Exploit tagged types to add constraints on standard values, with custom Json deserializers

gistfile1.scala

import play.api.libs.json._
import play.api.data.validation.ValidationError

object Test{
  
  type Tagged[U] = { type Tag = U }

  trait NonEmpty
  type NonEmptyString = String with Tagged[NonEmpty]

  case class Foo(str1: NonEmptyString, str2: String)
  
  implicit val NonEmptyStringReads = new Reads[NonEmptyString]{ 
    def reads(json: JsValue): JsResult[NonEmptyString] = 
      json.validate[String]
        .filterNot(ValidationError("String is empty!"))(_.isEmpty)
        .map(_.asInstanceOf[NonEmptyString]) 
  }
  
  implicit val FooReads = Json.reads[Foo]

  assert(Json.obj("str1" -> "not empty", "str2" -> "not empty").validate[Foo].asOpt.isDefined)
  assert(!Json.obj("str1" -> "", "str2" -> "not empty").validate[Foo].asOpt.isDefined)
  assert(Json.obj("str1" -> "not empty", "str2" -> "").validate[Foo].asOpt.isDefined)
  assert(!Json.obj("str1" -> "", "str2" -> "").validate[Foo].asOpt.isDefined)
  
  assert(Json.obj("str1" -> "not empty", "str2" -> "").as[Foo].str1.isInstanceOf[String])
}