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// You create the trait.
trait JavaScriptCompilerComponent {
  def compiler: JavaScriptCompiler

  trait JavaScriptCompiler {
    def compile(files: Seq[Source]): String
  }
}

// You implement it.
trait DummyJavaScriptParserComponent extends JavaScriptCompilerComponent {
  override val compiler = new DummyJavaScriptCompiler

  class DummyJavaScriptCompiler extends JavaScriptCompiler {
    override def compile(files: Seq[Source]) = {
      ...
    }
  }
}

trait PageRenderer {
  def render: Elem
}

// You add the dependecy.
class MyPageRenderer extends PageRenderer { this: JavaScriptCompilerComponent =>
  val jsFiles = Source.fromFile("test.js") :: Nil

  override def render =
    <html>
      <script>
        {compiler.compile(jsFiles)}
      </script>
    </html>
}

// And you inject it.
val page = new MyPageRenderer with DummyJavaScriptParserComponent
...
page.render