Quick demo of shapeless's Comapped /cc @lars_h @jdegoes

gistfile1.scala

scala> :paste
// Entering paste mode (ctrl-D to finish)

import shapeless._ ; import ops.hlist.Comapped

sealed trait Parser[A]
case class DummyParser[A]() extends Parser[A]

case class AllOf[L <: HList, O <: HList](parsers: L)
  (implicit val cm: Comapped.Aux[L, Parser, O]) extends Parser[O]

// Exiting paste mode, now interpreting.

import shapeless._
import ops.hlist.Comapped
defined trait Parser
defined class DummyParser
defined class AllOf

scala> AllOf((DummyParser[Int](): Parser[Int]) :: HNil): Parser[Int :: HNil]
res0: Parser[shapeless.::[Int,shapeless.HNil]] = AllOf(DummyParser() :: HNil)