Mapping tuples in shapeless HList

Question

You do

  import shapeless._ ; import poly._

  object fun extends (List ~>> (List, Int)) {
    override def apply[T](list: List[T]): (List, Int) = list -> list.size
  }

  println((List(1,2,3) :: List("a", "b", "c") :: HNil).map(fun))

to map every sublist into a couple. However, what if HList elements are more complex, like tutples, for instance? The natural attempt

  object fun extends ((String -> List) ~>> (List, Int)) {
    override def apply[T](list: (String -> List[T])): (List, Int) = list -> list.size

is rejected by the compiler. What do you do? Where can you learn that?

Answer 1

import shapeless._
import poly._

object fun2 extends Poly1 {
  implicit def caseTuple[T] =
    at[(String, List[T])](x => (x._2.tail, x._1.toInt))
}

Then:

println((("56", List(1,2,3)) :: ("78", List(4,2,3)) :: HNil).map(fun))

Or if you want to do it with original ~> it's yet still possible, but looks ugly to my taste:

object fun3 extends (({type L[T] = (String, List[T])})#L ~> ({type L[T] = (List[T], Int)})#L) {
  override def apply[T](f: (String, List[T])): (List[T], Int) = f match {
    case (str, list) => (list.tail, str.toInt)
  }
}

println((("56", List(1,2,3)) :: ("78", List(4,2,3)) :: HNil).map(fun3))

P.S. Also please note that your code example does not compile. You miss [T] in the first part, you need to use ~> and String -> List is a wrong type in scala in the last part. List is a type constructor, not type and you can't use it this way