Posts in the “scala” category

Here’s a quick Scala example that shows how to convert multiple spaces in a string to a single space:

scala> val before = "  foo  bar   baz bonk "
before: String = "  foo  bar   baz bonk "

scala> val after = before.trim.replaceAll(" +", " ")
after: String = foo bar baz bonk

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:

In this ZIO performance tuning article by Pierre Ricadat, there’s this nice tip about using ZIO.foreachDiscard instead of ZIO.foreach:

“One of ZIO's common operators is ZIO.foreach, which allows you to run an effect for each item in a collection. There's also ZIO.foreachDiscard that is faster if you don't need to collect results (e.g., if your effect returns Unit).”

For more details, see that article.

Scala performance FAQ: Is there a reliable way to determine the number of CPUs or cores when writing Scala code?

Solution

As I was reading this article on ZIO performance tuning, I decided to look into whether there is a reliable way to determine the number of CPUs or cores on a computer using Scala (and therefore Java, Kotlin, and other JVM languages).

The solution is that there seems to be a reliable way to determine the number of CPUs or cores in Scala. In short, use the Java Runtime class to get the information. This Scala example shows how to do it:

As a quick note to self, I wrote this Scala code as a way to (a) find the first element in a sequence and then (b) return that element without traversing the rest of the sequence.

I initially thought about writing this code with a while loop, for loop, or for expression — because I knew I needed a loop and a way to break out of a loop — but then I realized that an iterator would help me out here.

A first solution

So without any further ado, here’s this solution:

When it comes to generating random strings with the scala.util.Random class, I haven’t figured out yet how to properly use the nextString method. I’ve tried using it in several different ways, but I always get a string of question marks as output, like this:

Without any introduction or discussion, here’s a little Scala script I just wrote to read in a text file that contains stocks symbols, with one symbol per line, along with some blank lines; then convert those symbols to comma-separated output (CSV format) that I print to STDOUT:

I just had this problem in Scala where I wanted to concatenate two corresponding multiline strings into one final multiline string, such that the elements from the first string were always at the beginning of each line, and the lines from the second string were always second. (When I say corresponding, I mean that the two strings are of equal length.)

That is, given two Scala multiline strings like these:

If you want to get a value out of a Scala Option type, there are a few ways to do it. In this article I’ll start by showing those approaches, and then discuss the approach of using the fold method on an Option (which I’ve seen discussed recently on Twitter).

1) Getting the value out of an Option, with a backup/default value

As a first look at getting values out of an Option, a common way to extract the value out of a Scala Option is with a match expression:

I’ve been slowly working on a series of new Scala programming books, and today I’m proud to announce the first of these:

Starting today you can buy the PDF version of Learn Scala 3 The Fast Way! for just ten dollars — $10 (USD) — at this Gumroad.com URL.

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):

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.

ZIO 2 FAQ: How do I work with the ZIO.attempt function?

Solution

When you’re working with existing (legacy) Scala code that:

• is synchronous, and
• can throw an exception

wrap that code with ZIO.attempt. As you’re about to see, this creates a ZIO effect from that legacy code.

Not only does this create a ZIO effect, it also puts the exception in the ZIO “error channel,” i.e., the E parameter in the ZIO[R, E, A] type signature.

As a brief note today, here’s a Scala 3 function that counts the number of vowels in the String it is given as input:

def countVowels(s: String): Int =
    val vowels = Set('a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U')
    s.count(vowels.contains)

Note that this works because as I have mentioned in other places, a Scala Set can be used as a function — specifically as a predicate — and the count function on the Scala sequence classes expects a predicate.

Scala 2 solution

As a brief note today, here’s a Scala function to get the Unix epoch time for X days ago (5 days ago, 10 days ago, etc.):

/**
 * Returns a 10-digit Long (like 1585275929) representing the date/time.
 * Use it to get the time for 1 day ago, 2 days ago, etc. `0` will give
 * you the current time.
 */
def unixEpochTimeForNumberOfDaysAgo(numDaysAgo: Int): Long = {
    import java.time._
    val numDaysAgoDateTime: LocalDateTime = LocalDateTime.now().minusDays(numDaysAgo)
    val zdt: ZonedDateTime = numDaysAgoDateTime.atZone(ZoneId.of("America/Denver"))
    val numDaysAgoDateTimeInMillis = zdt.toInstant.toEpochMilli
    val unixEpochTime = numDaysAgoDateTimeInMillis / 1000L
    unixEpochTime
}

As shown in the comments, if you give it a 0 it will return the current epoch time. As shown by the function’s type signature, the function’s return type is a Long (which is a 64-bit two's complement integer).

Of course you can make the code shorter and better; I just wanted to show the steps in the approach using the Date/Time classes that were introduced in Java 8.

Scala FAQ: Using Scala, how do I generate random numbers without duplicate values, i.e., how do I generate a sequence of random, unique values?

Solution

To show the solution, here’s a Scala 3 function that generates a random sequence of unique integer values:

UPDATE: The latest version of this code is at github.com/alvinj/TextPlottingUtils.

Creating an ASCII version of a Sparkline chart — also known as a Win/Loss chart — in Scala turned out to take just a few lines of code, as shown in this source code:

As a brief note today, this is a sample Git .gitignore file that I use for Scala projects, including Scala CLI projects, sbt projects, and using VS Code or IntelliJ IDEA as your IDE:

If you ever need to send a text message on a macOS system when using Scala (or Java, Kotlin, or other JVM languages), I just tested this function, and it seems to work:

When using Scala (or Java, Kotlin, or other JVM languages), and (a) you want to put a String on the system clipboard and/or (b) get a String off the system clipboard — i.e., copying and pasting text strings — I just used these functions, and can confirm that they work: