Pickling `List[Product with Serializable with MyTrait]`

[mateuszdraco]

There is some problem with Set that what I understand is wrong. Let's define:

import scala.pickling._
import json._

trait MyTrait
case object valA extends MyTrait
case object valB extends MyTrait

Now, if try to pickle some collections:

List[MyTrait]().pickle // works
List(valA).pickle // works
List(valA, valB).pickle // doesn't work
List[MyTrait](valA, valB).pickle // works

The third List does not work, however it's not real issue, we just have to specify the type of an elements and we are good to go.

And now, the main problem is that I cannot pickle any filled set of values:

Set[MyTrait]().pickle // works
Set[MyTrait](valA).pickle

scala.NotImplementedError: an implementation is missing

And I have no idea what I should do to make it works :(

[alexandros-bourantas]

Same experience with Vector and other Iterable implementations. For my application it is ok to convert to and from List for pickling/unpickling. However, it would be nice to know why it doesn't work with those other Iterables and what can be done to make it work.

[eed3si9n]

There's bit of a type inference issue mixed in with this Pickling problem.

scala> :paste
// Entering paste mode (ctrl-D to finish)

trait MyTrait
case object valA extends MyTrait
case object valB extends MyTrait

// Exiting paste mode, now interpreting.

defined trait MyTrait
defined object valA
defined object valB

scala> List(valA, valB)
res0: List[Product with Serializable with MyTrait] = List(valA, valB)

The error I get when I pickle is:

scala> List(valA, valB).pickle
<console>:20: error: type mismatch;
 found   : Class[?0] where type ?0 <: Product
 required: ?{def ==(x$1: ? >: Class[Any](classOf[scala.Any])): Boolean}
Note that Class extends Any, not AnyRef.
Such types can participate in value classes, but instances
cannot appear in singleton types or in reference comparisons.
              List(valA, valB).pickle
                               ^