Maia.scala

package com.jspha.maia

import scala.language.higherKinds
import shapeless._
import shapeless.ops.hlist.Mapped

trait Maia {

  trait Api {
    type Fields <: HList
  }

  sealed trait Target
  sealed trait Atomic[A] extends Target
  sealed trait Nested[T] extends Target

  // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

  sealed trait Person extends Api {
    type Fields = Atomic[String] :: HNil
  }

  // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

  trait Request[T <: Api] {
    import Request.Elem

    type Rec <: HList
    val mapped: Mapped.Aux[T#Fields, Elem, Rec]
    val record: Rec
  }

  object Request {

    sealed trait Elem[_]
    case class ElemAtomic[A](a: A) extends Elem[Atomic[A]]
    case class ElemNested[T <: Api](rec: Request[T]) extends Elem[Nested[T]]

    type Aux[T <: Api, Rec0 <: HList] = Request[T] { type Rec = Rec0 }

    def build[T <: Api, Rec0 <: HList](record0: Rec0)(
        implicit mapped0: Mapped.Aux[T#Fields, Elem, Rec0]): Request[T] =
      new Request[T] {
        type Rec = Rec0
        val mapped = mapped0
        val record = record0
      }

    class Builder[T <: Api] {
      def apply[Rec0 <: HList](record0: Rec0)(
          implicit mapped0: Mapped.Aux[T#Fields, Elem, Rec0]): Request[T] =
        new Request[T] {
          type Rec = Rec0
          val mapped = mapped0
          val record = record0
        }
    }

    def apply[T <: Api]: Builder[T] =
      new Builder[T]

  }

  val req0: Request[Person] = new Request[Person] {
    import Request._

    type Rec = Request.Elem[Atomic[String]] :: HNil
    val mapped = implicitly[Mapped.Aux[Person#Fields, Elem, Rec]]
    val record = ElemAtomic("Hi") :: HNil
  }

  val req1: Request[Person] =
    Request.build[Person, Request.Elem[Atomic[String]] :: HNil](
      Request.ElemAtomic("Hi") :: HNil)

  val req2: Request[Person] =
    Request[Person](Request.ElemAtomic("Hi") :: HNil)

}

// [info] Compiling 1 Scala source to /Users/tel/Dropbox/proj/scala/maia2/target/scala-2.12/classes...
// [error] /Users/tel/Dropbox/proj/scala/maia2/src/main/scala-2.12/com/jspha/maia/Maia.scala:77: could not find implicit value for parameter mapped0: shapeless.ops.hlist.Mapped.Aux[Maia.this.Person#Fields,Maia.this.Request.Elem,shapeless.::[Maia.this.Request.ElemAtomic[String],shapeless.HNil]]
// [error]     Request[Person](Request.ElemAtomic("Hi") :: HNil)
// [error]                    ^
// [error] one error found
// [error] (compile:compileIncremental) Compilation failed
// [error] Total time: 0 s, completed Apr 7, 2017 5:41:27 PM