`implicit' modifier cannot be used for top-level objects

Question

I recently started to work on play & reactive mongo. Referred the reactive mongo documentation to create a SimpleAlbum. When I run the play app I am getting an error like "Implicit modifier cannot be used for top-level objects". Why am I getting this? Help me in resolving the issue. Thanks

package models

import org.joda.time.DateTime
import reactivemongo.bson._


case class SimpleAlbum(
                    title: String,
                    releaseYear: Int,
                    hiddenTrack: Option[String],
                    allMusicRating: Option[Double])

implicit object SimpleAlbumWriter extends BSONDocumentWriter[SimpleAlbum] {
def write(album: SimpleAlbum): BSONDocument = BSONDocument(
   "title" -> album.title,
   "releaseYear" -> album.releaseYear,
   "hiddenTrack" -> album.hiddenTrack,
   "allMusicRating" -> album.allMusicRating)
}

implicit object SimpleAlbumReader extends BSONDocumentReader[SimpleAlbum] {
def read(doc: BSONDocument): SimpleAlbum = {
SimpleAlbum(
  doc.getAs[String]("title").get,
  doc.getAs[Int]("releaseYear").get,
  doc.getAs[String]("hiddenTrack"),
  doc.getAs[Double]("allMusicRating"))
 }
}

Fregger

Answer 1

implicit cannot be used at the package level. You need to put your implicit objects inside another object that you can then import where you need the implicits, e.g.:

object MyImplicits {

   implicit object SimpleAlbumWriter ....

   implicit object SimpleAlbumReader ....
}

and then where you need access to the implicits just put

import MyImplicits._

as part of the package imports.

EDIT: As @m-z points out, using the package object, you can and define implicits at the package level like this:

package models

package object Implicits {

   implicit object SimpleAlbumWriter ....

   implicit object SimpleAlbumReader ....
}

which imports in the same way:

import models.Implicits._

Detest

One could also use a package object. – Ridglee

I was wondering about that. Wasn't sure if the top level constraint was enforced at the package object level – Detest

Yes, it's a special case where the object name can collide with the package path, and everything within the package object is in the scope of that package. eg., package object models { val a = 1 } makes a available throughout package models. – Ridglee

Answer 2

Implicit class need to add into the object:

object StudentTest {
 implicit class stringUtils(myString:String){
 def scalaWordCount(): Map[String, Int] ={
  val split=myString.split("\\s+")
  val grouped=split.groupBy(word=>word)
  val counterPerKey=grouped.mapValues(group=>group.length)
  counterPerKey
}

}

Refection