How to generically update a case class field using LabelledGeneric?

Question

Using shapeless, one can use LabelledGeneric to update case class fields like so:

case class Test(id: Option[Long], name: String)
val test = Test(None, "Name")
val gen = LabelledGeneric[Test]

scala> gen.from(gen.to(test) + ('id ->> Option(1L)))
res0: Test = Test(Some(1),Name)

I would like the Test class (and others) to extend an abstract class Model, that will implement a method withId that would use a LabelledGeneric similar to the above code to update the id field, should it have one (which it should).

My attempt adds an implicit parameter of a LabelledGeneric[A] to the constructor of Model, which materializes just fine. I also need to somehow provide evidence to the record syntax that the LabelledGeneric#Repr has the id field to replace. Adding an implicit Updater parameter to withId satisfies the compiler, so that the code below will compile, but it is not usable.

import shapeless._, record._, ops.record._, labelled._, syntax.singleton._, tag._

abstract class Model[A](implicit gen: LabelledGeneric[A] { type Repr <: HList }) { this: A =>

    def id: Option[Long]

    val idWitness = Witness("id")

    type F = FieldType[Symbol with Tagged[idWitness.T], Option[Long]]

    def withId(id: Long)(implicit u: Updater.Aux[gen.Repr, F, gen.Repr]) =
        gen.from(gen.to(this) + ('id ->> Option(id)))

}

case class Test(id: Option[Long], name: String) extends Model[Test]

When calling test.withId(1), the implicit Updater fails to materialize. The macro reports that gen.Repr isn't an HList type, when it in fact is. It seems that this match is the one that fails, where u baseType HConsSym returns <notype>. Equivalent to:

scala> weakTypeOf[test.gen.Repr].baseType(weakTypeOf[::[_, _]].typeConstructor.typeSymbol)
res12: reflect.runtime.universe.Type = <notype>

This is using shapeless 2.3, though it fails for different reasons in 2.2 (seems as though Updater had a large refactor).

Is it possible to accomplish this with shapeless, or am I way off target?

knutwalker knutwalker · Accepted Answer · 2016-04-12T09:40:59

The main issue here is that the refined result type of the LabelledGeneric (Repr) is lost. At Model, the only thing known about Repr is Repr <: HList. The implicit Updater.Aux[gen.Repr, F, gen.Repr] searches for something that is only known as _ <: HList and thus fails to materialize. You'd have to define Model with two type parameters abstract class Model[A, L <: HList](implicit gen: LabelledGeneric.Aux[A, L]) but this doesn't allow you to write class Test extends Model[Test] and you have to write the labelled generic type by hand.

If you instead move the gen down to withId, you can make it work:

object Model {
  private type IdField = Symbol with Tagged[Witness.`"id"`.T]
  private val  IdField = field[IdField]

  type F = FieldType[IdField, Option[Long]]
}
abstract class Model[A] { this: A =>
  import Model._

  def id: Option[Long]

  def withId[L <: HList](id: Long)(implicit   // L captures the fully refined `Repr`
    gen: LabelledGeneric.Aux[A, L],           // <- in here ^
    upd: Updater.Aux[L, F, L]                 // And can be used for the Updater
  ): A = {
    val idf = IdField(Option(id))
    gen.from(upd(gen.to(this), idf))
  }
}

case class Test(id: Option[Long], name: String) extends Model[Test]

If you're concerned with resolution performance, you can cache the value(s) in the companion of Test:

case class Test(id: Option[Long], name: String) extends Model[Test]
object Test {
  implicit val gen = LabelledGeneric[Test]
}

This would mean that code like this

val test = Test(None, "Name")
println("test.withId(12) = " + test.withId(12))
println("test.withId(12).withId(42) = " + test.withId(12).withId(42))

would use the definition of Test.gen instead of materializing a new LabelledGeneric every time.

This works for both, shapeless 2.2.x and 2.3.x.

How to generically update a case class field using LabelledGeneric?

1 Answers