How to remove repetitive case statements for implementations of a sealed trait

Question

I often find that I need to extract the type of a sealed trait before doing the same thing to each implementation:

sealed trait Trait
case class Foo() extends Trait
case class Bar() extends Trait
// ... lots of other implementations

// *must* take a `Trait`, not a `T <: Trait`
def thing(t: Trait): ??? = t match {
  case f: Foo => // something with the instance and specific type
  case b: Bar => // something with the instance and specific type 
  // ... same thing again for other implementations
}

for example

// typically provided by somebody else...
trait Thing[T] { def thingy: String }
implicit def thing[T]: Thing[T] = new Thing[T] { def thingy = "stuff" }

def thing(t: Trait): String = t match {
  case Foo() => implicitly[Thing[Foo]].thingy
  case Bar() => implicitly[Thing[Bar]].thingy
  // ...
}

I'd like to reduce the boilerplate involved in doing this.

Answer 1

UPDATE: nowadays we'd use typeclass derivation via shapeless. e.g. https://github.com/fommil/shapeless-for-mortals

It turns out that you can use shapeless' polymorphic functions and co-product to do this:

  object thing extends Poly1 {
    implicit def action[T <: Trait: Thing] = at[T](
      a => implicitly[Thing[T]].thingy
    )
    // magic that makes it work at the sealed trait level
    def apply(t: Trait): String =
      Generic[Trait].to(t).map(thing).unify
  }

which can then be used like

println(thing(Foo(): Trait))

I'd like to make this easier to write via an abstract class (let's forget about passing on implicit parameters to action for now), e.g.

abstract class MatchSealed[In, Out] extends Poly1 {
  implicit def poly[T <: In] = at[T](action)

  def action[T <: In](t: T): Out

  import ops.coproduct.Mapper
  def apply[R <: HList](in: In)(
    implicit
      g: Generic.Aux[In, R],
      m: Mapper[this.type, R]
  ): Out = {
    val self: this.type = this
    g.to(in).map(self).unify
  }
}

but this is failing with a missing Mapper[self.type, g.Repr] on the final line. I don't know which implicit is missing, but I suspect it is the self.type. I really want to capture realisedSelf.type but I don't know how to do that.

UPDATE: it turns out that it is not possible to obtain the Mapper because it needs access to the realised object Unable to map on HList