Trying to restrict types in shapeless' HList

[Oliver Ruebenacker]

     Hello,

  I want to restrict the types of the elements of a shapeless HList. I tried:

import shapeless.{HList, LUBConstraint}

case class A[L <: HList](list: L)(implicit val c: LUBConstraint[L, String]) {
  def head: String = list.head
}

  This does not compile, giving me:

Error:(21, 27) could not find implicit value for parameter c: shapeless.ops.hlist.IsHCons[L]
  def head: String = list.head
                          ^

Error:(21, 27) not enough arguments for method head: (implicit c: shapeless.ops.hlist.IsHCons[L])c.H.
Unspecified value parameter c.
  def head: String = list.head
                          ^

  Any idea? Thanks!

     Best, Oliver

--

Oliver Ruebenacker

Senior Software Engineer, Diabetes Portal, Broad Institute

[William Harvey]

Here you go:

case class A[L <: HList](list: String :: L)(implicit val c: LUBConstraint[L, String]) {

  def head: String = list.head
}

[Oliver Ruebenacker]

     Hello,

  Thanks, but that only restricts the type of the head, right?

  OK, my example was bad, because it doesn't make sense for Strings.

  How would I restrict all elements of an HList to be of type Seq[_]? Thanks!

     Best, Oliver

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[William Harvey]

It restricts the type of the head and tail.

If you want Seq[_] elements in your HList, try this:

    case class A[L <: HList](list: Seq[_] :: L)(implicit val c: LUBConstraint[L, Seq[_]]) {
      def head: Seq[_] = list.head
    }
   
    A(Seq("Hello") :: HNil)
    A(Seq("Hello") :: Seq("World") :: HNil)
    A(Seq("Hello") :: Seq(42, 42f, "Blah") :: HNil)
    val blah = A(Seq("Hello") :: Seq(42, 42f, "Blah", java.awt.Color.red) :: HNil)
   
    val theHead: Seq[Any] = blah.head
   
    // Doesn't compile, as expected.
    // A(42 :: HNil)

Does that help?

Cheers,

William

[Oliver Ruebenacker]

Awesome, thanks!