scala - generic unzip for HList

Question

I have the following Scala problem:

Write a function that will take a list of HLists

List(23 :: “a” :: 1.0d :: HNil, 24 :: “b” :: 2.0d :: HNil)    #          


        

Answer 1

To solve this you'll need a custom typeclass:

import shapeless._

trait Unzipper[ -input ]{
  type Output
  def unzip( input: input ): Output
}
object Unzipper {
  implicit def headTail
    [ head, tail <: HList ]
    ( implicit tailUnzipper: Unzipper[ List[ tail ] ]{ type Output <: HList } )
    =
    new Unzipper[ List[ head :: tail ] ]{
      type Output = List[ head ] :: tailUnzipper.Output
      def unzip( list: List[ head :: tail ] ) = 
        list.map(_.head) :: tailUnzipper.unzip(list.map(_.tail))
    }
  implicit val nil =
    new Unzipper[ List[ HNil ] ]{
      type Output = HNil
      def unzip( list: List[ HNil ] ) = HNil
    }
}

val list = List(23 :: "a" :: 1.0d :: HNil, 24 :: "b" :: 2.0d :: HNil)
println( implicitly[Unzipper[list.type]].unzip(list) )

Outputs:

List(23, 24) :: List(a, b) :: List(1.0, 2.0) :: HNil