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Contract Structure

Contract Structure

Each contract must contain 2 functions: main and cf.
The main function is run whenever a contract is used within a transaction.
The function will run both at the validation and at the generation of the transaction.
The structure of the main function is:
( txSkel : Zen.Types.txSkeleton )
( context : Zen.Types.context )
( contractId : Zen.Types.contractId )
( command : string )
( sender : Zen.Types.sender )
( messageBody : option )
( wallet : Zen.Types.wallet )
( state : option )
: Zen.Types.contractResult `Zen.Cost.t` n
where n must be an expression which evaluates to a natural number (nat).


  • txSkel : Zen.Types.txSkeleton : The partial transaction supplied as input to the contract.
  • context : Zen.Types.context : The blockchain context of the transaction, given by blockNumber (unsigned 32 bit integer), and timestamp - which is the UNIX Epoch time (unsigned 64 bit integer) of the block creation.
  • contractId : Zen.Types.contractId : The contract identifier. (version, hash)
  • command : string : String that the contract may use. Contains a command which tells the contract what to do. For example:
match command with
| "redeem" -> redeem txSkeleton contractId returnAddress wallet
| "buy" -> buy txSkeleton contractId returnAddress
  • sender : Zen.Types.sender : The sender identity. Can be any of the following:
    • Contract of Zen.Types.contractId: a contract, given by its ID
    • PK of Zen.Types.publicKey: a Public key
    • Anonymous: an anonymous sender
  • messageBody : option : The transaction may carry a message which can be any of the following:
    • Byte: of FStar.UInt8.t: 8 bit unsigned integer
    • U32 of FStar.UInt32.t: 32 bit unsigned integer
    • U64 of FStar.UInt64.t: 64 bit unsigned integer
    • I64 of FStar.Int64.t: 64 bit signed integer
    • ByteArray of Zen.Array.t FStar.UInt8.t: array of 8 bit unsigned integer
    • String of string: FStar.prims
    • Hash of Zen.Types.hash: 256-bit hash value
    • Lock of Zen.Types.lock: Lock
    • Signature of Zen.Types.signature: Signature
    • PublicKey of Zen.Types.publicKey: Public key
    • Collection of Zen.Types.dataCollection: Data collection
  • wallet : Zen.Types.wallet : Contains all the transaction inputs that were previously locked to the contract. In order for a contract to spend its own funds they need to come from contract wallet.
  • state : option : The contract current state. Can be either Some data (as the message body) or None.


The output of the contract is of the record type Zen.Types.contractReturn which has 3 fields:
  • state : Zen.Types.stateUpdate State update. Can be any of the following:
    • Delete - Delete the current state, resetting it to None
    • NoChange - Keeping the current state as it is, with no change.
    • Update of - Change the state to be the new given data.
  • tx : Zen.Types.txSkeleton The generated transaction structure.
  • message : option Zen.Types.message An optional message for invoking another contract. This is a record type which has 3 fields:
    • recipient: Zen.Types.contractId - The recipient contract.
    • command: string - The command given to the recipient contract.
    • body: option - The message body of given to the recipient contract.