Shapeless flatmap HList with Option yielding HList

Question

Given the following

case class A(value:Int)
case class B(value:String)

val h:Option[A] :: A :: Option[B] :: Option[A] :: HNil = Some(A(1)) :: A(2) :: Some(B("two")) :: (None:Option[B]) :: HNil

How can I get the following ?

A(1) :: A(2) :: B("two") :: HNil

My attempt below

trait a extends Poly1 {
    implicit def any[T] = at[T](_ :: HNil)
}
object f extends a {            
    implicit def some[T] = at[Option[T]](t => if (t.isDefined) t.get :: HNil else HNil)         
}

works for map

h map f

> A(1) :: HNil :: A(2) :: HNil :: B(two) :: HNil :: HNil :: HNil

but fail for flatMap

h flatMap f

> could not find implicit value for parameter mapper: shapeless.ops.hlist.FlatMapper[f.type,shapeless.::[Option[A],shapeless.::[A,shapeless.::[Option[B],shapeless.::[Option[B],shapeless.HNil]]]]]

Answer 1

Most likely the only thing you can do is define separate cases for Some and for None :

trait a extends Poly1 {
  implicit def default[T] = at[T](_ :: HNil)
}
object f extends a {
  implicit def caseSome[T] = at[Some[T]](_.get :: HNil)
  implicit def caseNone = at[None.type](_ => HNil)
}

It also means you can't use generic Option s in the types, it must be known at compile time whether each element is Some or None :

scala> (Some(A(1)) :: A(2) :: Some(B("two")) :: None :: HNil) flatMap f
res1: shapeless.::[A,shapeless.::[A,shapeless.::[B,shapeless.HNil]]] = A(1) :: A(2) :: B(two) :: HNil

This distinction defines the type of the resulting expression: Some(1) :: HNil flatMap f will have type ::[Int, HNil] , but None :: HNil flatMap f will have type just HNil .

This kind of type information can't be figured out at compile time from simple Option s: should (x: Option[T]) :: HNil flatMap f have type ::[T, HNil] or HNil ? We don't know until we actually run the program and see what the value of x is.

I'm not sure if there's some smart way to do it anyway and get an opaque HList , but at that point you'll abandon exact type information about each element and the length of the list and may as well cast it to a normal List (and maybe use cast from shapeless later if you know what exact type the final result will have)