Who can explain the meaning of this scala code

Question

I am reading this code for a long time. I typed it into REPL and it works as well.

but I don't have any idea of what's going on here. Why and how does this even work!!!

import shapeless._

case class Size[L <: HList](get : Int)
object Size {
    implicit val hnilSize = Size[HNil](0)
    implicit def hconsSize[H, T <: HList](implicit tailSize: Size[T]) = 
        Size[H :: T](1 + tailSize.get)
    def apply[L <: HList](l : L)(implicit size: Size[L]) : Int = size.get
}

Size(1 :: "Foo" :: true :: HNil)

Can someone explain this step by step and help me understand what is going on here.

Answer 1

Yeah, that's pretty thick stuff.

The mind-bender here is that hconsSize is recursive without actually being self referential.

Both apply and hconsSize pull in an implicit of type Size[X]. There are only two implicits that could fit that bill:

1. hnilSize, but only if X is type HNil
2. hconsSize itself

So apply pulls in the hconsSize implicit, which adds 1 to the stack and pulls in another hconsSize implicit (not necessarily in that order). This continues until we encounter an element of type HNil. Then the hnilSize implicit is pulled in, the get is zero, the stack is unrolled and all those 1's are added up.

Result: number of elements in the shapeless HList.