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