Posts in the “scala” category

Scala zip and zipWithIndex examples (with Stream)

I’ve known about using Scala’s zipWithIndex method for quite some time. I used it in for loops to replace counters, and it works like this:

scala> List("a", "b", "c").zipWithIndex
res0: List[(String, Int)] = List((a,0), (b,1), (c,2))

I learned about using zip with Stream last night while reading Joshua Suereth’s book, Scala In Depth. It works like this:

Scala List class examples: range, fill, tabulate, appending, foreach, more ...

Scala List FAQ: Can you share some Scala List class examples?

The Scala List class may be the most commonly used data structure in Scala applications. Therefore, it's very helpful to know how create lists, merge lists, select items from lists, operate on each element in a list, and so on.

In this tutorial, I'll share examples of the most common List operations (methods).

Scala/for/Option: How to process multiple Option values in a Scala ‘for’ loop

My last edits to the Scala Cookbook were in June, 2013, and after all this time there aren’t many things I wish I had added to the Cookbook. Yesterday I ran into one thing that I don’t think I included in the Cookbook: How to process multiple Option values in a Scala for loop (for comprehension). Here’s a quick look at how to do this.

For the impatient

For those who just want to see a for comprehension that processes multiple input Option values, here you go:

How to use multiple Futures in a Scala for-comprehension

If you want to create multiple Scala Futures and merge their results together to get a result in a for comprehension, the correct approach is to (a) first create the futures, (b) merge their results in a for comprehension, then (c) extract the result using onComplete or a similar technique.

Scala: What do “effect” and “effectful” mean in functional programming?

When you get started with functional programming (FP) a common question you’ll have is, “What is an effect in functional programming?” You’ll hear advanced FPers use the words effects and effectful, but it can be hard to find a definition of what these terms mean.

Effects are related to monads (but don’t worry)

A first step in the process of understanding effects is to say that they’re related to monads, so you have to know just a wee bit about monads to understand effects.

As I wrote in my book, Functional Programming, Simplified, a slight simplification is to say that in Scala, a monad is any class that implements the map and flatMap methods. Because of the way Scala for-expressions work, implementing those two methods lets instances of that class be chained together in for-expressions (i.e., for/yield expressions).

A Scala method to write a list of strings to a file

As a brief note today, here’s a Scala method that writes the strings in a list — more accurately, a Seq[String] — to a file:

def writeFile(filename: String, lines: Seq[String]): Unit = {
    val file = new File(filename)
    val bw = new BufferedWriter(new FileWriter(file))
    for (line <- lines) {
        bw.write(line)
    }
    bw.close()
}

How to define a default value for a method parameter that is a function

I was just reminded that functions are values just like Int, String, etc. Beyond the “typical” use cases, this is also true when it comes to supplying a default value for a function parameter that you’re passing into a method.

As some quick background, as I showed in the Scala Cookbook, you can define a method that provides default values for one or more of its parameters like this:

Creating a web browser with Scala Swing

Is Swing dead? I don’t know. I’ve written several Swing apps that I use every day, but I can’t speak for the rest of the world.

What I can say is that I really like Scala Swing. How I wish it was available ten years ago ... you’ll never know.

But getting to the point, after working with Scala Swing a little bit, I decided to see if I could write a web browser with it. The short answer is that here’s what the browser looks like:

Passing a block of code to a function in Scala (callbacks)

I've posted a lot of Scala source code examples out here lately, and as I keep trying to learn more about passing one function to another function in Scala (function callbacks), here's another example showing how you can use Scala's functional programming approach to clean up some Java Swing code:

Passing an anonymous function to SwingUtilities.invokeLater (in Scala)

As a quick note, if you ever need to call the invokeLater of the Java SwingUtilities class in Scala, you can pass it an anonymous function (lambda) like this:

SwingUtilities.invokeLater(() => {
    // your gui-updating code here, such as:
    for (c <- newsControllers) c.updateContent()
})

(I haven’t worked with Java 8 lambdas too much, but I assume that the Java lambda syntax is similar to that.)

Using Scala with Java Swing classes is pretty seamless

If you ever wanted to use Scala with Java Swing classes (like JFrame, JTextArea, JScrollPane, etc.), the process is pretty seamless. Here’s an example of a simple Scala/Swing application where I show a text area in a JFrame:

import java.awt.BorderLayout
import java.awt.Dimension
import javax.swing.JFrame
import javax.swing.JScrollPane
import javax.swing.JTextArea

object SwingExample extends App {

    val textArea = new JTextArea("Hello, Swing world")
    val scrollPane = new JScrollPane(textArea)

    val frame = new JFrame("Hello, Swing")
    frame.getContentPane.add(scrollPane, BorderLayout.CENTER)
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)
    frame.setSize(new Dimension(600, 400))
    frame.setLocationRelativeTo(null)
    frame.setVisible(true)

}

Technically you’ll want to display the JFrame the way I show in How to create, center, and display a Java JFrame, but this code gives you an idea of how the process works.

I’ve written much larger applications with Scala and Swing, but for today I just wanted to share some code to show that it’s a very straightforward process.

How to sort a Scala Array

Scala FAQ: How do I sort the elements in an Array in Scala?

Solution: If you’re working with an Array that holds elements that have an implicit Ordering, you can sort the Array in place using the scala.util.Sorting.quickSort method. For example, because the String class has an implicit Ordering, it can be used with quickSort:

Scala: How to use higher-order functions (HOFs) with Option (instead of match expressions)

I originally wrote a long introduction to this article 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.

Scala collections classes: Methods, organized by category

When I wrote the Scala Cookbook, I gave each recipe and then each chapter my full attention. I thought that if I wrote each recipe as well as possible, and included important recipes in each chapter, well, I wanted each chapter to be worth the price of the entire book. That was my goal.

As a result of this effort -- and perhaps to the chagrin of my editor -- the Scala collections chapters ended up being 130 pages in length.

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