Classes defined in the REPL have no default constructor

[danielkroeni]

reproduction steps

using Scala 2.13.2, when defining a class in the REPL, no default constructor is generated. Instead there is only the constructor which expects the reference to the surrounding object.

Welcome to Scala 2.13.2 (OpenJDK 64-Bit Server VM, Java 13.0.1).
Type in expressions for evaluation. Or try :help.

scala> class A
class A

scala> classOf[A].getConstructors()
val res2: Array[java.lang.reflect.Constructor[_]] = Array(public A($iw))

This was different in 2.13.1:

Welcome to Scala 2.13.1 (OpenJDK 64-Bit Server VM, Java 13.0.1).
Type in expressions for evaluation. Or try :help.

scala> class A
defined class A

scala> classOf[A].getConstructors()
res0: Array[java.lang.reflect.Constructor[_]] = Array(public A())

problem

I used to show akka examples in the repl for a course I am teaching. With Scala 2.13.2 it does not work anymore. I guess Props[PrintActor] searches for the zero argument constructor of PrintActor to create an instance - and can not find it.

Welcome to Scala 2.13.2 (OpenJDK 64-Bit Server VM, Java 13.0.1).
Type in expressions for evaluation. Or try :help.

scala>  import akka.actor._
import akka.actor._

scala>  val as = ActorSystem("as")
val as: akka.actor.ActorSystem = akka://as

scala> class PrintActor extends Actor {
     | def receive = { case msg: Int => println(msg) }
     | }
class PrintActor

scala> val printActor: ActorRef = as.actorOf(Props[PrintActor])
java.lang.IllegalArgumentException: no matching constructor found on class PrintActor for arguments []
  at akka.util.Reflect$.error$1(Reflect.scala:86)
  at akka.util.Reflect$.findConstructor(Reflect.scala:110)
  ... 37 elided

The example above works as expected with Scala 2.13.1.
Random note: -Yrepl-class-based does not help.

[som-snytt]

You want to turn off the option:

$ scala -Dscala.repl.info -Yrepl-class-based:false
[info] started at Tue Jun 09 09:57:35 PDT 2020
Welcome to Scala 2.13.2 (OpenJDK 64-Bit Server VM, Java 11.0.7).
Type in expressions for evaluation. Or try :help.

scala 2.13.2> class C
class C

scala 2.13.2> classOf[C].getConstructors
val res0: Array[java.lang.reflect.Constructor[_]] = Array(public C())

scala 2.13.2>

[SethTisue]

@dwijnand is this inherent to the class-based encoding?

I thought :paste -raw might be another way around it, but no. oops, actually that does work!

fyi @patriknw

[dwijnand]

Here's the summary of -Yrepl-class-based and the recent changes around it scala/scala#8712.

@dwijnand is this inherent to the class-based encoding?

Specifically for classes, it's not inherent that they are wrapped in classes - there's no impact on serialising REPL state nor impact in deadlocking. In the aggregate, it might create more confusion to have more variants that opt-out of class-based wrapping, which right now is just value classes. Also, there's the issue of what to do when there are multiple "things" defined, like class C; val x = 1 - when opting out value classes the solution decided was to just opt the whole snippet to object-based.

[Jasper-M]

I thought final classes avoided this outer pointer stuff, but apparently the pointer still gets passed to the constructor. It just doesn't get stored in a field.

:paste -raw works for me though. But you have to be careful: "raw" classes get shadowed by non-raw classes of the same name, and can't shadow other raw classes.

[SethTisue]

:paste -raw works for me though

oops, thanks for the correction, I was just confused before

[som-snytt]

can't shadow other raw classes

"raw hide".

[danielkroeni]

Thank you very much for your fast responses. Using -Yrepl-class-based:false solves the problem for me:

> scala -Yrepl-class-based:false -cp "lib/*" 
Welcome to Scala 2.13.2 (OpenJDK 64-Bit Server VM, Java 13.0.1).
Type in expressions for evaluation. Or try :help.

scala> import akka.actor._
     | val as = ActorSystem("as")
     | class PrintActor extends Actor {
     | def receive = { case msg: Int => println(msg) }
     | }
     | val printActor: ActorRef = as.actorOf(Props[PrintActor])
import akka.actor._
val as: akka.actor.ActorSystem = akka://as
class PrintActor
val printActor: akka.actor.ActorRef = Actor[akka://as/user/$a#-611164863]

I let the issue open since i am not sure if there is something you want to do about it.
Thanks again!