Concatenate .coffee files before compiling to .js

build.sbt

lazy val concatCoffeeScript = taskKey[Seq[File]]("Concatenate .coffee")
lazy val concatCoffeeScriptFiles = settingKey[Seq[(String, Seq[String])]]("Files to concatenate, in order")

concatCoffeeScriptFiles := Seq(
  "javascripts/app.coffee" → Seq("javascripts/product.coffee", "javascripts/person.coffee", "javascripts/main.coffee")
)

concatCoffeeScript <<= (sourceDirectory in Assets, concatCoffeeScriptFiles, WebKeys.webTarget) map { (d, fs, t) ⇒
  fs map { case (concatenated, parts) ⇒
      val cf = t / concatenated
      IO.delete(cf)
      IO.touch(cf)
      parts map { d / _ } filter { _.exists } foreach { pf ⇒
        IO.append(cf, IO.readBytes(pf))
        IO.append(cf, "\n")
      }
      cf
  }
}

import CoffeeScriptKeys._
(unmanagedSources in (Assets, coffeescript)) <<= concatCoffeeScript
(unmanagedSourceDirectories in (Assets, coffeescript)) <+= WebKeys.webTarget

coffeescript <<= coffeescript dependsOn concatCoffeeScript