Stages.scala

package org.felher.tmpc5C0ubzOfr

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

object Main {
  def main(args: Array[String]): Unit = {
    val a = ('name ->> "Alice") :: ('birthYear ->> 1980) :: HNil
    val b = ('name ->> "Bob") :: ('birthYear ->> 1990) :: HNil

    val ageAnnotater = makeAgeAnnotator(2020)
    println(
      List(a, b)
        .map(i => ageAnnotater.apply(i))
        .map(i => nameLenghtAnnotater.apply(i))
      );
  }

  def makeAgeAnnotator(nowYear: Int) = new Stage {
    type Required = Record.`'birthYear -> Int`.T
    type Out      = Record.`'age -> Int`.T

    def execute(r: Required): Out = {
      val age = nowYear - r('birthYear)
      ('age ->> age) :: HNil
    }
  }

  val nameLenghtAnnotater = new Stage {
    type Required = Record.`'name -> String`.T
    type Out      = Record.`'nameLength -> Int`.T

    def execute(r: Required): Out =
      ('nameLength ->> r('name).length) :: HNil
  }
  
}

abstract class Stage {
  type Required <: HList
  type Out <: HList

  def execute(r: Required): Out

  def apply[In <: HList](in: In)(implicit
    e: Extractor[In, Required],
    m: Merger[In, Out]
  ): m.Out = {
    val tmp = in.extract[Required]
    val added = execute(tmp)
    in.merge(added)
  }
}