Implicit conversion not applying

Question

I was trying to workout the classic example of converting arbitrary values into their Json representation and having compile time errors in case conversion is not defined.

So far, I have,

  trait Json

  trait ConvertableToJson[A] {
    def toJson: Json
  }

  object Json {

    case class Str(str: String) extends Json

    case class Int(int : Int) extends Json

    case class StrToJson(s: String) extends ConvertableToJson[StrToJson] {
      override def toJson: Json = Str(s)
    }

  }

  implicit def str2Json(s: String): StrToJson = StrToJson(s)

  def toJson[A <: ConvertableToJson[A]](a: A) = a.toJson

  println(toJson("some string"))

I expected the above code to work like:

toJson("some string") to fail to compile without the implicit def. Because String <: ConvertableToJson[String] is false.

But then to use, the implicit def and find Str2Json.

Str2Json <: ConvertableToJson[Str2Json] should be true.

However, this doesn't happen and compiler complains:

Error: Inferred type arguments [String] do not conform to method toJson's type parameter bounds [A <: scalaz.ConvertToJson.ConvertableToJson[A]]
  println(toJson("dhruv"))
          ^

It'll be great if someone can help me correct my understanding

iain · Accepted Answer · 2017-07-23T21:19:21.203

So there are two problems with your code. First of all String does not extend ConvertableToJson[String], which is what your last function call is trying to do.

Second case class StrToJson should extend ConvertableToJson[String] not ConvertableToJson[StrToJson].

Then your code be made to compile by using view-bounds <% (see the working example below). This however is a bad idea because view-bounds are being deprecated as a language feature, you should use type classes instead.

trait Json

trait ConvertableToJson[A] {
  def toJson: Json
}

object Json {

  case class Str(str: String) extends Json

  case class Int(int : Int) extends Json

  case class StrToJson(s: String) extends ConvertableToJson[String] {
    override def toJson: Json = Str(s)
  }

}

import Json._

implicit def str2Json(s: String): StrToJson = StrToJson(s)

def toJson[A <% ConvertableToJson[A]](a: A) = a.toJson

println(toJson("some string"))

Using typeclasses

trait Json

trait ConvertableToJson[A] {
  // NOTE: this now takes a parameter
  def toJson(a: A): Json
}

object Json {

  case class Str(str: String) extends Json

  case class Int(int : Int) extends Json
}

import Json._

// NOTE: Because toJson takes a parameter the implicit implementation can now be an object
implicit object Str2Json extends ConvertableToJson[String] {
  override def toJson(a: String): Json = Str(a)
}

// NOTE: If you want to support the a.toJson syntax this implicit class adds it for all types with an implicit ConvertableToJson
implicit class ConvertableToJsonSyntax[A](a: A)(implicit ev: ConvertableToJson[A]) {
  def toJson: Json = ev.toJson(a)
}

// NOTE: Now we use context-bounds instead of view-bounds
def toJson[A : ConvertableToJson](a: A) = a.toJson

// NOTE: we can expand the context-bounds
def toJson2[A](a: A)(implicit ev: ConvertableToJson[A]) = a.toJson

// NOTE: But since we have the type class instance now, we do not need the extra syntax
def toJson3[A](a: A)(implicit ev: ConvertableToJson[A]) = ev.toJson(a)

println(toJson("some string"))

I understand that String does not extend ConvertableToJson[String] which is why I defined an implicit def of str2Json with signature, String -> ConvertableToJson(StrToString). In my understanding, with this implicit def, scala compiler should be able to convert String into ConvertableToJson[StrToString] which should play well with toJson. Ofcourse it doesn't work and your implementation does, but I still don't see why mine doesn't work. Thanks! :) — Dhruv Kapur, Jul 24 '17 at 02:28
On second look it can work if you change the definition of `toJson` to `def toJson[A](a: ConvertableToJson[A]) = a.toJson`. Implicit conversions are only triggered when passing a value to into an variable/parameter that takes a different type. They are not triggered for generic parameters types. (You code will continue to work if you removed the generic parameter to `ConvertableToJson` as it is never used. — iain, Jul 24 '17 at 12:47

Implicit conversion not applying

1 Answers1