How to implicitly figure out the type at the head of a shapeless HList

Question

Lets say I have the following:

case class TestField(value: String)
case class TestField2(value: String)

implicit class ProductExtensions[T <: Product](val va         


        

Answer 1

Here is generalized version

implicit class ProductExtensions[T <: Product, L <: HList](val value: T) extends AnyVal {
  def mapTo[R <: Product](implicit tGen: Generic.Aux[T, L], rGen: Generic.Aux[R, L]): R = rGen.from(tGen.to(value))
}

The Type Astronaut’s Guide to Shapeless. 6.3 Case study: case class migrations https://books.underscore.io/shapeless-guide/shapeless-guide.html#sec:ops:migration

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New version

import shapeless.ops.hlist.IsHCons

implicit class ProductExtensions[T <: Product, L <: HList, U, L1 <: HList](val value: T) extends AnyVal {
  def mapTo[R <: Product](implicit 
                          tGen: Generic.Aux[T, L], 
                          rGen: Generic.Aux[R, L], 
                          isHCons: IsHCons.Aux[L, U, L1], 
                          o: OtherImplicit[U]
                         ): R = rGen.from(tGen.to(value))
}

4.3 Chaining dependent functions https://books.underscore.io/shapeless-guide/shapeless-guide.html#sec:type-level-programming:chaining

Scala shapeless Generic.Aux implicit parameter not found in unapply