a Plutus validator.

Typed Plutus Core and Plutus IR

Typed Plutus Core (TPLC) is the intrinsically typed counterpart of UPLC. It is based on higher-order polymorphic lambda calculus with isorecursive types (System Fωμ). TPLC serves as a low-level intermediate representation (IR) for the Plinth compiler. TPLC is closely related to UPLC, and compiling TPLC into UPLC is simply a matter of erasing types.

Plutus IR (PIR) is a high-level IR also used by the Plinth compiler. It extends TPLC by adding recursive bindings and recursive data types. The fact that recursion is explicit in PIR, rather than encoded using fixed point operators as in TPLC and UPLC, makes PIR significantly more readable than TPLC and UPLC. When optimizing the cost or size of Plutus scripts written in Plinth, it is usually useful to look into PIR.

Further reading

The formal details of Plutus Core are in its specification.

PIR is discussed in Unraveling recursion: compiling an IR with recursion to System F.