shapeless getters

shapeless-getters.scala

import shapeless._
import shapeless.labelled._

final case class Employee(
  name    : String,
  number  : Int,
  manager : Boolean
)

final case class IceCream(
  name        : String,
  numCherries : Int,
  inCone      : Boolean
)


trait Getter[A, B] {
  def apply(value: A): B
}

trait MkGetter[A, N] {
  type Out
  def create: Getter[A, Out]
}

object MkGetter {
  type Aux[A, N, B] = MkGetter[A, N] { type Out = B }
}

object Main extends Demo {
  val employee = Employee("Bill", 1, true)
  val iceCream = IceCream("Cornetto", 0, true)

  def getter[A, B](func: A => B): Getter[A, B] =
    new Getter[A, B] {
      def apply(value: A): B =
        func(value)
    }

  // implicit def hnilMkGetter[N, B]: MkGetter.Aux[HNil, N, B] =
  //   new MkGetter[HNil, N] {
  //     type Out = B
  //     def create: Getter[HNil, B] =
  //       getter(hnil => ???)
  //   }

  implicit def foundMkGetter[K, H, T <: HList, N](
    implicit
    ev: K <:< N
  ): MkGetter.Aux[FieldType[K, H] :: T, N, H] =
    new MkGetter[FieldType[K, H] :: T, N] {
      type Out = H
      def create: Getter[FieldType[K, H] :: T, H] =
        getter { case h :: t => h }
    }

  implicit def notFoundMkGetter[K, H, T <: HList, N, B](
    implicit
    tailMkGetter: MkGetter.Aux[T, N, B]
  ): MkGetter.Aux[FieldType[K, H] :: T, N, B] = {
    val tailGetter = tailMkGetter.create
    new MkGetter[FieldType[K, H] :: T, N] {
      type Out = B
      def create: Getter[FieldType[K, H] :: T, B] =
        getter { case h :: t => tailGetter(t) }
    }
  }

  implicit def genericMkGetter[A, R, N, B](
    implicit
    gen: LabelledGeneric.Aux[A, R],
    mkGetter: MkGetter.Aux[R, N, B]
  ): MkGetter.Aux[A, N, B] = {
    val rGetter = mkGetter.create
    new MkGetter[A, N] {
      type Out = B
      def create: Getter[A, B] =
        getter(a => rGetter(gen.to(a)))
    }
  }

  def getter[A](w: Witness)(
    implicit
    mkGetter: MkGetter[A, w.T]
  ): Getter[A, mkGetter.Out] =
    mkGetter.create

  val g1: Getter[Employee, String] = getter[Employee]('name)
  val g2: Getter[Employee, Int] = getter[Employee]('number)
  val g3: Getter[Employee, Boolean] = getter[Employee]('manager)
  // val g4 = getter[Employee]('bob)

  println(g1(employee))
  println(g2(employee))
  println(g3(employee))
  // println(g4(employee))

  // println(employee)
  // println(iceCream)
}