How do I best construct a shapeless record with a default value?

Question

Say I have a shapeless record:

trait T
case object Ti extends T
case object Ta extends T
case object To extends T

type R = Record.`Ta -> Option[String], Ti -> Option[Int]`.T
val r: R = (Ta ->> Option("plif")) :: (Ti ->> Option(4)) :: HNil

I'd like to write a function get such that:

get(Ta) == Some("plif")
get(Ti) == Some(4)
get(To) == None

What would be the best way to achieve this?

Answer 1

A simple solution is to provide your own Selector instance for a default case:

class DefaultSelector[R <: HList, K] extends Selector[R, K] {
  type Out = Option[Nothing]
  def apply(l: R): Out = None
}

def get[K, V](k: K)(
  implicit sel: Selector[R, K] = new DefaultSelector[R, K]
): sel.Out = sel(r)

But with that code Scala's compiler may have difficulties providing TypeTags for the result of the default case.

---

So to fix that you can also write a new typeclass DefaultSelector, which will default to None: Option[Nothing], if no Selector is found:

import shapeless._
import shapeless.ops.record._

trait DefaultSelector[R <: HList, K] {
  type Out
  def apply(r: R): Out
}

sealed trait LowPriorityDefaultSelector {
  type Aux[R <: HList, K, V] = DefaultSelector[R, K] { type Out = V }

  case class Impl[R <: HList, K, V](get: R => V) extends DefaultSelector[R, K] {
    type Out = V
    def apply(r: R): Out = get(r)
  }

  implicit def default[R <: HList, K, V](
    implicit ev: Option[Nothing] =:= V  // tricking Scala's implicit resolution
  ): Aux[R, K, V] =
    Impl[R, K, V](Function.const(None))
}

object DefaultSelector extends LowPriorityDefaultSelector {
  implicit def existing[R <: HList, K, V](
    implicit sel: Selector.Aux[R, K, V]
  ): Aux[R, K, V] =
    Impl[R, K, V](sel.apply)
}

Then the get function becomes:

def get[K, V](k: K)(implicit sel: DefaultSelector[R, K]): sel.Out = sel(r)

---

And the result (for both solutions) is:

scala> get(Ti)
res0: Option[Int] = Some(4)

scala> get(Ta)
res1: Option[String] = Some(plif)

scala> get(To)
res2: Option[Nothing] = None