Mapping over HList raises AbstractMethodError

问题

I am trying a shapeless example in REPL and getting a runtime error:

scala> import shapeless._
import shapeless._

scala> import shapeless.poly._
import shapeless.poly._

scala> object choose extends (Set ~> Option) {
     |   def apply[T](set: Set[T]) = set.headOption
     | }
defined object choose

scala> val sets = Set(1) :: Set(0) :: HNil
sets: shapeless.::[scala.collection.immutable.Set[Int],shapeless.::[scala.collection.immutable.Set[Int],shapeless.HNil]] = Set(1) :: Set(0) :: HNil

scala> sets map choose
java.lang.AbstractMethodError: choose$.caseUniv()Lshapeless/PolyDefns$Case;
  ... 42 elided

Do you know why it doesn't work and how to fix it ?

来源：https://stackoverflow.com/questions/39374015/mapping-over-hlist-raises-abstractmethoderror