How to use shapeless to convert generic Map[String

Question

I'm trying to follow the answer here https://stackoverflow.com/a/31641779/1586965

That is, I want to be able to convert (potentially nested) Map[String, Any] to a case class.

scalaVersion := "2.11.8"
val shapelessV = "2.3.3"

If I try to wrap the code in the above answer in another generic function, I can't seem to get it to compile

import shapeless._, labelled._
import FromMap._

def usesGenerics[P](map: Map[String, Any]): P = {
  to[P].from(mp).get
}

I get the following compile error

could not find implicit value for parameter gen: shapeless.LabelledGeneric.Aux[P,R]

FULL ANSWER

This incorporates both Travis's initial idea and Dmytros fix, and finally some simplification

import shapeless._, labelled.{FieldType, field}

trait FromMap[L <: HList] {
  def apply(m: Map[String, Any]): Option[L]
}

trait LowPriorityFromMap {
  implicit def hconsFromMap1[K <: Symbol, V, T <: HList](implicit
                                                         witness: Witness.Aux[K],
                                                         typeable: Typeable[V],
                                                         fromMapT: Lazy[FromMap[T]]
                                                        ): FromMap[FieldType[K, V] :: T] = new FromMap[FieldType[K, V] :: T] {
    def apply(m: Map[String, Any]): Option[FieldType[K, V] :: T] = for {
      v <- m.get(witness.value.name)
      h <- typeable.cast(v)
      t <- fromMapT.value(m)
    } yield field[K](h) :: t
  }
}

object FromMap extends LowPriorityFromMap {
  implicit val hnilFromMap: FromMap[HNil] = new FromMap[HNil] {
    def apply(m: Map[String, Any]): Option[HNil] = Some(HNil)
  }

  implicit def hconsFromMap0[K <: Symbol, V, R <: HList, T <: HList](implicit
                                                                     witness: Witness.Aux[K],
                                                                     gen: LabelledGeneric.Aux[V, R],
                                                                     fromMapH: FromMap[R],
                                                                     fromMapT: FromMap[T]
                                                                    ): FromMap[FieldType[K, V] :: T] =
    new FromMap[FieldType[K, V] :: T] {
      def apply(m: Map[String, Any]): Option[FieldType[K, V] :: T] = for {
        v <- m.get(witness.value.name)
        r <- Typeable[Map[String, Any]].cast(v)
        h <- fromMapH(r)
        t <- fromMapT(m)
      } yield field[K](gen.from(h)) :: t
    }
}

trait CaseClassFromMap[P <: Product] {
  def apply(m: Map[String, Any]): Option[P]
}

object CaseClassFromMap {
  implicit def mk[P <: Product, R <: HList](implicit gen: LabelledGeneric.Aux[P, R],
                                            fromMap: FromMap[R]): CaseClassFromMap[P] = new CaseClassFromMap[P] {
    def apply(m: Map[String, Any]): Option[P] = fromMap(m).map(gen.from)
  }

  def apply[P <: Product](map: Map[String, Any])(implicit fromMap: CaseClassFromMap[P]): P = fromMap(map).get
}

Answer 1

Does this work for you?

def usesGenerics[P, R <: HList](map: Map[String, Any])(implicit gen: LabelledGeneric.Aux[P, R], fromMap: FromMap[R]): P = {
  to[P].from[R](map).get
}

---

You need one more type class

  trait CaseClassFromMap[P <: Product] {
    def apply(m: Map[String, Any]): Option[P]
  }
  object CaseClassFromMap {
    implicit def mk[P <: Product, R <: HList](implicit
      gen: LabelledGeneric.Aux[P, R],
      fromMap: FromMap[R]
      ): CaseClassFromMap[P] = new CaseClassFromMap[P] {
      def apply(m: Map[String, Any]): Option[P] = to[P].from[R](m)
    }
  }

  def usesGenerics[P <: Product](map: Map[String, Any])(implicit fromMap: CaseClassFromMap[P]): P = {
    fromMap(map).get
  }

  usesGenerics[Person](mp)