External Functions in Solidity

pragma solidity >=0.4.22 <0.7.0;


contract Oracle {
    struct Request {
        bytes data;
        function(uint) external callback;
    }

    Request[] private requests;
    event NewRequest(uint);

    function query(bytes memory data, function(uint) external callback) public {
        requests.push(Request(data, callback));
        emit NewRequest(requests.length - 1);
    }

    function reply(uint requestID, uint response) public {
        // Here goes the check that the reply comes from a trusted source
        requests[requestID].callback(response);
    }
}


contract OracleUser {
    Oracle constant private ORACLE_CONST = Oracle(0x1234567); // known contract
    uint private exchangeRate;

    function buySomething() public {
        ORACLE_CONST.query("USD", this.oracleResponse);
    }

    function oracleResponse(uint response) public {
        require(
            msg.sender == address(ORACLE_CONST),
            "Only oracle can call this."
        );
        exchangeRate = response;
    }
}