match

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

By Alvin Alexander. Last updated: February 16 2020

I originally wrote a long introduction to this article about how to work with the Scala Option, but I decided to keep that introduction for a future second article in this series. 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

An example of using enums in Scala 3 (Dotty)

By Alvin Alexander. Last updated: December 14 2019

Here’s a quick example of how to use Scala 3 (Dotty) enums, including using them as constructor and method parameters, and in a match expression. First, some Scala 3 enums for a pizza store application:

A Scala 3 (Dotty) enum match/case expression example

By Alvin Alexander. Last updated: August 22 2019

Here’s a little Scala 3 (Dotty) enum match/case expression example:

Scala match/case expressions (syntax, examples)

By Alvin Alexander. Last updated: August 9 2019

Scala match expressions
Aside: A quick look at Scala methods
Using a match expression as the body of a method
Handling alternate cases
Using if expressions in case statements
Even more ...

Table of Contents1 - Scala match expressions2 - Aside: A quick look at Scala methods3 - Using a match expression as the body of a method4 - Handling alternate cases5 - Using if expressions in case statements6 - Even more ...

This is a lesson on Scala match/case expressions from my book, Hello, Scala.

Scala match expressions

Scala has a concept of a match expression. In the most simple case you can use a match expression like a Java switch statement:

// i is an integer
i match {
    case 1  => println("January")
    case 2  => println("February")
    case 3  => println("March")
    case 4  => println("April")
    case 5  => println("May")
    case 6  => println("June")
    case 7  => println("July")
    case 8  => println("August")
    case 9  => println("September")
    case 10 => println("October")
    case 11 => println("November")
    case 12 => println("December")
    // catch the default with a variable so you can print it
    case _  => println("Invalid month")
}

As shown, with a match expression you write a number of case statements that you use to match possible values. In this example I match the integer values 1 through 12. Any other value falls down to the _ case, which is the catch-all, default case.

My Scala Sed project: More features, returning strings

By Alvin Alexander. Last updated: July 11 2019

Basic use
Using a Map
Match expressions
Sed limitations
My Sed project
Bonus: Factories and HOFs

Table of Contents1 - Basic use2 - Using a Map3 - Match expressions4 - Sed limitations5 - My Sed project6 - Bonus: Factories and HOFs

My Scala Sed project is still a work in progress, but I made some progress on a new version this week. My initial need this week was to have Sed return a String rather than printing directly to STDOUT. This change gave me more ability to post-process a file. After that I realized it would really be useful if the custom function I pass to Sed had two more pieces of information available to it:

The line number of the string Sed passed to it
A Map of key/value pairs the helper function could use while processing the file

Note: In this article “Sed” refers to my project, and “sed” refers to the Unix command-line utility.

Basic use

In a “basic use” scenario, this is how I use the new version of Sed in a Scala shell script to change the “layout:” lines in 55 Markdown files whose names are in the files-to-process.txt file:

A good reason to use sealed traits and classes in Scala

By Alvin Alexander. Last updated: March 28 2019

Read more about A good reason to use sealed traits and classes in Scala

This scala-lang.org documentation page shares a good reason to use “sealed” traits and classes: When you created sealed traits, the compiler can easily tell all of the subtypes of your class or trait, and as just one benefit, you don’t need to add a default, “catch-all” case in your Scala match expressions.

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Scala: How to use regex pattern matching in a match expression

By Alvin Alexander. Last updated: July 11 2019

Scala FAQ: How can I use regular expression (regex) pattern matching in a match expression (a Scala match/case expression)?

As I wrote in my Scala sed class post earlier today, Jon Pretty’s Kaleidoscope project lets you use string pattern-matching code in Scala match expressions. This enables regex pattern-matching code like this:

A little Scala `sed` class

By Alvin Alexander. Last updated: July 6 2019

A few times during the past year I got tired of trying to remember the Unix/Linux sed syntax while wanting to make edits to many files, so this weekend I wrote a little sed-like Scala class.

Scala: How to use fold on an Option (syntax)

By Alvin Alexander. Last updated: February 16 2020

Directly extracting the value out of the Option
Applying a function while extracting the value from the Option
Discussion: Using fold on Option
How fold works on a sequential collection
More

Scala/Java: How to write a pattern that matches a minimum to maximum number of specified characters

By Alvin Alexander. Last updated: April 24 2018

If you’re using Java or Scala and need to write a pattern that matches a range of characters, where those characters occur between a minimum and maximum number of times in the pattern, the following example shows a solution I’m currently using.

The idea is that the pattern "[a-zA-Z0-9]{1,4}" means, “Match a string that has only the characters a-z, A-Z, and 0-9, where those characters occur a minimum of one time and a maximum of four times.” The following tests in the Scala REPL shows how this works:

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