How to match a Scala Object type, like None or Nil, in a Shapeless Poly?

Question

Say you have this HList

val l = Some(1) :: Nil :: HNil

and you want to create a shapeless Poly function that will match with one implicit on the Nil, but with a different implicit on the Some. Naively, we might think this would work:

object matcher extends Poly1 {
  implicit def caseNil = at[Nil.type](x => 0)
  implicit def caseSome[T] = at[Some[T]](x => 1)
}

However this fails spectacularly and try as I might I can't seem to create a shapeless Poly that will match a Scala Object type, like Nil or None. It's easy enough to match a generic List[T] however matching Nil seems impossible.

Is it possible to create a shapeless Poly case that will match a Scala Object type? If so, how? If not, why not?

Your code seems to work for me, can you add an example how "this fails spectacularly" ? — Peter Neyens
Huh, well that was unexpected. Turns out I was actually trying to figure out how match inside a tuple. What I ended up needing was a type alias. — Mark Kegel

Haspemulator Haspemulator · Accepted Answer · 2016-07-01T10:14:29

It compiles just fine for me:

@ object matcher extends Poly1 {
    implicit def caseNil = at[Nil.type](x => 0)
    implicit def caseSome[T] = at[Some[T]](x => 1)
  }
defined object matcher

@ val l = Some(1) :: Nil :: HNil
l: Some[Int] :: Nil.type :: HNil = Some(1) :: List() :: HNil

@ l.map(matcher)
res11: Int :: Int :: HNil = 1 :: 0 :: HNil

How to match a Scala Object type, like None or Nil, in a Shapeless Poly?

1 Answers