Convert a List[String] to a case class using Shapeless

Question

I was wondering if anyone could provide some insight on a problem I'm having. I've made a gist with some code and explanation of my problem: https://gist.github.com/tbrown1979/9993f07c8f4fa2786c83

Basically I'm trying to make something that will allow me to convert List[String] to a case class. I've made a Reader that will allow me to do so, but I've run into the issue where a Reader defined for a case class can't contain a reader for a separate case class.

Looking at the 'non-working example' below - I encounter an issue where, when reading, I don't know how many items to pull out of the list. With Bar, which holds a Test, I would need to pull 2 elements out (because Test has two parameters). Is there a way for me to know the amount of fields a case class has just from its type? Is there a better way to do this?

Here is an example of how to use the Reader. I've included a non-working example as well.

  ////Working Example////
  case class Foo(a: Int, s: String)
  object Foo {
    implicit val FooReader : Reader[Foo] =
      Reader[Int :: String :: HNil].map(Generic[Foo].from _)
  }

  val read: ValidationNel[String, Foo] = Reader.read[Foo](List("12","text"))
  println(read)//Success(Foo(12, "text"))
  ///////////////////////////

  ////Non-working Example////
  case class Test(a: Int, b: String)
  object Test {
    implicit val TestReader: Reader[Test] =
      Reader[Int :: String :: HNil].map(Generic[Test].from _)
  }

  case class Bar(c: Test)
  object Bar {
    implicit val BarReader: Reader[Bar] =
      Reader[Test :: HNil].map(Generic[Bar].from _)
  }

  val barRead = Reader.read[Bar](List("21", "someString"))
  println(barRead) //Failure(NonEmptyList("Invalid String: List()", "Exepected empty, but contained value"))
  //////////////////////////

Answer 1

Something like this works for me (modification of this)

object ShapelessStringToTypeConverters {

  import cats._, implicits._, data.ValidatedNel
  import mouse._, string._, option._
  import shapeless._, labelled._

  private type Result[A] = ValidatedNel[ParseFailure, A]

  case class ParseFailure(error: String)

  trait Convert[V] {
    def parse(input: String): Result[V]
  }

  object Convert {
    def to[V](input: String)(implicit C: Convert[V]): Result[V] =
      C.parse(input)

    def instance[V](body: String => Result[V]): Convert[V] = new Convert[V] {
      def parse(input: String): Result[V] = body(input)
    }

    implicit def booleans: Convert[Boolean] =
      Convert.instance(
        s =>
          s.parseBooleanValidated
            .leftMap(e => ParseFailure(s"Not a Boolean ${e.getMessage}"))
            .toValidatedNel)

    implicit def ints: Convert[Int] =
      Convert.instance(
        s =>
          s.parseIntValidated
            .leftMap(e => ParseFailure(s"Not an Int ${e.getMessage}"))
            .toValidatedNel)

    implicit def longs: Convert[Long] =
      Convert.instance(
        s =>
          s.parseLongValidated
            .leftMap(e => ParseFailure(s"Not an Long ${e.getMessage}"))
            .toValidatedNel)

    implicit def doubles: Convert[Double] =
      Convert.instance(
        s =>
          s.parseDoubleValidated
            .leftMap(e => ParseFailure(s"Not an Double ${e.getMessage}"))
            .toValidatedNel)

    implicit def strings: Convert[String] = Convert.instance(s => s.validNel)
  }

  //////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

  sealed trait SchemaMap[A] {
    def readFrom(input: Map[String, String]): ValidatedNel[ParseFailure, A]
  }

  object SchemaMap {
    def of[A](implicit s: SchemaMap[A]): SchemaMap[A] = s

    private def instance[A](body: Map[String, String] => Result[A]): SchemaMap[A] = new SchemaMap[A] {
      def readFrom(input: Map[String, String]): Result[A] =
        body(input)
    }

    implicit val noOp: SchemaMap[HNil] =
      SchemaMap.instance(_ => HNil.validNel)

    implicit def parsing[K <: Symbol, V: Convert, T <: HList](implicit key: Witness.Aux[K], next: SchemaMap[T]): SchemaMap[FieldType[K, V] :: T] =
      SchemaMap.instance { input =>
        val fieldName = key.value.name
        val parsedField = input
          .get(fieldName)
          .cata(entry => Convert.to[V](entry), ParseFailure(s"$fieldName is missing").invalidNel)
          .map(f => field[K](f))

        (parsedField, next.readFrom(input)).mapN(_ :: _)
      }

    implicit def classes[A, R <: HList](implicit repr: LabelledGeneric.Aux[A, R], schema: SchemaMap[R]): SchemaMap[A] =
      SchemaMap.instance { input =>
        schema.readFrom(input).map(x => repr.from(x))
      }
  }

  //////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

  sealed trait SchemaList[A] {
    def readFrom(input: List[String]): ValidatedNel[ParseFailure, A]
  }

  object SchemaList {
    def of[A](implicit s: SchemaList[A]): SchemaList[A] = s

    private def instance[A](body: List[String] => Result[A]): SchemaList[A] = new SchemaList[A] {
      def readFrom(input: List[String]): Result[A] = body(input)
    }

    implicit val noOp: SchemaList[HNil] =
      SchemaList.instance(_ => HNil.validNel)

    implicit def parsing[K <: Symbol, V: Convert, T <: HList](implicit key: Witness.Aux[K], next: SchemaList[T]): SchemaList[FieldType[K, V] :: T] =
      SchemaList.instance { input =>
        val fieldName = key.value.name
        val parsedField = input
          .headOption
          .cata(entry => Convert.to[V](entry), ParseFailure(s"$fieldName is missing").invalidNel)
          .map(f => field[K](f))

        (parsedField, next.readFrom(input.tail)).mapN(_ :: _)
      }

    implicit def classes[A, R <: HList](implicit repr: LabelledGeneric.Aux[A, R], schema: SchemaList[R]): SchemaList[A] =
      SchemaList.instance { input =>
        schema.readFrom(input).map(x => repr.from(x))
      }
  }
}

/*
    case class Foo(a: String, b: Int, c: Boolean)
    def m: Map[String, String] = Map("a" -> "hello", "c" -> "true", "b" -> "100")
    def e: Map[String, String] = Map("c" -> "true", "b" -> "a100")
    val result = SchemaMap.of[Foo].readFrom(m)

    val lst = List("145164983", "0.01862523", "16.11681596", "21:38:57", "bid")
    case class Trade0(tid: Long, price: Double, amount: Double, time: String, tpe: String)
    val result2 = SchemaList.of[Trade0].readFrom(lst)
*/