Dynamically sanitizing Strings (XSS concerns) based on type tags

TaggedPlayJsonExample.scala

import scalaz.{ @@, Tag }

import org.apache.commons.lang.StringEscapeUtils
import org.joda.time.DateTime
import play.api.libs.json._

trait DomRenderable

def RenderableString(s: String): String @@ DomRenderable = Tag[String, DomRenderable](s)

trait StringDate

/* This will only get called when a tagged type is required but an untagged is given */
implicit def EpochTime(dt: DateTime): DateTime @@ StringDate = Tag[DateTime, StringDate](dt)

case class ResponseView(
  messageSubject: String @@ DomRenderable,
  messageBody: String @@ DomRenderable,
  friendlyDescription: String,
  dateCreated: DateTime,
  dateModified: DateTime @@ StringDate
)

/* Just an example - could be more than just true or false */
implicit val browserUserAgent: Boolean = true

implicit def domRenderableWrites(implicit userAgent: Boolean) =
  Writes[String @@ DomRenderable] { taggedString =>
    JsString(
      (userAgent, Tag.unwrap(taggedString)) match {
        case (true, s) => StringEscapeUtils.escapeHtml(s)
        case (false, s) => s
      }
    )
  }

implicit val epochDateWrites = Writes[DateTime @@ StringDate](dt => JsString(Tag.unwrap(dt).toString))

implicit val strImpliesRenderable: (String => String @@ DomRenderable) = s => RenderableString(s)

val responseView = ResponseView(
  "<>", "<>", "<>", new DateTime, new DateTime
)

implicit val responseViewWrites = Json.writes[ResponseView]
val x = responseViewWrites.writes(responseView).toString()
/*
{
  "messageSubject": "&lt;&gt;",
  "messageBody": "&lt;&gt;",
  "friendlyDescription": "<>",
  "dateCreated": 1479175039392,
  "dateModified": "2016-11-14T20:57:19.392-05:00"
}
 */