Map a single type in a Coproduct. Lift Function1 to work on Coproducts.

Replace.scala

import shapeless._

/** In coproduct `C`, replace `A` with `B`. */
trait Replace[C <: Coproduct, A, B] extends DepFn2[C, A => B] {
  type Out <: Coproduct

  def lift(f: A => B): C => Out =
    c => apply(c, f)
}

object Replace {
  type Aux[C <: Coproduct, A, B, D <: Coproduct] = Replace[C, A, B] { type Out = D }

  implicit def inl[H1, T <: Coproduct, H2]: Aux[H1 :+: T, H1, H2, H2 :+: T] =
    new Replace[H1 :+: T, H1, H2] {
      type Out = H2 :+: T

      def apply(c: H1 :+: T, f: H1 => H2): Out = c match {
        case Inl(h) => Inl(f(h))
        case Inr(t) => Inr(t)
      }
    }

  implicit def inr[H, T <: Coproduct, A, B](
    implicit replace: Replace[T, A, B]
  ): Aux[H :+: T, A, B, H :+: replace.Out] = new Replace[H :+: T, A, B] {
    type Out = H :+: replace.Out

    def apply(c: H :+: T, f: A => B): Out = c match {
      case Inl(h) => Inl(h)
      case Inr(t) => Inr(replace(t, f))
    }
  }
}

case class CoproductExt[C <: Coproduct](c: C) extends AnyVal {
  def replace[A, B](f: A => B)(implicit replace: Replace[C, A, B]) = replace(c, f)
}

object CoproductExt {
  import scala.language.implicitConversions

  implicit def coproductExt[C <: Coproduct](c: C) = CoproductExt(c)
}

case class Function1Ext[A, B](f: A => B) extends AnyVal {
  def lift[C <: Coproduct, D <: Coproduct](implicit replace: Replace.Aux[C, A, B, D]): C => D = replace.lift(f)
  def liftTo[C <: Coproduct](implicit replace: Replace[C, A, B]): C => replace.Out = replace.lift(f)
}

object Function1Ext {
  import scala.language.implicitConversions

  implicit def function1Ext[A, B](f: A => B) = Function1Ext(f)
}

object Test extends App {
  import CoproductExt._
  import Function1Ext._

  type ISB = Int :+: String :+: Boolean :+: CNil
  type IDB = Int :+: Double :+: Boolean :+: CNil

  val isb = Coproduct[ISB]("foo")
  val f: String => Double = x => 0.42

  val idb = isb replace f
  println(idb) // 0.42

  val g: ISB => IDB = f.lift
  println(g(isb)) // 0.42

  val h = f.liftTo[ISB]
  println(h(isb)) // 0.42
}