Posts in the “scala” category

This is an excerpt from the Scala Cookbook, 2nd Edition. This is Recipe 23.9, Simulating Dynamic Typing with Union Types.

Problem

When using Scala 3, you have a situation where it would be helpful if a value could represent one of several different types, without requiring those types to be part of a class hierarchy. Because the types aren’t part of a class hierarchy, you’re essentially declaring them in a dynamic way, even though Scala is a statically-typed language.

As a “note to self” about using ZIO 2, here are a few ZIO examples that show the ZIO.fail, ZIO.succeed, ZIO.attempt, orElseFail, and orDie methods and functions.

Before looking at the following code, it’s important to know that it uses the following Scala 3 enum:

When I first saw Scala generic type and multiple-parameter group code like this ~12 years ago, my initial thought was, “Wow, maybe I need to think about a different career”:

def race[A, B](lh: IO[A], rh: IO[B])(implicit cs: ContextShift[IO]): IO[Either[A, B]]

But in the end, as Rocky once said, it ain’t so bad. :)

Here are a few nice blog posts on functional error-handling in Scala:

• Easing into functional error handling in Scala
• How do I error handle thee?
• Designing fail-fast error handling
• My own Functional error handling in Scala
• My free “functional error handling in Scala” training video

Note: This is an excerpt from my book, Functional Programming, Simplified. (No one has reviewed this text yet. All mistakes are definitely my own.)

Defining “Functional Programming”

It’s surprisingly hard to find a consistent definition of functional programming. As just one example, some people say that functional programming (FP) is about writing pure functions — which is a good start — but then they add something else like, “The programming language must be lazy.” Really? Does a programming language really have to be lazy (non-strict) to be FP? (The correct answer is “no.”)

I share links to many definitions at the end of this lesson, but I think you can define FP with just two statements:

I originally wrote a long introduction to this tutorial about how to work with the Scala Option/Some/None classes, but I decided to keep that introduction for a future article. For this article I’ll just say:

• idiomatic Scala code involves never using null values
• because you never use nulls, it’s important for you to become an expert at using Option, Some, and None
• initially you may want to use match expressions to handle Option values
• as you become more proficient with Scala and Options, you’ll find that match expressions tend to be verbose
• becoming proficient with higher-order functions (HOFs) like map, filter, fold, and many others are the cure for that verbosity

Given that background, the purpose of this article is to show how to use HOFs rather than match expressions when working with Option values.

As a brief note today, here is some source code for a ZIO ZLayer application using Scala 3. In this code I use the ZLayer framework to handle some dependency injection for a small application. (Note that I don’t like to use the word “simple” when writing about computer programming, but I have tried to make this as simple as I can.)

I’ve commented the code below as multiple “parts” so you can see the thought process of creating an application that uses ZLayer. Basically the idea is that your application needs some sort of service — which might be like a database connection pool, HTTP framework, etc. — and then you make that service available to your application with ZLayer’s provideLayer function (or one of its other functions).

The ZLayer example

Given that small introduction, here’s my ZIO ZLayer example, with many notes shown in the comments inside the code:

This is an excerpt from the Scala Cookbook, 2nd Edition. This is Recipe 24.7, Building Modular Systems with Scala 3.

Problem

You’re familiar with Martin Odersky’s statement that Scala developers should use “functions for the logic, and objects for the modularity,” so you want to know how to build modules in Scala.

This page contains a large collection of examples of how to use the methods on the Scala List class.

Scala List class introduction

The List class is an immutable, linear, linked-list class. It’s very efficient when it makes sense for your algorithms to (a) prepend all new elements, (b) work with it in terms of its head and tail elements, and (c) use functional methods that traverse the list from beginning to end, such as filter, map, foldLeft, reduceLeft.

In today’s installation of “how to have fun with Scala,” if you want to (a) define a Scala method that takes an input parameter, and (b) that parameter has a generic type, and (c) you further want to further declare that the parameter must extend some base type, then (d) use this “bounds” syntax:

def getName[A <: RequiredBaseType](a: A) = ???

This example can be read as, “The parameter a has the generic type A, and A must be a subtype of RequiredBaseType.”

A complete Scala bounds example

As a concrete example of how using bounds works, start with a simple base type, such as this Scala trait:

trait SentientBeing {
    def name: String
}

Next, extend that base trait with a few more traits:

trait AnimalWithLegs extends SentientBeing
trait TwoLeggedAnimal extends AnimalWithLegs
trait FourLeggedAnimal extends AnimalWithLegs

Then extend those with some concrete case classes:

case class Dog(name: String) extends FourLeggedAnimal
case class Person(name: String, age: Int) extends TwoLeggedAnimal
case class Snake(name: String) extends SentientBeing

Notice that Snake extends SentientBeing, but not AnimalWithLegs.

