Explicit type for a Shapeless record.

gistfile1.scala

object TestExplicitRecordType {
  import shapeless._, record._, syntax.singleton._

  object testF extends Poly1 {
    implicit def atFieldType[F, V](implicit wk: shapeless.Witness.Aux[F]) = at[FieldType[F, V]] {
      f => wk.value.toString
    }
  }

  // Is there more straighforward way to give an explicit type for a record?
  val k1  = Witness("k1")
  val k2  = Witness("k2")
  type Error = FieldType[k1.T, String] :: FieldType[k2.T, Long] :: HNil

  // That seems to work...
  val err1        = "k1" ->> "1" :: "k2" ->> 1L :: HNil
  val err2: Error = "k1" ->> "1" :: "k2" ->> 1L :: HNil

  // ... except. Looks like evidence for Witness.Aux is lost.
  testF(err1.head)  // OK
  testF(err2.head)  // could not find implicit value for parameter cse: shapeless.poly.Case[testF.type,shapeless.::[shapeless.record.FieldType[k1.T,String],shapeless.HNil]]
}

I think this is now addressed via the new record type literals in shapeless 2.1.0 ... does this resolve the problem for you?

object TestExplicitRecordType {
  import shapeless._, labelled._, record._, syntax.singleton._

  object testF extends Poly1 {
    implicit def atFieldType[F, V](implicit wk: shapeless.Witness.Aux[F]) = at[FieldType[F, V]] {
      f => wk.value.toString
    }
  }

  type Error = Record.`"k1" -> String, "k2" -> Long`.T

  // That seems to work...
  val err1        = "k1" ->> "1" :: "k2" ->> 1L :: HNil
  val err2: Error = "k1" ->> "1" :: "k2" ->> 1L :: HNil

  testF(err1.head)  // OK
  testF(err2.head)  // OK
}