retain key information in implicit LabelledGeneric - scala

Question

I have this code that convert an arbitrary size of class to an HList
class Record[H <: HList](val hs: H)
object Record {
import shapeless.LabelledGeneric
def apply[P <: Product, L <: HList](p: P)(implicit gen: LabelledGeneric.Aux[P, L]) = new Record[L](gen.to(p))
}
I would like to use it as follows:
import shapeless._
import syntax.singleton._
val h = Record('a ->> 1, 'b ->> 2)
# h.hs
res91: cmd90.<refinement>.this.type.Out = 1 :: 2 :: HNil
However the keys are not retained after the conversion. As opposed to creating the same HList:
# h.hs.keys //error: could not find implicit value for parameter keys: shapeless.ops.record.Keys[this.Out]
# ('a ->> 1 :: 'b ->> 2 :: HNil).keys //works ok
The question is how to retain the key information in the record.

Answer 1

You use records incorrectly.
Things like 'a ->> 1, 'b ->> 2 shouldn't be used separately, they should be used inside HLists like 'a ->> 1 :: 'b ->> 2 :: HNil.
When you write Record('a ->> 1, 'b ->> 2) the argument of Record is considered as Tuple2. Since Tuple2 is a case class, LabelledGeneric transforms it implicitly into an HList (actually, a shapeless' record). Look at your h, its type is very strange (pseudocode):
Record['_1 -> 'a -> Int :: '_2 -> 'b -> Int :: HNil]
(_1 and _2 are field names of Tuple2).
So how to fix your code depends on what you really wanted to get (I'm not sure). One possible way is
// class Record[H <: HList](val hs: H)
object Record {
def apply[P <: Product, L <: HList](p: P)(implicit gen: LabelledGeneric.Aux[P, L]) = /*new Record[L](*/gen.to(p)/*)*/
}
case class MyCaseClass(a: Int, b: Int)
val h = Record(MyCaseClass(1, 2))
h/*.hs*/ // 1 :: 2 :: HNil
h/*.hs*/.keys // 'a :: 'b :: HNil
('a ->> 1 :: 'b ->> 2 :: HNil).keys // 'a :: 'b :: HNil
I would like to have something like Record('a ->> 1, 'b ->> 2) ... or potentially Record(('a', 1), ('b', 2))
as an input, and an HList 'a ->> 1 :: 'b ->> 2 :: HNil as an output.
Then you don't need LabelledGeneric. You can transform Record('a ->> 1, 'b ->> 2) into just a tuple ('a ->> 1, 'b ->> 2) (it's Record.unapply(...).get if you defined case class Record(p: Product)) and then this tuple into HList:
import shapeless.{::, HNil}
import shapeless.syntax.std.tuple._
import shapeless.syntax.singleton._
('a ->> 1, 'b ->> 2).productElements == 'a ->> 1 :: 'b ->> 2 :: HNil // true