Now that you have all the types you need, define a method that takes a parameter that has a generic type that must extend some base type. To see how everything works, define it this way the first time:

def getName[A <: SentientBeing](a: A): String = a.name

Because the base type (or “super type”) is SentientBeing, all of these calls work just fine:

getName(Person("Fred", 20))
getName(Dog("Rover"))
getName(Snake("Noodles"))

(Copy and paste those into the Scala REPL if you want to verify they work as advertised.)

Now extend AnimalWithLegs

Next, change getName so the generic type A must be a subtype of AnimalWithLegs:

def getName[A <: AnimalWithLegs](a: A): String = a.name

Now, when you run the same three method calls again, you’ll see that the Snake example fails because it doesn’t extend AnimalWithLegs:

getName(Person("Fred", 20))
getName(Dog("Rover"))
getName(Snake("Noodles"))   //error

Here’s what the two sets of getName examples look like in the Scala REPL:

(Right-click that image and select “View image” to see it larger.)

The Scala “type parameter bounds” error

As shown, the second Snake example results in this error message:

scala> getName(Snake("Noodles"))
<console>:15: error: inferred type arguments [Snake] do not conform to method getName's 
type parameter bounds [A <: AnimalWithLegs]
       getName(Snake("Noodles"))
       ^
<console>:15: error: type mismatch;
 found   : Snake
 required: A
       getName(Snake("Noodles"))
                    ^

This is because the type Snake does not extend AnimalWithLegs. (At least not in my world.)

Technical matters: ‘A’ has an “upper bound”

Technically what’s happening here is that I’m defining A with an “upper bound.” Bounds let you place restrictions on type parameters, and in this example I’m saying, “A must be a subtype of the type AnimalWithLegs”:

def getName[A <: AnimalWithLegs](a: A): String = a.name

More generally, this is how you say, “A must be a subtype of B”:

A <: B

I write more about this topic in, An introduction to Scala Types, so please see that article for a few more details.

A “bounds” video

If you’re interested in more information on this topic, see my free “Scala 3 Bounds” training video.

Here’s an example of Union Types in Scala 3 (Dotty). This image comes from this Martin Odersky video.

UPDATE: For more information, see my free “Scala 3 Union Types” video.

As a brief note today, here’s an example of making an HTTP GET request using ZIO 2 and the Scala sttp library. I also let the user specify a “timeout” value, so the request will timeout, rather than hanging.

As a very important note, this is a blocking approach, not a non-blocking approach.

ZIO 2 HTTP GET request with a timeout example

Here’s the source code and Scala 3 + ZIO 2 + sttp function:

ZIO FAQ: How do I create a very simple ZLayer in a ZIO 2 application?

Solution

As a wee bit of background, the ZIO Zlayer approach provides several important purposes, including:

• Dependency injection
• Modularity and composability
• Resource management
• Testability
• Separation of concerns
• Type safety

Scala/Ammonite FAQ: How do I use ZIO 2 in the Ammonite REPL?

Solution

ZIO can be added into the Ammonite REPL as a managed dependency by using Ammonite’s import $ivy syntax:

Without much explanation, here’s a Scala 3 and ZIO 2 example that shows how to process command-line arguments (command-line input) in a ZIO 2 application:

A cool feature of the Scala REPL is that you can reset/clear a REPL session. To do so, just issue the :reset command, like this:

scala> :reset
Resetting interpreter state.
Forgetting this session history:

Assuming that you already have at least a little history in your REPL session, the :reset command will show you everything that it dumps, so the full output looks more like this:

When you want to test a multiline command/statement in the Scala REPL, you can easily run into a problem where the REPL gets confused, or more accurately, it attempts to evaluate your statement too early.

As a simple example of this, imagine that you want to test the Scala "if" statement syntax. You begin typing your if statement normally, but when you hit [Enter] after your second line, you'll see that everything blows up:

As a brief note today, this post includes an example that uses Scala 3, scala-cli, and a Typesafe HOCON configuration file. As a bit of an aside, I’m using Scala code like this to create “video” tags for the HTML files in my Scala 3 and ZIO 2 video training courses.

First, here’s the HOCON configuration file, which I named videos.conf:

As I mention in my free functional programming videos, if you had the desire to see your code as math, you might have discovered FP techniques yourself.

As an example, 20 years ago I couldn’t get my sysadmin to install a server-side spam filter at my consulting firm because he was always “too busy.”

So I wrote my own spam-filtering algorithm, and then someone with a CS degree looked at it and said, “That’s a Bayesian algorithm.” I had no idea what that was, I just knew that I was sick and tired of seeing spam all the time.

I later presented this solution at the 2004 Borland Conference (demonstrating how JBuilder helped at that time).

This page contains over 170 examples of how to use the Scala Vector class, covering most of the methods that are available on a Vector, including map, filter, foldLeft, reduceLeft, and many more.

Scala Vector class video

As a brief update, if you prefer videos, here is my Scala Vector class training video.

The Scala Vector class

Use the Vector class when you want to use a general-purpose, immutable indexed sequence in your Scala applications: