Our first attempt to implement a command process that may look similar to the following:

function process_command(command::String, args)
    if command == "open"
        # open a file
    elseif command == "close"
        # close current file
    elseif command == "exit"
        # exit program
    elseif command == "help"
        # pops up a help dialog
    else
        error("bug - this should have never happened.")
    end
end

The process_command function simply takes the command as a string. Then, depending on the value of the string, it will call the respective function. The args argument may be passed by the GUI code for additional information; for example, the path of the file that is being opened or closed.

There is nothing wrong with this code from a logical perspective, but it can be improved, as follows:

• The code contains a list of if-then-else statements. In this example, we only have to support four functions. In practice, we will probably have to handle many more functions. Having such a large if-then-else block makes the code very ugly and hard to maintain. 
• Whenever we need to add a new command, we have to modify this function to include a new condition.

Fortunately, we can make it better using singleton types and dynamic dispatch. We'll go over that next.