Integrate MainAnnotation with scala-cli scripts

[anatoliykmetyuk]

scala-cli can be used to execute scripts. A CLI script file, *.sc, performs the same role as the @main annotation in a normal Scala program: it marks a block of code as a runnable program.

There is a pending PR to Dotty that aims to allow users to define their own annotations similar to @main (docs in PR). This mechanism can be used e.g. by framework authors to provide their users with a standard way to run the program based on their library. For example, in Cask, to define an application you need to extend cask.MainRoutes class.

It would be good to also allow scala-cli scripts to support the customization mechanism like that. For example, take the following file from Cask documentation:

object MinimalApplication extends cask.MainRoutes{
  @cask.get("/")
  def hello() = {
    "Hello World!"
  }

  @cask.post("/do-thing")
  def doThing(request: cask.Request) = {
    request.text().reverse
  }

  initialize()
}

I propose to be able to write it as a Scala CLI script as follows:

//> using Scala 3.1.2
//> MainAnnotation: com.haoyi.cask.CustomMainAnnotationToBeImplemented   // or another similar way to configure it

@cask.get("/")
def hello() = {
  "Hello World!"
}

@cask.post("/do-thing")
def doThing(request: cask.Request) = {
  request.text().reverse
}

Notice how the class definition and the initialize() call go away – abstracted to the referenced macro.

The impact of the above is a simplified way to get started not only with basic scripts but more complex programs like a web server.

[bjornregnell]

Look's like a useful feature, and in the Scala spirit of generalizing concepts and allowing libraries to enable features. Nice! :)

[kubukoz]

Could this mechanism also be used for cats-effect's IOApp?

In .scala:

//foo.scala
import cats.effect._
object Main extends IOApp.Simple {
  override def run: IO[Unit] = IO.println("foo")
}

In .sc:

//foo.sc
//> MainAnnotation: cats.effect.IOApp

IO.println("foo")