Scala Compiler (2.11.7) anomaly with Play JSON Writes - json

Question

So I have the following code:
Base:
import play.api.libs.json.{JsNull, Json, JsValue, Writes}
case class Cost(cost: Option[Double])
This compiles:
case object Cost {
def writes = new Writes[Cost] {
override def writes(r: Cost): JsValue = {
val cost = r.cost.map(Json.toJson(_)).getOrElse(JsNull)
Json.obj(
"cost" -> cost
)
}
}
}
But this doesn't compile
case object Cost {
def writes = new Writes[Cost] {
override def writes(r: Cost): JsValue = {
Json.obj(
"cost" -> r.cost.map(Json.toJson(_)).getOrElse(JsNull)
)
}
}
}
Compiler error is the following:
type mismatch;
[error] found : Object
[error] required: play.api.libs.json.Json.JsValueWrapper
[error] "cost" -> r.cost.map(Json.toJson(_)).getOrElse(JsNull)
In the latter if I use .asInstanceOf[JsValue] it works, but with IntelliJ grays it out saying it's unnecessary as it can't be anything else that JsValue anyway. What might be the reason that Scala compiler (2.11.7) doesn't detect the Class properly?

Answer 1

I am pretty sure the issue comes from .getOrElse(JsNull)
I have successfully compiled this code:
import play.api.libs.json.{JsNull, Json, JsValue, Writes}
case class Cost(cost: Option[Double])
case object Cost {
def writes = new Writes[Cost] {
override def writes(r: Cost): JsValue = {
Json.obj(
"cost" -> r.cost.map(Json.toJson(_))
)
}
}
}
and parsed the output:
scala> Cost(Some(5))
res2: Cost = Cost(Some(5.0))
scala> Json.toJson(res2)(Cost.writes)
res5: play.api.libs.json.JsValue = {"cost":5}
Looking for the source of the issue you can check couple of additional solutions, assuming the writes function:
val cost = r.cost.map(t => Json.toJson(t))
Json.obj(
"cost" -> cost
)
if you want to resolve the Option of cost value with getOrElse you can either cast (as you have tried) or provide the type:
cost.getOrElse[JsValue](JsNull)
cost.getOrElse(JsNull).asInstanceOf[JsValue]
and without the type specification sbt always gives an error saying:
[error] (...) type mismatch;
[error] found : Object
[error] required: play.api.libs.json.Json.JsValueWrapper
which must come from some SBT's compiler bug.