xsbt coffee script plugin (example) using sourceGenerators

CoffeeScriptPlugin.scala

import sbt._
import Keys._
import Process._
import Project.Initialize

import java.io.File

object CoffeeScriptPlugin extends Plugin {

  val coffeeSource = SettingKey[File]("coffee-source", "Directory containing coffee files.")
  val coffeeTarget = SettingKey[File]("coffee-target", "Output directory for translated coffee scripts.")
  val coffeeClean = TaskKey[Unit]("coffee-clean", "Clean just the files generated from coffee sources.")
  val coffee = TaskKey[Unit]("coffee", "Compile the coffee sources.")

  private def base (file: File) = file.getName.toString.substring(0, file.getName.toString.lastIndexOf("."))

  private def javascript(coffee: File, targetDir: File) =
    new File(targetDir, base(coffee) + ".js")

  private def outdated (coffee: File, javascript: File) =
    (! javascript.exists) || (coffee.lastModified > javascript.lastModified)

  private def compile (coffee: File, target: File, out: Logger) {
    val cmd = "coffee --compile --output %s %s".format(target, coffee)
    out.info(cmd)
    val rc = Process(cmd).run.exitValue()
  }

  private def compileChanged (sources: File, target: File, out: Logger) = {
    for (coffee <- sources.listFiles)
      if (outdated(coffee, javascript(coffee, target)))
        compile(coffee, target, out)
    Seq()
  }

  private def coffeeCleanTask: Initialize[Task[Unit]] =
    (streams, coffeeTarget) map {
      (out, target) =>
        out.log.info("Cleaning " + target)
        IO.delete(target)
    }

  // Note: the sourceGenerator task returns an empty list because we
  // don't want SBT to invoke the scala compiler on Javscript files.

  private def coffeeSourceGeneratorTask: Initialize[Task[Seq[File]]] =
    (streams, coffeeSource, coffeeTarget) map {
      (out, sourceDir, targetDir) =>
        compileChanged(sourceDir, targetDir, out.log)
    }

  private def coffeeTask: Initialize[Task[Unit]] =
    (streams, coffeeSource, coffeeTarget) map {
      (out, sourceDir, targetDir) =>
        compileChanged(sourceDir, targetDir, out.log)
    }

  // Use declarations here just as you would in build.sbt.

  override def settings = Seq (
    coffeeSource <<= (baseDirectory) { (d) => new File(d, "/src/main/coffee") },
    coffeeTarget <<= (baseDirectory) { (d) => new File(d, "/src/main/www/js") },
    cleanFiles <+= (coffeeTarget) { (t) => t},
    coffeeClean <<= coffeeCleanTask,
    coffee <<= coffeeTask,
    sourceGenerators in Compile <+= coffeeSourceGeneratorTask
  )

}