How to get Option[T] from object using Shapeless

Question

I was trying to get Option[Author] from book in getAuthor method:

  import shapeless.ops.record._

  case class Book(title: String, author: Option[Author])
  case class Author(name: String)
  val book = Book("Types and Programming Languages", Option(Author("Benjamin Pierce")))

  val w = Witness('author)
  def getAuthor[T1, T2 <: HList, T3](t: T1)(implicit gen: LabelledGeneric.Aux[T1, T2], s: Selector.Aux[T2, w.T, Option[T3]]): s.Out = {
    val repr = gen.to(t)
    val opt = s.apply(repr)
    opt.foreach {
      ??? // do something
    }
    opt
  }

  val author = getAuthor(book)
  println(author)

but this code cause following compilation error:

could not find implicit value for parameter s: shapeless.ops.record.Selector.Aux[T2,shapeless.DuckTyping.w.T,Option[T3]] (No field shapeless.DuckTyping.w.T in record T2)
  val author = getAuthor(book)

I want to avoid compilation error and specify Option[T] as Selector#apply return value.

Can anyone help, please?

Dmytro Mitin · Answer 1 · 2021-09-25T10:45:59.950

You seem to over-constrain implicits.

Try to modify method signature

def getAuthor[T1, T2 <: HList, T3](t: T1)(implicit
  gen: LabelledGeneric.Aux[T1, T2],
  s: Selector.Aux[T2, w.T, T3],
  ev: T3 <:< Option[_]
): s.Out /* T3 */

HList foldLeft with tuple as zero

Why is this implicit resolution failing?

Scala shapeless Generic.Aux implicit parameter not found in unapply

Extract FieldType key and value from HList

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How to get Option[T] from object using Shapeless

1 Answers1