Play JSON Helpers

package.scala

import play.api.libs.json._

package object json extends JsObjectOps
                    with JsPathImplicits
                    with ReadOps {

  object formats extends FullDateTimeFormat
                 with MapFormats
                 with RefinedFormats
                 with SingletonFormats

  implicit class RichReads[A](val reads: Reads[A]) extends AnyVal {

    def flatMapResult[B](f: A => JsResult[B]): Reads[B] = ReadOps.flatMapResult(reads)(f)

    // Workaround for https://github.com/playframework/play-json/issues/171
    def composeKeepPath[B <: JsValue](rb: Reads[B]): Reads[A] = Reads { js =>
      rb.reads(js) match {
        case JsSuccess(b, _) => reads.reads(b)
        case JsError(e)      => JsError(e)
      }
    }

    // Workaround for https://github.com/playframework/play-json/issues/171
    def andThenKeepPath[B](rb: Reads[B])(implicit witness: A <:< JsValue): Reads[B] = {
      rb.composeKeepPath(reads.map(witness))
    }
  }

  implicit class RichJsObjectReads(val reads: Reads[JsObject]) extends AnyVal {

    def orElseEmpty: Reads[JsObject] = reads.orElse(Reads.pure(JsObject.empty))
  }

  implicit class RichJsArrayReads(val reads: Reads[JsArray]) extends AnyVal {

    def orElseEmpty: Reads[JsArray] = reads.orElse(Reads.pure(JsArray.empty))
  }

}

ReadOps.scala

import play.api.libs.functional._
import play.api.libs.json.Reads._
import play.api.libs.json._

import scala.collection.GenTraversableOnce

trait ReadOps {

  def updateFieldValues[A <: JsValue](reads: Reads[A]): Reads[JsObject] = {
    of[JsObject].flatMap { obj =>
      reduce(updateObjectFieldValues(obj)(reads))
    }
  }

  private def updateObjectFieldValues[A <: JsValue](obj: JsObject)(reads: Reads[A]): Iterable[Reads[JsObject]] = {
    obj.keys.map { key =>
      val path = __ \ key
      path.json.copyFrom {
        path.json.pick andThen __.json.update(reads)
      }
    }
  }

  def mapFieldValues[A <: JsValue](reads: Reads[A]): Reads[JsObject] = {
    of[JsObject].flatMap { obj =>
      reduce(mapObjectFieldValues(obj)(reads))
    }
  }

  def mapFieldValuesWithKey[A <: JsValue](reads: String => Reads[A]): Reads[JsObject] = {
    of[JsObject].flatMap { obj =>
      reduce(obj.keys.map { key =>
        val path = __ \ key
        path.json.copyFrom(path.read(reads(key)))
      })
    }
  }

  private def mapObjectFieldValues[A <: JsValue](obj: JsObject)(reads: Reads[A]): Iterable[Reads[JsObject]] = {
    obj.keys.map { key =>
      val path = __ \ key
      path.json.copyFrom(path.read(reads))
    }
  }

  def updateItems[A <: JsValue](reads: Reads[A]): Reads[JsArray] = {
    of[JsArray].flatMap { obj =>
      reduce(updateArrayItems(obj)(reads))
    }
  }

  private def updateArrayItems[A <: JsValue](array: JsArray)(reads: Reads[A]): Iterable[Reads[JsValue]] = {
    array.value.indices.map { i =>
      (__ \ i).read[JsObject].flatMapResult(_.transform(reads).map[JsValue](identity).repath(__ \ i))
    }
  }

  def mapItems[A <: JsValue](reads: Reads[A]): Reads[JsArray] = {
    of[JsArray].flatMap { obj =>
      reduce(mapArrayItems(obj)(reads))
    }
  }

  private def mapArrayItems[A <: JsValue](array: JsArray)(reads: Reads[A]): Iterable[Reads[JsValue]] = {
    array.value.indices.map { i =>
      (__ \ i).read(reads).map[JsValue](identity)
    }
  }

  def flatMapResult[A, B](reads: Reads[A])(f: A => JsResult[B]): Reads[B] = {
    reads.flatMap { a =>
      Reads { _ =>
        f(a)
      }
    }
  }

  def reduce[M[_], A, B](mas: GenTraversableOnce[M[A]])
                        (implicit applicative: Applicative[M],
                         monoid: Monoid[B],
                         reducer: Reducer[A, B]): M[B] = {
    mas.foldLeft(applicative.pure(monoid.identity)) {
      case (mb, ma) => combine(ma, mb)
    }
  }

  private[json] def combine[M[_], A, B](ma: M[A], mb: M[B])
                                       (implicit applicative: Applicative[M], reducer: Reducer[A, B]): M[B] = {
    applicative.apply(applicative.map(ma, (a: A) => (b: B) => reducer.prepend(a, b)), mb)
  }
}

object ReadOps extends ReadOps