How to create an instances for typeclass with dependent type using shapeless

Question

I\'m trying to derive a tuple instance for a type class with dependent type. I\'m using shapeless to create summon the type class for the tuple elements. I\'m having trouble mat

Answer 1

Try Aux pattern

trait Identifiable[M] {
  type K
  def identify(id: M): K
}

object Identifiable {
  type Aux[M, K0] = Identifiable[M] { type K = K0 }

  implicit def identifiableTuple[M1, K1, M2, K2](
    implicit
    identifiable1: Identifiable.Aux[M1, K1],
    identifiable2: Identifiable.Aux[M2, K2]
  ): Identifiable.Aux[(M1, M2), (K1, K1)] = new Identifiable[(M1, M2)] {
    type K = (K1, K2)
    def identify(id: (M1, M2)): (K1, K2) =
      identifiable1.identify(id._1) -> identifiable2.identify(id._2)
  }
}

Aux pattern was invented because

• it is easier for human to read it
• I think(?) compiler used to have problems with deriving type classes for path-dependent types... but not for their aliases

So just use Aux to derive things.