Burnable Token

Problem

I need token holders to be able to permanently destroy (burn) their own tokens, reducing total supply.

Solution

token BurnableToken {
    name: "Burnable Token"
    symbol: "BTK"
    decimals: 18
    supply: 10_000_000 tokens
    initial_holder: deployer

    event Burned(from: address indexed, amount: amount)
    error InsufficientBalance()
    error ZeroAmount()

    action burn(amount: amount) {
        given amount > 0 or revert_with ZeroAmount()
        given balances[caller] >= amount or revert_with InsufficientBalance()

        balances[caller] = balances[caller] - amount
        total_supply = total_supply - amount

        emit Burned(caller, amount)
    }

    action burn_from(owner: address, amount: amount) {
        given amount > 0 or revert_with ZeroAmount()
        given allowances[owner][caller] >= amount or revert_with Unauthorized()
        given balances[owner] >= amount or revert_with InsufficientBalance()

        allowances[owner][caller] = allowances[owner][caller] - amount
        balances[owner] = balances[owner] - amount
        total_supply = total_supply - amount

        emit Burned(owner, amount)
    }
}

Explanation

burn lets callers destroy their own tokens. burn_from uses the allowance mechanism (standard ERC-20 approve/transferFrom) to let approved spenders burn on behalf of a holder — useful for DeFi protocols (lending, AMMs) that need to destroy collateral tokens.

Both actions decrement total_supply, making burns visible to anyone querying supply.

Gas Estimate

Operation	Gas
`burn`	~35,000
`burn_from`	~45,000

Common Pitfalls

Not decrementing total_supply: If you only zero the balance, total_supply becomes inconsistent. Always update both.
Burning more than balance: The >= check prevents underflow. Without it, Covenant’s amount type would revert on underflow, but the error message is less clear.
Missing allowance decrement in burn_from: The allowance must be reduced before the balance — CEI pattern.
No burn_from -> no DeFi compatibility: Protocols like Aave expect burnFrom to exist. Include it even if you don’t plan to use it immediately.

Variations

Owner-only burning

action burn(target: address, amount: amount) only owner {
    balances[target] = balances[target] - amount
    total_supply = total_supply - amount
    emit Burned(target, amount)
}