Reactive

These are very rough personal notes from a Coursera course I took in 2015 called Principles of Reactive Programming by Martin Odersky, Erik Meijer, Roland Kuhn.

The link to the version of the course that I took is https://class.coursera.org/reactive-002.

General information about the course and possible future sessions is found at https://www.coursera.org/course/reactive.

Getting Started! (Assignment 0)

This is the first assignment. The goal is to familiarize yourself with the infrastructure and the tools required during this class. The grade in this assignment will be excluded from your final grade for the course.

Part 0: Install Required Tools

Before anything else, we need to make sure that all tools are correctly installed. Take a look at the Tools Setup page and verify that all of the listed tools work.

(If you already did all this for the progfun course, you don’t need to do anything more.)

Part 1: Obtain the Project Files

Download the example.zip handout archive file and extract it somewhere on your machine.

Part 2: Make sure the Scala REPL is working

During this class we will always launch the Scala REPL (the interactive Scala console) through sbt, so we don’t need to install the Scala distribution—having sbt is enough. (In case you prefer to have the scala command, you can download the Scala distribution from the scala-lang.org website.)

Note that sbt can only be started inside a project directory, so we first navigate to the example directory created in Part 1, then enter sbt.

In the future, we can type console to get the Scala REPL, but for now, just type

> eclipse

at the sbt prompt to generate the files needed to load the project into Eclipse. See next section for more details. You can exit sbt with ctrl-c.

(If something goes wrong with sbt, open the Sbt Tutorial page and follow the first steps until “Running the Scala Interpreter.”)

Part 3: Opening the Project in Eclipse

To work on the source code of the project, we must import it into Eclipse.

3.1. Creating the Eclipse project from sbt

IMPORTANT Before importing the project into Eclipse, we must first create the Eclipse project from the sbt command line as mentioned above.

$ sbt
> eclipse
...

3.2. Importing the Project in Eclipse

Once the Eclipse project has been created from sbt, you can import it in Eclipse. Follow these steps to work on the project using the Scala IDE:

  1. Start up Eclipse
  2. Select “File” - “Import” from the menu
  3. In the folder “General”, select the item “Existing Projects into Workspace” and click “Next >”
  4. In the textfield “Select root directory:” select the directory where you unpacked the downloaded handout archive
  5. Click “Finish”.

3.3. Working with Eclipse

In the folder src/main/scala, open the package example and double-click the file Lists.scala. There are two methods in this file that need to be implemented (sum and max).

When working on an assignment, it is important that you don’t change any existing method, class or object names or types. Doing so will prevent our automated grading tools from working and you have a high risk of not obtaining any points for your solution.

To learn how to use the Scala IDE, we recommend you to watch the official tutorial video which is available here: http://scala-ide.org/docs/current-user-doc/gettingstarted/index.html. This website also contains a lot more useful information and handy tricks that make working with eclipse more efficient.

3.4. Running Tests inside Eclipse

You can easily execute the test suites directly inside eclipse. Simply navigate to source file of the test suite in src/test/scala, right-click on it and select “Run As” - “JUnit Test”.

The JUnit window will display the result of each individual test.

Part 4: Running your Code

Once you start writing some code, you might want to run your code on a few examples to see if it works correctly. We present two possibilities to run the methods you implemented.

4.1. Using the Scala REPL

In the sbt console, start the Scala REPL by typing console.

> console
[info] Starting scala interpreter...

scala>

The classes of the assignment are available inside the REPL, so you can for instance import all the methods from object Lists: 

scala> import example.Lists._
import example.Lists._

scala> max(List(1,3,2))
res1: Int = 3

4.2. Using a Main Object

Another way to run your code is to create a new Main object that can be executed by the Java Virtual Machine.

  1. In eclipse, right-click on the package example in src/main/scala and select “New” - “Scala Object”
  2. Use Main as the object name (any other name would also work)
  3. Confirm by clicking “Finish”

In order to make the object executable it has to extend the type App. Change the object definition to the following: 

object Main extends App {
  println(Lists.max(List(1,3,2)))
}

Now the Main object can be executed. In order to do so in eclipse:

  1. Right-click on the file Main.scala
  2. Select “Run As” - “Scala Application”

You can also run the Main object in the sbt console by simply using the command run.

Part 5: Writing Tests

Throughout the assignments of this course we will require you to write unit tests for the code that you write. Unit tests are the preferred way to test your code because unlike REPL commands, unit tests are saved and can be re-executed as often as required. This is a great way to make sure that nothing breaks when you have go back later to change some code that you wrote earlier on.

We will be using the ScalaTest testing framework to write our unit tests. In eclipse, navigate to the folder src/test/scala and open the file ListsSuite.scala in package example. This file contains a step-by-step tutorial to learn how to write and execute ScalaTest unit tests.

Here is a copy of the ListSuite.scala file.

Part 6: Submitting your Solution

The only way to submit your solution is through sbt.

6.1. Generate a new password

In order to submit, you need to have your coursera username and your submission password. Note that the submission password is a special password you must generate using a button on the assignments page.

6.2. Start sbt

Important: Open a terminal, change to your project directory, and start the sbt console. That is, you must start sbt from within your project directory.

6.3. Issue the command for submitting in sbt:

> submit your.email@domain.com submissionPassword
[info] Connecting to coursera. Obtaining challenge...
[info] Computing challenge response...
[info] Submitting solution...
[success] Your code was successfully submitted: Your submission has been accepted and will be graded shortly.
[success] Total time: 2 s, completed Aug 30, 2012 4:30:10 PM
>

You are allowed to resubmit a maximum of 5 times! Once you submit your solution, you should see your grade and a feedback about your code on the Coursera website within 10 minutes. If you want to improve your grade, just submit an improved solution. The best of all your submissions will count as the final grade.