How to flatten an HLists of HLists

Question

So at runtime I get an Hlist of Hlists which looks like:

(2 :: HNil) :: (1001 :: HNil) :: (1001 :: HNil) :: HNil

Here the type of the resultant Hlist is :

(Int :: HNil) :: (Long :: HNil) :: (Long :: HNil) :: HNil

but this could have been any other type at runtime. For example, it could have been

(Int :: HNil) :: (String :: HNil) :: HNil

Is there a way to flatten this HList like so:

2 :: 1001 :: 1001 :: HNil

with the following type:

Int :: Long :: Long :: HNil

Answer 1

You can do it applying functional operations to HList. Take a look to how Poly works. First, we declare the Poly for (Int::HNil) and (Long::HNil):

  import shapeless._

  object myPoly extends Poly1 {
    implicit val tupleIntCase: Case.Aux[(Int :: HNil), Int] =
      at(position => position.head)
    implicit val tupleLongCase: Case.Aux[(Long :: HNil), Long] =
      at(position => position.head)
  }

and using the map function you can extract and flatten the HList:

((2 :: HNil) :: (1001L :: HNil) :: (1001 :: HNil) :: HNil).map(myPoly)

It gives the desired result:

2 :: 1001 :: 1001 :: HNil

Note that the resulting type here is:

Int :: Long :: Int :: HNil