Scala Pickling: Writing a custom pickler / unpickler for nested structures

Question

I'm trying to write a custom SPickler / Unpickler pair to work around some the current limitations of scala-pickling. The data type I'm trying to pickle is a case class, where some of the fields already have their own SPickler and Unpickler instances. I'd like to use these instances in my custom pickler, but I don't know how.

Here's an example of what I mean:

// Here's a class for which I want a custom SPickler / Unpickler.
// One of its fields can already be pickled, so I'd like to reuse that logic.
case class MyClass[A: SPickler: Unpickler: FastTypeTag](myString: String, a: A)

// Here's my custom pickler.
class MyClassPickler[A: SPickler: Unpickler: FastTypeTag](
  implicit val format: PickleFormat) extends SPickler[MyClass[A]] with Unpickler[MyClass[A]] {
  override def pickle(
    picklee: MyClass[A],
    builder: PBuilder) {
    builder.beginEntry(picklee)

    // Here we save `myString` in some custom way.
    builder.putField(
      "mySpecialPickler",
      b => b.hintTag(FastTypeTag.ScalaString).beginEntry(
        picklee.myString).endEntry())

    // Now we need to save `a`, which has an implicit SPickler.
    // But how do we use it?

    builder.endEntry()
  }

  override def unpickle(
    tag: => FastTypeTag[_],
    reader: PReader): MyClass[A] = {
    reader.beginEntry()

    // First we read the string.
    val myString = reader.readField("mySpecialPickler").unpickle[String]

    // Now we need to read `a`, which has an implicit Unpickler.
    // But how do we use it?
    val a: A = ???

    reader.endEntry()

    MyClass(myString, a)
  }
}

I would really appreciate a working example. Thanks!

Answer 1

Here is a working example:

case class MyClass[A](myString: String, a: A)

Note that the type parameter of MyClass does not need context bounds. Only the custom pickler class needs the corresponding implicits:

class MyClassPickler[A](implicit val format: PickleFormat, aTypeTag: FastTypeTag[A],
                                     aPickler: SPickler[A], aUnpickler: Unpickler[A])
  extends SPickler[MyClass[A]] with Unpickler[MyClass[A]] {

  private val stringUnpickler = implicitly[Unpickler[String]]

  override def pickle(picklee: MyClass[A], builder: PBuilder) = {
    builder.beginEntry(picklee)

    builder.putField("myString",
      b => b.hintTag(FastTypeTag.ScalaString).beginEntry(picklee.myString).endEntry()
    )

    builder.putField("a",
      b => {      
        b.hintTag(aTypeTag)
        aPickler.pickle(picklee.a, b)
      }
    )

    builder.endEntry()
  }

  override def unpickle(tag: => FastTypeTag[_], reader: PReader): MyClass[A] = {
    reader.hintTag(FastTypeTag.ScalaString)
    val tag = reader.beginEntry()
    val myStringUnpickled = stringUnpickler.unpickle(tag, reader).asInstanceOf[String]
    reader.endEntry()

    reader.hintTag(aTypeTag)
    val aTag = reader.beginEntry()
    val aUnpickled = aUnpickler.unpickle(aTag, reader).asInstanceOf[A]
    reader.endEntry()

    MyClass(myStringUnpickled, aUnpickled)
  }

}

In addition to the custom pickler class, we also need an implicit def which returns a pickler instance specialized for concrete type arguments:

implicit def myClassPickler[A: SPickler: Unpickler: FastTypeTag](implicit pf: PickleFormat) =
  new MyClassPickler