Example from Sablier to explain external calls with if statements

Sablier.sol

pragma solidity =0.5.17;

// import statements (omitted for brievity) ...

/**
 * @title Sablier
 * @author Sablier
 * @notice Money streaming.
 */
contract Sablier is ISablier, ReentrancyGuard, CarefulMath {

    // functions and events (omitted for brievity) ....

    /**
     * @notice Cancels the stream and transfers the tokens back on a pro rata basis.
     * @dev Throws if the id does not point to a valid stream.
     *  Throws if the caller is not the sender or the recipient of the stream.
     *  Throws if there is a token transfer failure.
     * @param streamId The id of the stream to cancel.
     * @return bool true=success, otherwise false.
     */
    function cancelStream(uint256 streamId)
        external
        nonReentrant
        streamExists(streamId)
        onlySenderOrRecipient(streamId)
        returns (bool)
    {
        Types.Stream memory stream = streams[streamId];
        uint256 senderBalance = balanceOf(streamId, stream.sender);
        uint256 recipientBalance = balanceOf(streamId, stream.recipient);

        delete streams[streamId];

        IERC20 token = IERC20(stream.tokenAddress);
        if (recipientBalance > 0) token.safeTransfer(stream.recipient, recipientBalance);
        if (senderBalance > 0) token.safeTransfer(stream.sender, senderBalance);

        emit CancelStream(streamId, stream.sender, stream.recipient, senderBalance, recipientBalance);
        return true;
    }

}