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Scala Scripts with instant startup

scala-cli allows to easly compile and run Scala Scripts. It also allows for straightforward compilation with Scala Native. Scala Native is an ahead-of-time compiler to native binary allowing for instant startup times, meaning that along with scala-cli, it should perfectly suit the needs of a fast scripting tool.

Using Scala Native​

As an example, let’s build a script printing files from a directory with sizes bigger than a passed value.

size-higher-than.scala
//> using scala "3.1.1"
//> using lib "com.lihaoyi::os-lib::0.8.1"
 
@main
def sizeHigherThan(dir: String, minSizeMB: Int) =
  val wd = os.pwd / dir
  val files = os.walk.attrs(wd).collect{
    case (p, attrs) if attrs.size > minSizeMB * 10E6 => p
  }
  files.foreach(println(_))

Running this for a dir directory and 20 MB as a lower limit with scala-cli size-higher-than.scala – dir 20 can give us for example:

scala-cli size-higher-than.scala -- dir 20
Compiling project (Scala 3.1.1, JVM)
Compiled project (Scala 3.1.1, JVM)
/Users/user/Documents/workspace/dir/large-file.txt

A keen eye will notice that we have not yet compiled to Scala Native. We are still running on the JVM! We can fix that by either running with a —-native option, or, in this case, by including an additional using directive:

size-higher-than.scala
//> using scala "3.1.1"
//> using lib "com.lihaoyi::os-lib::0.8.1"
//> using platform "scala-native"
 
@main
def sizeHigherThan(dir: String, minSizeMB: Int) =
  val wd = os.pwd / dir
  val files = os.walk.attrs(wd).collect{
    case (p, attrs) if attrs.size > minSizeMB * 10E6 => p
  }
  files.foreach(println(_))

After rerunning, you may notice that while the initial compilation took a little longer, subsequent runs will severely cut on the startup time compared to the JVM.

Optimization options​

We can make the runtime itself even faster, using various Scala Native optimization options:

  • debug - what was used by default up to this point, fast compilation with a slower runtime
  • release-fast - moderate compilation time with a faster runtime
  • release-full - slow compilation time with the fastest runtime

We pass these using a -–native-mode scala-cli option or, like previously, by adding a using directive:

size-higher-than.scala
//> using scala "3.1.1"
//> using lib "com.lihaoyi::os-lib::0.8.1"
//> using platform "scala-native"
//> using nativeMode "release-full"
 
@main
def sizeHigherThan(dir: String, minSizeMB: Int) =
  val wd = os.pwd / dir
  val files = os.walk.attrs(wd).collect{
    case (p, attrs) if attrs.size > minSizeMB * 10E6 => p
  }
  files.foreach(println(_))

We can also package this script into a separate binary with the package command, useful especially on Windows where typically shebangs won’t work:

Additional considerations​

Some things to look out for when working with Scala Native:

  • dependencies - libraries have to be published separately for Scala Native. Notice the org::project::version double colon syntax used for os-lib - it basically hides org::project_native[Scala Native binary version]:version underneath. Fortunately, many libraries are already available for Scala Native. However, Java dependencies will not work altogether.
  • some differences exist when compared to Scala on the JVM.