Custom type serialization with bimap() error

[martin-ockajak]

Task List

• Steps to reproduce provided
• Stacktrace (if present) provided
• Example that reproduces the problem uploaded to Github
• Full description of the issue provided (see below)

Commit or Tag Version

v1.9.1

Expected Behaviour

Serializing a value of custom type using an implicit FromTo instance created with bimap() should work.

Note: The equivalent code for uPickle works as expected.

Actual Behaviour

The following error occurs using weePickle 1.9.1 with Scala 2.13.12:

Exception in thread "main" java.lang.ClassCastException: class com.rallyhealth.weepickle.v1.core.Types$To$Delegate
cannot be cast to class com.rallyhealth.weepickle.v1.core.Types$Tagged 
(com.rallyhealth.weepickle.v1.core.Types$To$Delegate and com.rallyhealth.weepickle.v1.core.Types$Tagged are in unnamed module of loader 'app')
        at com.rallyhealth.weepickle.v1.core.Types.com$rallyhealth$weepickle$v1$core$Types$$findTagName(Types.scala:133)
        at com.rallyhealth.weepickle.v1.core.Types$TaggedTo$Node.<init>(Types.scala:286)
        at com.rallyhealth.weepickle.v1.core.Types$To$.merge(Types.scala:97)
        at stdin$.delayedEndpoint$stdin$1(stdin:9)
        at stdin$delayedInit$body.apply(stdin:65534)
        at scala.Function0.apply$mcV$sp(Function0.scala:42)
        at scala.Function0.apply$mcV$sp$(Function0.scala:42)
        at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:17)
        at scala.App.$anonfun$main$1(App.scala:98)
        at scala.App.$anonfun$main$1$adapted(App.scala:98)
        at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:576)
        at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:574)
        at scala.collection.AbstractIterable.foreach(Iterable.scala:933)
        at scala.App.main(App.scala:98)
        at scala.App.main$(App.scala:96)
        at stdin$.main(stdin:65534)
        at stdin.main(stdin)

Steps to Reproduce

Paste this snippet into a shell (requires Scala CLI):

scala-cli run - << END

//> using scala 2.13.12
//> using dep com.rallyhealth::weepickle-v1:1.9.1
import com.rallyhealth.weejson.v1.{Str, Value}
import com.rallyhealth.weepickle.v1.WeePickle.{FromScala, FromTo, macroFromTo}

class Custom(a: String)

implicit val fromTo: FromTo[Custom] = macroFromTo[Value].bimap[Custom](
  _ => Str(""),
  _ => new Custom(""),
)

FromScala(Custom(""))

END

Related Code

import com.rallyhealth.weejson.v1.{Str, Value}
import com.rallyhealth.weepickle.v1.WeePickle.{FromScala, FromTo, macroFromTo}
class Custom(a: String)

implicit val fromTo: FromTo[Custom] = macroFromTo[Value].bimap[Custom](
  _ => Str(""),
  _ => new Custom(""),
)

FromScala(Custom(""))

Further comments

The type cast error is caused by this function in Types.scala:

    def merge[T](rws: FromTo[_ <: T]*): TaggedFromTo[T] = {
      new TaggedFromTo.Node(rws.asInstanceOf[Seq[TaggedFromTo[T]]]: _*)
    }

The function assumes that will always recieve a sequence of TaggedFromTo but this is apparently not guaranteed.

This function seems to be invoked by this part of the FromTo derivation macro in MacroImplicits.scala:

    override def mergeTrait(subtrees: Seq[Tree], subtypes: Seq[Type], targetType: Type): Tree = {
      q"${c.prefix}.To.merge[$targetType](..$subtrees)"
    }

[rremple]

The issue here is that Value is a sealed trait, but does not declare the discriminator that macroFromTo requires of sealed traits. However, you shouldn't need it -- Value is already defined as its own visitor, and FromValue and ToValue are already defined by default. If, instead of using macroFromTo[Value] here, you used fromTo[Value] (which will implicitly join the implicit FromValue and ToValue definitions as a FromTo), then everything should work as expected. (And it should compile a tiny bit faster, since no macro is required.)