Picking the Scala version with scala-cli

By default, scala-cli runs the latest supported scala version by Scala CLI. See our list of Supported Scala Versions in Scala CLI.

To demonstrate how this works, here’s a universal piece of code that detects the Scala version at runtime. The code is a bit complicated, so we suggest that you skip reading the whole file, and just focus on what it prints:

ScalaVersion.scala

object ScalaVersion extends App {
  def props(url: java.net.URL): java.util.Properties = {
    val properties = new java.util.Properties()
    val is = url.openStream()
    try {
      properties.load(is)
      properties
    } finally is.close()
  }

  def scala2Version: String =
    props(getClass.getResource("/library.properties")).getProperty("version.number")

  def checkScala3(res: java.util.Enumeration[java.net.URL]): String =
    if (!res.hasMoreElements) scala2Version else {
      val manifest = props(res.nextElement)
      manifest.getProperty("Specification-Title") match {
        case "scala3-library-bootstrapped" =>
          manifest.getProperty("Implementation-Version")
        case _ => checkScala3(res)
      }
    }
  val manifests = getClass.getClassLoader.getResources("META-INF/MANIFEST.MF")

  val scalaVersion = checkScala3(manifests)
  val javaVersion = System.getProperty("java.version")

  println(s"Scala: $scalaVersion")
}

When this application is run without specifying a Scala version, it uses the latest stable release of Scala — 3.1.0 at the time of writing this doc:

scala-cli ScalaVersion.scala

When you want to control the Scala version, you can control it from the command line using the --scala option (with -S and --scala-version aliases):

scala-cli -S 2.13.5 ScalaVersion.scala

In many cases you won't care for a precise Scala version and will want "any Scala 2" or "any 2.13 release." For this situation, scala-cli accepts version prefixes like this:

scala-cli -S 2 ScalaVersion.scala

and this:

scala-cli -S 2.12 ScalaVersion.scala

In the first example (-S 2), the application picks up the latest Scala 2 stable release (2.13.9 at the time of this writing). In the second example, the application picks up the latest stable release of 2.12 (which is 2.12.17 at the time of this writing).

You can also pin the version of the language within a .scala file with using directives.

info

The using directives syntax is still experimental, and may change in future versions of scala-cli.

Here’s an example of a source code file named version.scala that contains a using directive:

version.scala

//> using scala "2.12"

object OldCode
//rest of the config

Now when you compile that code along with the previous ScalaVersion.scala file:

scala-cli ScalaVersion.scala version.scala

The output at the time of this writing is "2.12.17".

The scala-cli philosophy is “command line first,” so any configuration information that’s passed to the command line will override using directives. So when you run this command with the -S option:

scala-cli -S 2.13.10 ScalaVersion.scala version.scala

the result is "2.13.10" (as opposed to "2.12.17" in the previous example).

note

See our Using Directives Guide for more details on using directives.

When should I provide a full version of Scala?

For prototyping, scripting, and other use cases that won’t need to be run multiple times in the future, providing a Scala version generally isn’t necessary.

Scala is source and binary compatible within each major version (e.g., 2.12.x or 3.1.x) so specifying the version in epoch.major form (e.g., 2.12, 2.13, or 3.1) should be perfectly fine for most use cases. When your Scala code contains more advanced features that may be more sensitive for changes in minor version (e.g., from 2.13.4 to 2.13.5) we recommend specifying the complete Scala version.