trait BlogSystem[A] {
  val postsDirectory: File
  def postFile[A](title: String): Option[File] =
      postsDirectory.some.filter(f => f.isFile && f.canRead)
}

object BlogPost {
  def unapply[A](title: String)(implicit ev:BlogSystem[A]): Option[BlogPost] = {
    ev.postFile(title).map(BlogPost(_))
  }
}

class BlogPlan[A] extends Plan with Scalate {
  def intent = Intent {
    case Path(Seg("post" :: BlogPost(post) :: Nil)) => Ok ~> ResponseString(post.file.toString)
  }
}