"Exception occurred while executing macro expansion." #971
Description
Play JSON Version (2.5.x / etc)
2.10.4
API (Scala / Java / Neither / Both)
Scala
Operating System (Ubuntu 15.10 / MacOS 10.10 / Windows 10)
MacOS 14.2.1
bwbecker@beta playjson % uname -a
Darwin beta 23.2.0 Darwin Kernel Version 23.2.0: Wed Nov 15 21:53:18 PST 2023; root:xnu-10002.61.3~2/RELEASE_ARM64_T6000 arm64
JDK (Oracle 1.8.0_72, OpenJDK 1.8.x, Azul Zing)
bwbecker@beta playjson % java -version
openjdk version "17.0.9" 2023-10-17
OpenJDK Runtime Environment Homebrew (build 17.0.9+0)
OpenJDK 64-Bit Server VM Homebrew (build 17.0.9+0, mixed mode, sharing)
Library Dependencies
None.
Expected Behavior
I expect to be able to combine a type parameter and a default value in a case class and for Play-Json to be able to automatically derive the appropriate Reads and Writes.
// Version 3 -- Fails with "Exception occurred while executing macro expansion."
case class Data[T](
data: T,
descr: String = "Default value"
)
object Data {
implicit def errorWrites[T](implicit writes:Writes[T]): Writes[Data[T]] = Json.writes[Data[T]]
implicit def errorReads[T](implicit reads:Reads[T]): Reads[Data[T]] = Json.reads[Data[T]]
}
Actual Behavior
The compiler throws the following error:
bwbecker@beta playjson % scala-cli error.scala
Compiling project (Scala 3.3.1, JVM (17))
[error] ./error.scala:58:81
[error] Exception occurred while executing macro expansion.
[error] java.lang.Exception: Expected an expression. This is a partially applied Term. Try eta-expanding the term first.
[error] at scala.quoted.runtime.impl.QuotesImpl$reflect$TreeMethods$.asExpr(QuotesImpl.scala:110)
[error] at scala.quoted.runtime.impl.QuotesImpl$reflect$TreeMethods$.asExprOf(QuotesImpl.scala:116)
[error] at scala.quoted.runtime.impl.QuotesImpl$reflect$TreeMethods$.asExprOf(QuotesImpl.scala:115)
[error] at play.api.libs.json.JsMacroImpl$ReadsHelper$$anon$2.applyOrElse(JsMacroImpl.scala:332)
[error] at scala.PartialFunction$Lifted.apply(PartialFunction.scala:338)
[error] at scala.PartialFunction$Lifted.apply(PartialFunction.scala:334)
[error] at scala.Option.collect(Option.scala:462)
...
Reproducible Test Case
Place the following in error.scala and then run it with scala-cli error.scala.
It contains three examples; remove/replace comments as appropriate for each one.
Example 1: Works as expected, demonstrating use of a type variable, T, for data.
Example 2: Works as expected, demonstrating the use of a default value for descr.
Example 3: Combines the two above examples, generating a compile-time exception.
//> using dep com.typesafe.play::play-json:2.10.4
import play.api.libs.json._
import Data._
object Main {
def main(args:Array[String]):Unit = {
println("Hello, world!")
// Versions 1 and 3
val err = Data[Int](400, "Some error")
val jsonString = Json.toJson(err).toString
println(jsonString)
val obj = Json.parse(jsonString).validate[Data[Int]].get
println(obj)
// Version 2
// val err = Data(400)
// val jsonString = Json.toJson(err).toString
// println(jsonString)
// val obj = Json.parse(jsonString).validate[Data].get
// println(obj)
}
}
// Version 1 -- Works as expected with type parameter
// case class Data[T](
// data: T,
// descr: String
// )
// object Data {
// implicit def errorWrites[T](implicit writes:Writes[T]): Writes[Data[T]] = Json.writes[Data[T]]
// implicit def errorReads[T](implicit reads:Reads[T]): Reads[Data[T]] = Json.reads[Data[T]]
// }
// Version 2 -- Works as expected with default parameter
// case class Data(
// data: Int,
// descr: String = "Default value"
// )
// object Data {
// implicit def errorWrites: Writes[Data] = Json.writes[Data]
// implicit def errorReads: Reads[Data] = Json.reads[Data]
// }
// Version 3 -- Fails with "Exception occurred while executing macro expansion."
case class Data[T](
data: T,
descr: String = "Default value"
)
object Data {
implicit def errorWrites[T](implicit writes:Writes[T]): Writes[Data[T]] = Json.writes[Data[T]]
implicit def errorReads[T](implicit reads:Reads[T]): Reads[Data[T]] = Json.reads[Data[T]]
}