LoveLetter-send.sol

contract LoveLetter {
    ...

    function send(address to, string memory message)
        external
        payable
        returns (uint256 id)
    {
        id = totalLetters;
        senders[id] = msg.sender;
        receivers[id] = to;
        letters[id] = Letter({
            message: message,
            etherAmount: msg.value,
            opened: false
        });
        console.log("[send]", id, msg.value);
        totalLetters++;
        emit Sent(msg.sender, to, id, msg.value);
    }

    ...
}