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Etch smart contracts

Quote

A smart contract is a computer protocol intended to digitally facilitate, verify, or enforce the negotiation or performance of a contract. Smart contracts allow the performance of credible transactions without third parties. These transactions are trackable and irreversible. Source: Wikipedia.

Smart contract code runs on the etch virtual machine.

All ledger nodes maintain the etch VM and smart contract code.

The identity of a smart contract is calculated by performing a SHA256 hash on the contract code string as an initial step. Next, a further SHA256 hash is calculated from the previous result concatenated with a public key Address which represents the contract owner.

On the ledger, the etch VM stores the contract identity, the contract source code, and the data resources that are mapped by a data.json file.

With this information, the etch VM performs a modulo 16 calculation to decide where to store the data on the ledger, i.e. onto which shard.

Note

Coming soon: details on how developers may dictate the sharding storage design for a smart contract.

Smart contract structure

Smart contract functions are annotated depending on the activity they perform.

@init

The @init function defines a contract constructor that sets the state of the contract prior to any operations performed on it. It is called once and once only on contract initialisation/deployment.

The name of the @init function can be anything you like.

For example, the following function initialises a contract by creating a State type to represent the owner's account which then receives an initial supply of FET tokens.

This happens once and once only at contract deployment.

@init
function initialise(owner: Address)

    var INITIAL_SUPPLY = 100000000000u64;
    var account = State<UInt64>("owner");
    account.set(INITIAL_SUPPLY);

endfunction

function main()

    var owner = Address("2ifr5dSFRAnXexBMC3HYEVp3JHSuz7KBPXWDRBV4xdFrqGy6R9");
    initialise(owner);

endfunction

@action

The @action annotation signifies a function which performs a transaction.

You cannot create a smart contract in etch without an @action function and it is these functions that trigger the charging rules for data persistence fees.

The following function performs a transaction between two parties.

@action
function transfer(from: Address, to: Address, amount: UInt64)

    var from_balance = State<UInt64>(from); 
    from_balance.set(1000u64);
    var to_balance = State<UInt64>(to);
    to_balance.set(0u64);

    // check if all the conditions are valid
    if (from_balance.get() <= amount)
        panic("Argh!");
    endif

    from_balance.set(from_balance.get() - amount);
    to_balance.set(to_balance.get() + amount);

endfunction

function main()

    var owner = Address("2ifr5dSFRAnXexBMC3HYEVp3JHSuz7KBPXWDRBV4xdFrqGy6R9");
    var user = Address("2ifr5dSFRAnXexBMC3HYEVp3JHSuz7KBPXWDRBV4xdFrqGy6R9");
    transfer(owner, user, 100u64);

endfunction

In the worst case, the above function needs two shards for data storage.

@query

Query functions are read-only functions that allow you to view data residing on the ledger.

The following function queries the balance of an Address.

@query
function balance(address : Address) : UInt64

    var account = State<UInt64>(address);
    account.set(100u64);
    return account.get();

endfunction

function main()

    var owner = Address("2ifr5dSFRAnXexBMC3HYEVp3JHSuz7KBPXWDRBV4xdFrqGy6R9");
    var owner_balance = balance(owner);
    printLn(owner_balance);

endfunction

Data confirmation

If you run an etch contract in the simulator containing one or more State types and flag the compiler with -data and a filename, it will create a json file containing the details of the data that will be stored on the ledger.

./vm-lang *filename* -data data.json

If we run one of the above code examples in this way, data.json may contain the following:

{"2ifr5dSFRAnXexBMC3HYEVp3JHSuz7KBPXWDRBV4xdFrqGy6R9": "8403000000000000"}

Utility functions

getBlockNumber() : returns the number of the current block in UInt64.

You need a node running to test this. As well as that, you can only get a result when the function is embedded within smart contract code in Python.

Details for running a node are here.

More details on the Python libraries for smart contracts are in the next sections.