Now I'll show you how you can write your own swirl courses and lessons. In order to do that, you're going to need a package call swirlify, which I'll load up right now. All right, so swirlify is always aware of your current working directory. My working directory right now is just my home folder, so I'm going to set my directory. I have a special directory on my desktop called Courses where I like to keep some of my swirl courses and develop them. So you can start writing a new swirl course and a new lesson with the new lesson function. I'm going to name this lesson one and I'll name the course My First Course. All right, so executing that function did several things. The first thing that it does is that it opens up this YAML file. All swirl lessons are written in YAML. YAML is a fairly easy to understand markup language. The way that swirl uses it is in this way where each question is demarcated by a hyphen, and then there is a set of key value pairs that specifies each type of question, and what goes into that question in your swirl lesson. So just to show you, just to sort of give you a look, let's go to My Courses folder. So, this is the course I just created, called My First Course. Opening it up there is one lesson inside of it, called Lesson_1. Each course is its own folder and each lesson is its own folder inside of a course. And then that new lesson function created a couple files. The first file, customTests.R is where you can specify your own functions for testing the correctness of swirl questions. We'll talk about answer testing a little bit later. Dependson.txt is a text file that you can use to list the names of functions, excuse me, the names of R packages that are on Cram. That's where we'll check to make sure R installed before the lesson begins. initLesson.R is an R script, and whatever you write in that R script will happen before a lesson starts, so you can load the data inside of initLesson.R or you can create functions that will soon then be used in the lesson. And then this lesson.yaml file is exactly what we're looking at here inside of RStudio. I like using RStudio for writing swirl lessons because having a text editor and an R console next to each other on the same screen works for me. I certainly recommend using RStudio but different people have their different work flows. So I have this new lesson all set up. Now I want to start adding questions. So you can use the wq series of functions that come with swirl find. And as you can see, there's several different kinds of questions that you can include in a lesson. The most simple kind of question that you can include is what's called a message question. So I'm just going to execute this wq message function and go over to my lesson.yaml file, and as you can see, it's added the template for a message question. So I'm going to edit this message question and say, Welcome to lesson 1! And then that's all I need to do in order to write a simple message question. I'm then going to add another question, this time a command question. So a command question will prompt the student to actually enter some R code into the R console. And as you can see, the question template is added to the lesson.yaml file. So I'm going to ask a student a simple question like add 2 and 2 using the plus operator. And then the CorrectAnswer is just the very simple R expression of 2 + 2. And then this AnswerTests is a function that tests the correctness of what the student enters into the console. Omnitest is a function that comes packaged in swirl and for 80-90% of the questions you're going to be asking, you want to use Omnitest. So in this case I'm just going to write 2 + 2 inside of this CorrectExpression argument, and then I'm going to give a very simple hint to the user and say Just type 2 + 2. All right, so I've written some questions, now time to start testing this lesson. Whenever I add a lesson to a course, you should add it to what's called the course manifest. You can do this very easily by using the add_to_manifest function. And just to show you what that did is that created this manifest file. This is so we're inside of the course directory right now. That is the course which is my first course. And this manifest file just keeps track of the order that you want your lessons to be displayed in. And it also keeps track of what the lessons in the course are. That way if you want to put data or other files inside of this course folder, you can do that and those files won't show up as a lesson that the student can select when they're going, when they're choosing a lesson to work on inside of swirl. So this manifest file just kind of keeps things organized. So I've added this lesson to the manifest. Now I'm, now let's test out this lesson. So just to check the general formatting of this lesson, I'm going to use the test lesson function. If I didn't get any warnings or any messages, that means that this lesson passed some formatting checks that come with swirlify. Then I want to actually use this lesson inside of swirl, so I'm going to use the demo lesson function. And as you can see, that takes us right into the lesson that we're writing. And I don't need to go through swirl's menu, so Welcome to lesson 1 is the first question. Add 2 and 2 using a plus operator. I'll do that. And then that's the end of the lesson.
