Contractum: smart-contract language

Contractum is a functional declarative programming language designed for developing smart contracts which run on Bitcoin and Lightning network using RGB technology.

Declarative

Define contract state and operations in a simple declarative form, which is easy to write and understand

Strong-typed

State and contract operations take only known structured types

Abstract

Leverage interfaces, type libraries, schemata and schema hierarchy for code-reuse and reducing risks of mistakes

Contractum differs from other smart contract programming languages in a fact that as functional as Haskell and nearly as close to the bare metal as Rust at the same time, filling in the space which was not accessible for the smart contracts before:

Contractum on the landscape of other languages

RGB smart contracts

Contractum is the language to write RGB contracts. RGB is a technology which allows creation of arbitrary-complex ("Turing-complete") smart contracts that run on bitcoin and, most imporangly, Lightning network. RGB contracts are confidential, scalable (up to the speed of the Lightning transactions, with small data footprint) and robust.

Confidential

• No blockchain footprint

• No chainanalysis possible: contract breaks transaction graphs

• Extensive use of zero-knowledge (confidential transactions, bulletproofs)

Scalable

• No transaction size increase, no block space usage

• Sharded: data kept by contract parties and not each node

• Operate with the speed of Lightning Network

Turing-complete

• Arbitrary complex business logic

• #BiFi: bitcoin finance, which is much more robust and reliable than DeF

• No tokens or gas required; bitcoin is the native currency

Contracts written with Contractum are verified with client-side-validation, which does not adds data to a bitcoin blockchain and may be thought as a sharding technology, enhanced with zero-knowledge. Client-side-validation also breaks transaction graphs, unlinking contract evolution from blockchain transactions, making chain analysis impossible.

Learn more about RGB smart contracts on the RGB FAQ website.

Work in progress

The contractum is a work in progress: the language design is under active development at the LNP/BP Association. Everyone is welcome to join the effort; a good starting point can be reading and writing to the language design discussions group in GitHub.

To understand and participate in contractum design it is important to learn more about technologies which are used by RGB smart contracts:

Single-use-seals

Client-side-validation relies on a deterministic commitment history named single-use-seals

Learn more

AluVM

Contractum runs code on AluVM virtual machine: functional registry-based RISC machine

Learn more

Strict encoding

RGB contracts uses special deterministic portable binary data type system and encoding

Learn more

The feel of the language

If you'd like to get a feel of the language, here is a sample contract written in Contractum:

types Did
   data PgpKey :: curve U8, key Bytes

schema DecentralizedIdentity
   -- This defines the atom of the contract state called `Identity` which has data 
   -- type `PgpKey`.
   -- The `owned` keyword means that there is always a party which owns the identity
   owned Identity :: Did.PgpKey

   owned IOYIssue :: Zk64
   -- `Zk64` means 64-bit unsigned integer hidden with zero-knowledge
   owned IOYTokens :: Zk64

   global IOYTicker :: String
   global IOYName :: String

   -- This says that to construct contract the user must provide information about 
   -- exactly one identity and its IOY token
   genesis :: Identity, IOYTicker, IOYName

   -- Now let's define what a owner of identity can do by executing his/her rights 
   -- via state transitions ("operation" on the state) of predefined forms, like
   op Revocation :: old Identity -> new Identity
   -- which does what it says: it revokes existing identity and creates a new one.
   
   -- This issues new IOY promises in tokenized form
   op Promise :: used IOYIssue -> given [IOYTokens]?, remaining IOYIssue?
      assert used == sum given + (remaining ?? 0)

   -- This transfers IOY tokens from one owner to another owner
   op Transfer :: spent {IOYTokens} -> received [IOYTokens]
      assert sum spent == sum received
   
interface FungibleToken:
   global Ticker -> String -- this is similar to schema definition; in fact
                           -- it is a requirement that the schema must provide
                           -- a global state of the String type and link it to
                           -- the "Ticker" name
   global Name -> String

   owned Inflation :: Zk64 -- pretty much the same applies to assigned state
   owned Asset :: Zk64

   op Issue :: Inflation -> [Asset]?, Inflation? -- and operations
   op Transfer :: {Asset} -> [Asset]

interface PgpIdentity
  owned Identity :: PgpKey
  exec Revocation :: old Identity -> new Identity

-- Specific schema state may use different naming, for instance because a schema
-- can define multiple assets with different names; in that case we will have
-- multiple interface implementations referencing different state.
implement FungibleToken for DecentralizedIdentity
   global Ticker := IOYTicker -- this creates a _binding_ of the state defined
                              -- in the schema (*IOYTicker* in this case) to the
                              -- interface 
   global Name := IOYName
   owned Inflation := IOYIssue
   owned Asset := IOYTokens
   op Issue := Promise
   op Transfer -- here we skip `:=` part since the interface operation name
               -- matches the name used in the schema. In such cases we can also
               -- skip the declaration at whole

implement PgpIdentity for DecentralizedIdentity
  -- we do not need to put anything here since schema state and operation names
  -- matches interface requirements and the compiler is able to guess the bindings
  
contract meSatoshiNakamoto implements DecentralizedIdentity
   set IOYTicker := "SATN"
   set IOYName := "Satoshi Promises"
   -- this defines a genesis state and assigns it to a single-use-seal
   assign orig Identity := 
     (0xfac503c4641c3deda72a2d00bc9d6ff1094b15276c386efea403746a91436772, 1) 
   -> PgpKey(0, 0x028730eeeec41802621d177507b086f390ae600ba3ca5e428b13913af4c2cd25b3)

transition iLostMyKey executes Revocation
   via meSatoshiNakamoto.orig -- specifies the single-use-seal we close to match
                              -- requirements on the valid operation execution
                              -- conditions
   assign upd Identity := (~, 2) -- here we use txid of the bitcoin transaction
                                 -- which will be created to hold the commitment to
                                 -- this state transition, called "seal witness".
                                 -- Since we can not know the txid upfront we use 
                                 -- `~` sign to indicate the witness transaction id
   -> PgpKey(0, 0x0219db0a4e0eb8cb833608c08d76b9b279ec44a851ab82cc6fd68a9b32624bfa8b)
   -- the above defines new state and assigns it to a single-use-seal

About

Contractum development is managed by a non-profit LNP/BP Standards Association. The language design and compiler implementation is lead by Dr Maxim Orlovsky.