Matthias Langer Matthias Langer - 2 years ago 199
Scala Question

How to generically extract field names with Shapeless?

Given a case class

I can extract its field names with Shapeless using the following snippet:

val fieldNames: List[String] = {
import shapeless._
import shapeless.ops.record.Keys

val gen = LabelledGeneric[A]
val keys = Keys[gen.Repr].apply

This works all nice, but how can I implement this in a more generic manner, so that I can conveniently use this technique for arbitrary classes, like

val fields: List[String] = fieldNames[AnyCaseClass]

Is there a library that already does this for me?

Answer Source

Something like this maybe, slightly modified version of this example:

import shapeless._
import shapeless.ops.record._
import shapeless.ops.hlist.ToTraversable

trait FieldNames[T] {
  def apply(): List[String]

implicit def toNames[T, Repr <: HList, KeysRepr <: HList](
  implicit gen: LabelledGeneric.Aux[T, Repr],
  keys: Keys.Aux[Repr, KeysRepr],
  traversable: ToTraversable.Aux[KeysRepr, List, Symbol]
): FieldNames[T] = new FieldNames[T] {
  def apply() = keys()

def fieldNames[T](implicit h : FieldNames[T]) = h()
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