Scala, Java, Unix, MacOS tutorials (page 56)

I was just reminded that the “object MyApp extends App” syntax is basically deprecated in Dotty. The correct approach in Scala 3 is to use an @main method, or to manually define a main method.

In my case I was trying to run an old Scala 2 futures example with Dotty without updating the App syntax, and ran into a problem. Once I switched to using the @main method everything worked as expected.

Many thanks to Guillaume Martres for his help!

As a brief note to self, if you need to run a main method in a JAR file created with Dotty (Scala 3), and you want to run execute that application using the java command, the syntax is the same as before, you just have to figure out which JAR files to include in your classpath.

I just created an example using Scala futures and found that I could execute/run the example using this command:

java -cp "/path/to/dotty-library_0.27-0.27.0-RC1.jar:/path/to/scala-library-2.13.3.jar:target/scala-0.27/multiplefutures2_0.27-0.1.0.jar" futures.MultipleFutures2

In that command, multiplefutures2_0.27-0.1.0.jar is the name of my JAR file, and futures.MultipleFutures2 is the name of my package and main method, respectively.

If you want to execute a JAR file built with Dotty, I hope this example is helpful

As a brief note, this example shows how to use the Scala 3 infix annotation — @infix. This solution also shows how to use the infix annotation with Scala 3 extension methods, because that’s what I need for my current situation:

import scala.annotation.infix

extension (i: Int)
   @infix def plus(j: Int) = i + j
   @infix def times(j: Int) = i + j

Given those infix + extension method definitions, this is how you use those infix methods:

1 plus 1    // 2
2 times 2   // 4

The REPL confirms this:

scala> 1 plus 1
val res0: Int = 2

scala> 2 times 2
val res1: Int = 4

Before I go, here’s a note from the Dotty / Scala 3 documentation related to infix methods:

A method annotation that suggests that the annotated method should be used as an infix operator. Infix operations with alphanumeric operator names require the operator to be annotated with @infix

In summary, if you wanted to see how to create a Scala 3 infix method, particularly as an extension method, I hope this example is helpful.

Here’s an example of how to use the Scala 3 @infix annotation to create an infix method, while using the technique with extension methods at the same time:

import scala.annotation.infix

extension (i: Int)
   @infix def plus(j: Int) = i + j
   @infix def times(j: Int) = i + j

That code adds the extension methods plus and times to the Scala Int class, while also letting them be used as infix methods. The REPL shows the result:

scala> 1 plus 1
val res0: Int = 2

scala> 2 times 2
val res1: Int = 4

In summary, if you wanted to see how to write infix methods and extension methods with Scala 3, I hope this example is helpful