Unable to derive instances of the following type class

[julienrf]

I couldn’t find a way to derive instances of the following typeclass. I would like to know if this is intended by design, or if you think there is something missing in the library to support my use case.

Consider the typeclass Record, defined as follows:

trait Records {
  type Field[A]
  type Record[A]
  extension [A] (recordA: Record[A])
    def zip[B](recordB: Record[B]): Record[(A, B)]
    def xmap[B](f: A => B)(g: B => A): Record[B]

  def field[A](name: String)(using Field[A]): Record[A]

  given int: Field[Int]
  given string: Field[String]
}

We can use it like so to manually create an instance of a data type User:

case class User(name: String, age: Int)

trait ManualInstance extends Records {

  val user: Record[User] =
    field[String]("name")
      .zip(field[Int]("age"))
      .xmap((name, age) => User(name, age))(user => (user.name, user.age))

}

I would like to automate the derivation process: for every field t: T of a case class, I want to call field[T]("t"), and then I want to combine the records together with the zip method, and finally I want to transform them to a case class with xmap.

I wrote the following, but I am stuck:

trait GenericInstances extends Records {

  def derivedRecord[A](using instances: K0.ProductInstances[Field, A]): Record[A] =
    ???

  // and then...

  derivedRecord[User]

}

It seems that none of the methods on K0.ProductInstances would work for me.

[joroKr21]

More missing methods: medeia/medeia#259

We're definitely missing construct with applicative semantics.
But I can't tell if that would be enough for your case.