    function Exchange(
        address _baseToken,
        address _dai,
        address _oracle,
        uint256 _ethRateN,
        uint256 _ethRateD,
        uint256 _daiRateN,
        uint256 _daiRateD
    ) public {
        baseToken = ERC20(_baseToken);
        dai = ERC20(_dai);
        addRole(_oracle, ROLE_ORACLE);
        oracle = _oracle;
        ethRate = R.Rational(_ethRateN, _ethRateD);
        daiRate = R.Rational(_daiRateN, _daiRateD);
    }