Fundamentals

Chapter 3 / 15

Actions, Events & Errors

emit, transfer, revert_with — the full action vocabulary.

Actions in Covenant carry four responsibilities: validate inputs, update state, emit events, and transfer value. Covenant's revert_with attaches typed error payloads — readable by any ABI-aware client without extra decoding.

bank.cov
record Bank {
    balances: map(address => amount);

    // Named events
    event Deposited(who: address, value: amount);
    event Withdrawn(who: address, to: address, value: amount);

    // Typed errors
    error InsufficientFunds(requested: amount, available: amount);
    error ZeroTransfer();

    action deposit() {
        let val = msg.value;
        if val == 0 {
            revert_with ZeroTransfer();
        }
        self.balances[msg.sender] += val;
        emit Deposited(msg.sender, val);
    }

    action withdraw(to: address, value: amount) {
        let avail = self.balances[msg.sender];
        if value == 0 {
            revert_with ZeroTransfer();
        }
        if value > avail {
            revert_with InsufficientFunds(value, avail);
        }
        // Checks-Effects-Interactions order
        self.balances[msg.sender] -= value;
        emit Withdrawn(msg.sender, to, value);
        transfer(to, value);
    }

    view balance_of(who: address) -> amount {
        return self.balances[who];
    }
}

Annotations

eventdeclares a log event with named, typed fields. The ABI selector is derived automatically.
errordeclares a typed revert payload — EIP-838 compatible. Arguments are ABI-encoded into revert data.
revert_with E(...)reverts the transaction and encodes the error payload. Equivalent to Solidity's revert E(...).
msg.valuethe ether attached to the call, as an amount.
transfer(to, value)sends ether using call{value: v}('') with a forward-gas cap. Reverts on failure.

Key takeaways