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:

main
    ( txSkel      : Zen.Types.txSkeleton  )
    ( context     : Zen.Types.context     )
    ( contractId  : Zen.Types.contractId  )
    ( command     : string                )
    ( sender      : Zen.Types.sender      )
    ( messageBody : option Zen.Types.data )
    ( wallet      : Zen.Types.wallet      )
    ( state       : option Zen.Types.data )
    : Zen.Types.contractResult `Zen.Cost.t` n

where n must be an expression which evaluates to a natural number (nat).

Parameters¶

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 Public key

Anonymous An anonymous sender

messageBody : option Zen.Types.data

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`	Byte array
`String`	`of string`	String
`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 Zen.Types.data

The contract current state. Can be either Some data (as the message body) or None.

Output¶

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 Zen.Types.data 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 Zen.Types.data`	The message body of given to the recipient contract.

`Delete`		Delete the current state, resetting it to `None`
`NoChange`		Keeping the current state as it is, with no change.
`Update`	`of Zen.Types.data`	Change the state to be the new given `data`.