Scala Scripting

BetterWay.sh

#!/usr/bin/env sbt -Dsbt.version=1.5.0 -Dsbt.main.class=sbt.ScriptMain -Dsbt.supershell=false -error

// Note that `ScriptMain` doesn't work with Scala 3 yet, and won't unless a
// volunteer steps up: https://github.com/sbt/sbt/issues/6274

/***
scalaVersion := "2.13.5"
onLoadMessage := ""
scalacOptions ++= Seq(
 "-deprecation", "-unchecked", "-feature", "-Werror")
libraryDependencies ++= Seq(
  "org.http4s" %% "http4s-ember-client" % "1.0.0-M21",
)
*/

import cats.effect._
import cats.effect.std.Console

object HelloWorld extends IOApp.Simple {
  val run = Console[IO].println("Hello world!")
}

HelloWorld.main(Array())

Scripting.scala

import cats.syntax.all._
import cats.effect._
import scala.concurrent.duration._
import java.nio.file.Paths

object Main extends IOApp {
  val console = cats.effect.std.Console.make[IO]
  def run(args: List[String]): IO[ExitCode] = {
    for {
      _ <- IO(println(s"Args: $args"))
      path <- IO(Paths.get(args.last))
      fileContent <- fs2.io.file.Files[IO].readAll(path, 512)
        .through(fs2.text.utf8Decode)
        .compile
        .string
      parsed <- Parser.simpleParser(fileContent).liftTo[IO]
      _ <- console.println(parsed)
      
    } yield ExitCode.Success
    //
    //Args: List(./test.scsh)
    //(List((interpreter, IOApp.Simple), (scala, 2.13.5), (dependency, "org.http4s" %% "http4s-ember-client" % "1.0.0-M21")),
    //
    //import cats.effect._
    //import cats.effect.std.Console
    //
    //def main: IO[Unit] = Console[IO].println("Hello world!"))

  }

}
// Optional #!
// Uninterrupted section of comments broken by newlines
// Terminated by /n/n or /r/n/r/n
// name/scala/sbt/interpreter/dependency headers defined
object Parser {
  def simpleParser(inputText: String): Either[Throwable, (List[(String, String)], String)] = Either.catchNonFatal{
    val text = {
      val base = inputText
      if (base.startsWith("#!")) {
        val idx = base.indexOf("\n")+1
        val out = base.substring(idx)
        out
      } else base
    }
    // println(s"Text: $text")
    val (startText, restText) = {
      val idx = text.indexOf("\n\n")
      if (idx == -1) throw new Throwable("No Headers Found Require name/scala/interpreter")
      else text.splitAt(idx)
    }
    val headersLines = fs2.Stream(startText)
      .through(fs2.text.lines)
      .takeWhile(_.startsWith("//"))
      .filter(x => x.contains(":")) // Comments are allowed that dont follow x:z
      .compile
      .to(List)
    val headers = headersLines.map{
      s =>
        val idx = s.indexOf(":") // :space is required in for a valid header
        val header = s.slice(2, idx).trim()
        val value = s.slice(idx + 1, s.length())
        (header, value)
    }
    (headers, restText)
  }
}

object Files {
  def buildFile(name: String, scala: String, dependencies: List[String]): String = {
    s"""
    |scalaVersion := "$scala"
    |name := "$name"
    |
    |${dependencies.map(s => s"libraryDependencies += $s").intercalate("\n")}
    |""".stripMargin
  }
  def buildProperties(sbt: String) = s"sbt.version=$sbt\n"
}

test.scsh

#!./core/target/universal/stage/bin/scripting
// interpreter: IOApp.Simple
// scala: 2.13.5
// dependency: "org.http4s" %% "http4s-ember-client" % "1.0.0-M21"

import cats.effect._
import cats.effect.std.Console

def main: IO[Unit] = Console[IO].println("Hello world!")