Posts in the “scala” category

If you’re finding Akka Typed hard to learn, fear not, it took me almost two days to get the simplest of their Scala examples working. And I had already written the original Scala Cookbook, and already knew how to use Akka Classic!

As a remedy to this problem, I wrote about how to use Akka Typed in the 2021 version of the Scala Cookbook. In the book I show some of the simplest possible Akka Typed examples, so you won’t have to waste two days of your life trying to get a simple example running.

Here in August and September, 2021, the Scala Cookbook is a best-selling new programming book in both the OOP and FP categories. And as I found out yesterday, a great thing about writing for O’Reilly is that if you write a book for them, they’ll send you a bunch of free copies, even when the book is almost 800 pages long. Makes for a great unboxing experience!

I’m happy to report that the Scala Cookbook for Scala 3 — a.k.a., the Scala 3 Cookbook — is now the #1 new release in Amazon’s functional programming category, as of late August, 2021. This comes on the heels of already being the #1 new OOP book.

As a brief note today, if you want to have multiple packages and classes in one file in Scala 3, this syntax should do the trick for you:

package com.acme.foo:
    class Foo:
        override def toString = "Foo"

package com.acme.bar:
    import com.acme.foo.Foo
    class Bar:
        val foo = Foo()

package com.acme.common:
    import com.acme.bar.Bar
    @main def multiPackageTest =
        val b = Bar()
        println(b.foo)

In the process of writing the Scala Cookbook for Scala 3, there were days when it was hard to find a working testing framework. As a result, I decided to write my own Scala 3 testing framework. Calling it a framework is a bit of a joke, as it’s only about 88 lines of source code. I named that library SimpleTest, and you can find it at github.com/alvinj/SimpleTest.

So I can easily find the source code later, I’m putting Version 0.3 of the source code here:

Wow, the following code might be the largest Scala SBT build.sbt configuration file I’ve ever seen. It comes from this project.

Here’s the build.sbt file contents:

Nothing major here, I just wanted to note the use of several scalacOptions in the following build.sbt example:

As I noted yesterday, I got a Play Framework application running on a Raspberry Pi (RPI) this weekend. It was pretty easy; I just deployed the application to a production server as usual, and on the RPI I only had to install Java, and control the RAM usage.

I have a 19" monitor on the counter between my kitchen and living room, and it’s powered by a Raspberry Pi. I use the Linux Phosphor screen saver to show a scrolling “news and stock ticker” on the display, which I’ve programmed to show news from several different sources (Atom and Rss feeds, along with other news and data sources). An old version of the display looks like this:

Today I added a new “Word of the day” feature to the display, and as with all of the other code, I wrote a Scala shell script to generate the output.

As I noted in this tweet on July 26, 2021, my book, Functional Programming, Simplified, was the #2 best-seller in Amazon’s functional programming category that is actually about functional programming.

Dateline July 15, 2021:

• I reduced the price of the print version of “Hello, Scala” so I only make ten cents per copy
• I reduced the Kindle version of the book to its minimum price, $2.99 (US)

The book is about 250 pages long, and I could easily charge $25 for it. But I want to make them available at as low a cost as possible to make it as inexpensive to learn Scala as possible.

Note that this book was released in October, 2018, and covers Scala 2. If you want a free version of a similar book, the online version of the Scala 3 Book is completely free.

A lot of times people ask me, “How can I learn Scala 3?” Or, “I have this Scala 3 question, can you answer it for me?” I don’t mind doing that when I can, but there are still a lot of things I don’t know off the top of my head.

So, in an effort to help people find Scala 3 resources, here’s a quick list:

• The Scala 3 Book (which I co-wrote) is freely available online
• The main Scala 3 documentation website starts at this link
• Under that link, you’ll find:
  • How to get started with Scala 3
  • What’s new in Scala 3
  • The Scala 2 to Scala 3 migration guide
  • The Scala 3 API docs (which are likely to move)
  • The Scala 3 Reference, which is also likely to move
• I’ve written the Scala Cookbook (for Scala 3), which will be out in August, 2021

Also, if you’re using Scala 3 and have a specific question about it, I recommend asking that question here:

• The Scala 3 Gitter channel

If you have a more general question about Scala 3, or even an idea on how to improve it, I recommend asking those types of questions here:

• The Scala 3 Contributors Forum

In summary, if you need some direct links to Scala 3 learning resources, I hope those are helpful.

Akka Actors Problem: You want to see the proper way to shut down Akka Typed actors.

(Note: This tutorial is written for Akka 2.6.)

Solution

There are two main ways to shut down Akka Typed actors:

• Send them a “graceful shutdown” message, and let them shut themselves down by calling Behaviors.stopped

• As a parent, stop a child actor with actorContext.stop(actorRef)

Akka Actors Problem: You know that an actor that “blocking” is bad for your Akka system, so you want to create actors that don’t block.

Solution

When using Akka actors, the mantra is always, “Delegate, delegate, delegate.” It’s important that high-level actors delegate work to lower-level actors, so the high-level actors can be free to receive new messages and respond to them immediately. This example shows how to implement non-blocking actors.

As I was writing the Scala Cookbook, 2nd Edition — and debating writing a book about Scala.js — I had to start thinking about how to implement Scala.js applications in the real world. When I did that, I began to look at these Scala.js libraries:

Akka Problem: In some situations you can’t pass an ActorRef to another actor, so you want to see how to “look up” an Akka Typed actor so you can send a message to it.

Solution

There are at least two ways to find other Akka Typed actors, and both involve using the Akka Receptionist. The example shown here in the solution shows how to find an actor asynchronously, and the example shown in the Discussion shows how to use the ask method to find another actor synchronously.

In both examples I’ll use the idea of creating an actor system that works like Amazon Echo devices. The basic idea is that a device like this has ears to listen to you, a mouth to speak to you, and a brain to do all of its work. In these examples the Brain actor will need to discover the Mouth actor.

May 17, 2021: At the moment I don’t expect/plan to write a book on Scala.js, but in my research I came across this terrific set of short videos by Mark Lewis on the Play Framework and creating web applications with Scala. At video #69 he starts to talk about Scala.js.

As a note to self, if you see this sbt error message with sbt 1.4.8, it’s a known issue:

java.lang.NoClassDefFoundError: org/fusesource/jansi/AnsiOutputStream

The short solution to the problem is to upgrade to sbt 1.4.9 or newer. The problem and solution are discussed in this Github sbt issue.

The next release of Functional Programming, Simplified should be available by the end of the day (my day here in Colorado) Tuesday, September 12, 2017. If I add everything in, it will contain over 30 new lessons.

Version 0.1.4 of “Functional Programming, Simplified” is now available. The latest changes are:

- 13 new chapters on the StateT monad and monad transformers
- Ten new chapters on Domain Modeling
- Two new chapters on ScalaCheck
- New appendices on Anonymous Functions, and using def vs val in traits
- The PDF now has small 113 chapters, four appendices, and is 983 pages long

To accompany this latest update, the book is on sale for a few days.