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Practice Scala

Submission deadline: Wednesday, September 28 at 11:59pm
Critique deadline: Friday, September 30 at 11:59pm

The goal of this assignment is to help you become familiar with Scala and the development tools we'll be using this semester. When you're done, you should:

  • feel you've had good practice with the basics of Scala: variables, data structures, functions, tests
  • have lots of questions about why Scala was designed the way it was

Credits: Much of this assignment is based on the Scala Labs project. Normally, it would be good practice to fork the Scala Labs repository, then modify the fork. But I didn't want our pull requests to accidentally go to Scala Labs, so I cloned the lab instead. We'll do only a small part of the lab, so I removed the other parts.

Instructions

Read the WHOLE document (and the linked pages) very carefully, before you start coding.

Here's what to do:

  • Get things set up

  • Work on the assignment

    • Read the lab instructions, hints, and tips, below.
    • Complete the exercises in the following files (found in the src/main/scala directory):
      • 00_HelloWorld.scala
      • 01_Echo.scala
      • 02_Fibonacci.scala
      • 03_Exponent.scala
      • 04_Options.scala
      • 05_Patterns.scala
      • 06_Collections.scala
      • 07_Palindrome.scala
    • Possibly complete the optional exercises in the following files:
      • Rainfall.scala
      • WebScraper.scala
    • Review Ben's solutions (in a pull request)
    • Possibly review the solutions from ScalaLabs.
    • Write about Scala in scala-thoughts.md

  • Submit the assignment

    • Submit your work (by issuing a pull request).
    • Review and comment on other people's code.

Resources

You'll need to learn a fair bit of Scala on your own, to do the exercises. In fact, that's the whole point of this assignment :). Many of the exercises contain links to helpful documentation / information. Here are some additional, general resources on learning Scala that might help:

Please also ask questions in class, on Piazza, and in office hours!

Working on the exercises

In general, for each exercise, do the following:

  1. Open the appropriate code file.
  2. Browse through the file to see what's required.
  3. Look at and run the corresponding tests.
  4. Work on the exercise, reading documentation, and running and augmenting the tests as needed.
  5. Compare your work to solutions.

Loading a project into ScalaIDE

If you want use ScalaIDE to work on your project, run sbt eclipse in the top directory (the one with build.sbt in it), then import the project.

Running code and tests with ScalaIDE

None of the exercises are meant to be run as programs. Instead, you can run the tests for each exercise. To run all the tests, right-click on the src/test/scala/exercises package (in the "Package Explorer, on the left side of the screen) and select Run as ▸ ScalaTest - Package.

To run a single test, open the test file and right click in the Editor (or you can just right click on the test file's name in the Package Explorer) and select either:

  • Run as ▸ ScalaTest - File (if the file contains ScalaTest tests). The test results will show up in the ScalaTest window, usually found along the left side of the screen.
  • Run as ▸ Scala - Application (if the file contains ScalaCheck tests). The test results will show up in the Console window, usually found along the bottom of the screen.

Running code and tests with sbt

None of the exercises are meant to be run as programs. Instead, you can run the tests for each exercise. To run all tests in sbt, you can type the following command in the terminal: sbt test

If you'd like to run a specific test, type something like the following command: sbt test-only exercises.HelloWorldTest This example runs the tests for the HelloWorld exercise. If you want to run tests for other exercises, use the appropriate class name instead of HelloWorld.

As always, you can run the tests continually, by starting sbt and using the tilde ( ~) symbol before the sbt command. Here's an example of continually running the HelloWorld test:

$ sbt
[info] Loading global plugins from /Users/ben/.sbt/0.13/plugins
[info] Loading project definition from ...
[info] Set current project to Practice Scala (in build file: ...)
> ~test-only exercises.HelloWorldTest
[info] HelloWorldTest:
[info] - Hello world *** FAILED ***
[info]   "[Change me]" did not equal "[Hello world!]" (00_HelloWorldTest.scala:22)
[info] ScalaCheck
[info] Passed: Total 0, Failed 0, Errors 0, Passed 0
[info] ScalaTest
[info] Run completed in 240 milliseconds.
[info] Total number of tests run: 1
[info] Suites: completed 1, aborted 0
[info] Tests: succeeded 0, failed 1, canceled 0, ignored 0, pending 0
[info] *** 1 TEST FAILED ***
[error] Failed: Total 1, Failed 1, Errors 0, Passed 0
[error] Failed tests:
[error]   exercises.HelloWorldTest
[error] (test:testOnly) sbt.TestsFailedException: Tests unsuccessful
[error] Total time: 1 s, completed Sep 21, 2016 11:02:20 AM
1. Waiting for source changes... (press enter to interrupt)

Write about Scala

After completing the exercises, take about 20 minutes to collect and write about your initial impressions of Scala. Write about them in scala-thoughts.md Some possible topics include:

  • What's easy to do in Scala? What's not?
  • What is/are your favorite language design choice(s) that the designers of Scala made? Why?
  • What is/are your least favorite language design choice(s)? Why? And why do you think the designers made that / those choice(s)?
  • What Scala features would you like to learn more about?

Peer-review other people's work

You'll comment on at least one other person's work. Look at the peer review table to see whom you should review. You should look through their solutions to each of the exercises, as well as their scala-thoughts.md file. You don't need to comment on everything, but find some interesting parts of their work to evaluate. Some questions you might consider / answer when providing feedback:

  • How does this team's solution compare to yours? To the provided solutions? To other people's solutions? Is there anything they did that you like better? Is there any way to improve their solution?
  • How readable is their code? How elegant is their solution? If you think it's inelegant, was that because the team was trying to show an alternative solution?
  • How Scala-y is the code? Most of us are just learning the language, and we can help each other spot cases when a functional approach would have worked in place of an imperative one, etc.
  • If a team has raised a question in the comments of their code, or in their writings about Scala, try to answer it!

Bonus exercises

There are two bonus exercises: Rainfall.scala and Webscraper.scala. These exercises are for your enjoyment, if you'd like to play with Scala a little bit more. There's no extra credit for them; they're just fun.

The Rainfall.scala exercise is mostly self-explanatory: it's similar in nature to the Palindrome.scala exercise.

The Webscraper.scala exercise is a chance to play around with a(nother) Scala library that implements an internal DSL, in this case for parsing web pages. To get started, head over the the home page for the library. Follow the instructions to add the library's dependency to sbt. (Note: if you're using ScalaIDE, you'll need to re-run sbt eclipse. Then, you can refresh the project in ScalaIDE, to make sure the new library is included in your project.) After you've added the web-scraping library, look at some examples and play around with writing a program that parses a webpage and produces interesting results.

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