ModifierSideEffects.sol

/* Bad */
contract BadGuy {
    function isLove(address _addr) external returns(bool) {}
}

contract BadGirl {
    BadGuy badguy;
    modifier isCheck(address _addr) {
        require(badguy.isLove(_addr));
        _;
    }
    function check() isCheck(msg.sender) public {}
}

/* Better */
contract GoodGirl {
    mapping(address => bool) love;
    modifier isCheck(address _addr) {
        require(love[msg.sender],"Insufficient Love, try again");
        _;
    }
    function check() isCheck(msg.sender) public {}
}