How does flashloan guarantee the borrowed ether/token will be paid back?

Question

I'm wondering how does the contract ensure the borrowed money will be paid back? Can anyone provide an example?

Answer 1

it's all done in one function, so you can revert if the money isn't paid back. (e.g. I send you 100 bucks, you do what you want with it, but the txn reverts if it's not back in my hand by the end of the block). Here's the standard:

function flashLoan(
    IERC3156FlashBorrower receiver,
    address token,
    uint256 amount,
    bytes calldata data
) external override returns (bool) {
    require(token == address(this), "FlashMinter: Unsupported currency");

    uint256 fee = flashFee(token, amount);
    _mint(address(receiver), amount);

    bytes32 CALLBACK_SUCCESS = keccak256("ERC3156FlashBorrower.onFlashLoan");
    require(
        receiver.onFlashLoan(msg.sender, token, amount, fee, data) == CALLBACK_SUCCESS,
        "FlashMinter: Callback failed"
    );

    uint256 _allowance = allowance(address(receiver), address(this));
    require(_allowance >= (amount + fee), "FlashMinter: Repay not approved");

    _approve(address(receiver), address(this), _allowance - (amount + fee));
    _burn(address(receiver), amount + fee);

   

Source: https://soliditydeveloper.com/eip-3156