The following is a simple example of a subcurrency which implements functionality to mint and send a token. It is a ledger-based token, i.e. the contract maintains a ledger of user account balances.
Being a ledger-based token, this example does not use Fuel's native asset system. It is not recommended to actually use ledger-based tokens in production; this example is here purely for illustrative purposes.
contract;
use std::hash::sha256;
////////////////////////////////////////
// Event declarations
////////////////////////////////////////
//
// Events allow clients to react to changes in the contract.
// Unlike Solidity, events are simply structs.
//
/// Emitted when a token is sent.
struct Sent {
from: Address,
to: Address,
amount: u64,
}
////////////////////////////////////////
// ABI method declarations
////////////////////////////////////////
/// ABI for a subcurrency.
abi Token {
// Mint new tokens and send to an address.
// Can only be called by the contract creator.
#[storage(read, write)]
fn mint(receiver: Address, amount: u64);
// Sends an amount of an existing token.
// Can be called from any address.
#[storage(read, write)]
fn send(receiver: Address, amount: u64);
}
////////////////////////////////////////
// Constants
////////////////////////////////////////
/// Address of contract creator.
const MINTER = Address::from(0x9299da6c73e6dc03eeabcce242bb347de3f5f56cd1c70926d76526d7ed199b8b);
////////////////////////////////////////
// Contract storage
////////////////////////////////////////
// Contract storage persists across transactions.
storage {
balances: StorageMap<Address, u64> = StorageMap {},
}
////////////////////////////////////////
// ABI definitions
////////////////////////////////////////
/// Contract implements the `Token` ABI.
impl Token for Contract {
#[storage(read, write)]
fn mint(receiver: Address, amount: u64) {
let sender = msg_sender().unwrap();
let sender: Address = match sender {
Identity::Address(addr) => {
assert(addr == MINTER);
addr
},
_ => revert(0),
};
// Increase the balance of receiver
storage.balances.insert(receiver, storage.balances.get(receiver).try_read().unwrap_or(0) + amount);
}
#[storage(read, write)]
fn send(receiver: Address, amount: u64) {
let sender = msg_sender().unwrap();
let sender = match sender {
Identity::Address(addr) => addr,
_ => revert(0),
};
// Reduce the balance of sender
let sender_amount = storage.balances.get(sender).try_read().unwrap_or(0);
assert(sender_amount > amount);
storage.balances.insert(sender, sender_amount - amount);
// Increase the balance of receiver
storage.balances.insert(receiver, storage.balances.get(receiver).try_read().unwrap_or(0) + amount);
log(Sent {
from: sender,
to: receiver,
amount: amount,
});
}
}