Posts in the “scala” category

Scala Either FAQ: How do I get the value out of a Scala Either?

Solution

In this recipe I show many different ways to get the value out of a Scala Either, while accounting for the fact that an Either value may be a success value (a Right) or a failure value (a Left).

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:

I just wrote the following ZIO 2 question about how to use ZIO.cond to a friend, and get the answer shown. I’ve also added in my own comments where they make sense.

ZIO.done question

Hey, I’m trying to understand why the failWithMsgEffect doesn’t seem to get run in the following code example?

I have learned that there are better ways to handle this, but I’ve found that if I don’t understand something like this, it will come back to bite me later. Here’s the code:

As a little ZIO 2 example with Scala 3, here’s some code that starts to show how to use ZIO.timeout along with ZIO.attempt while accessing an internet URL with Scala’s Source.fromURL.

Basically I attempt to access a URL using Scala’s Source.fromURL, and then I add a timeout to that, specifically a ZIO##timeout:

As a little note here today, here’s an example of how to use a Scala Either inside a for-expression when using ZIO 2. Specifically I convert each Either into a ZIO in each line of the for-expression.

April, 2024 Update: This ZIO cheatsheet is currently being updated to ZIO 2.x, but it still needs a lot of work.

If you want a good cheat sheet right now, see this one on github. I’m creating my own as I learn ZIO and read the ZIOnomicon book. During the learning process I find that it’s much better to create your own by hand, that way you get something that’s meaningful to you.

Note that almost all of these initial examples come from the ZIOnomicon book and the video that I link to later.

The ZIO Scaladoc

Here’s a link to the ZIO Scaladoc. That’s for the companion object, and this link is for the companion trait.

As I work more with ZIO 2, I also find myself thinking a lot about the ZIO 2 mental model, by which I partially mean “a way of thinking about your code” and also “ZIO 2 best practices.”

Here are my initial notes. Also, I hope that most of this code is mine, but several snippets are from other sources that I don’t remember, including the official ZIO 2 docs.

Scala URL FAQ: How do I download the contents of a URL to a String or file in Scala?

I ran a few tests last night in the Scala REPL to see if I could think of different ways to download the contents of a URL to a String or file in Scala, and came up with a couple of different solutions, which I'll share here.

Download URL contents to a String in Scala

The best way I could think of to download the contents of a URL to a String looks like this:

As a brief Scala note today, in Scala 3 it doesn’t look like there’s a way to use something like a package object to make some initial code available to all sub-packages of a high-level package. That is, I’d like to be able to define some types in a top-level package like this:

As a brief Scala ZIO 2 note, if you are using ZIO.attempt and want to show the actual exception you are working with (instead of just Throwable), use refineToOrDie, as shown in this example:

Scala math FAQ: How do I square a number in Scala, such as squaring an Int, Double, Long, or Float?

Solution

You can square a number in Scala in at least two different ways:

1. Multiply the number by itself
2. Call the Java Math.pow function or the scala.math.pow function

Scala 3 FAQ: What are opaque types in Scala?

Discussion

I previously wrote a little about Opaque Types in Scala 3, and today, as I’m working on a new video about opaque types, I thought I’d add some more information about them.

Scala dates FAQ: How do I calculate the difference between two dates? That is, while using Scala — Scala 2 or Scala 3 — you need to determine the difference between two dates. Also, you want to use the newest Java date/time API for this work, such as the date/time API in Java 8, 11, 14, 17, etc.

Solution: Calculating the difference between two dates (in Scala and Java)

If you need to determine the number of days between two dates in Scala — or Java or Kotlin — the DAYS enum constant of the java.time.temporal.ChronoUnit class provides the easiest solution:

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:

NOV., 2022: My new book, Learn Functional Programming The Fast Way, is currently an Amazon Java and functional programming #1 new release. The book is now available in three formats:

As a quick note, if you want to embed a Scala source code example in your Scaladoc comments, just put the source code block in between {{{ and }}} characters in your comments, as shown in this example:

I know that it’s fashionable to say that Scala is dead or something similar to that, but I’ve gotten a few royalty checks from O’Reilly this year for the Scala Cookbook that are amazingly good (💰) for a 3-year old programming book.

Shoot, I’d be happy to receive these royalty payments during the initial months after the book’s release.

As Bjarne Stroustrup said, “There are only two kinds of programming languages: the ones people complain about and the ones nobody uses.”

Functional Programming, Simplified — currently 5-star rated on Gumroad.com, 4.5-star rated on Amazon, and one of the all-time best-selling books on functional programming — is currently on sale in three formats (prices shown in USD):

Ammonite FAQ: How do I load managed dependencies into the Ammonite REPL? What is the syntax for loading dependencies (and JAR files, too)?

Solution

If you want to import a managed dependency into the Ammonite REPL, the solution is to use its import $ivy syntax:

As a brief note today, if you want to see an example of the ZIO collectAllPar method, the Scala 3 source code below shows one possible example that uses collectAllPar in a for expression.

First, here’s a small snippet of code that shows just the key parts: