shapeless calling value on lazy make a StackOverflowError - scala

Question

While learning shapeless via this useful guide
I was trying to merge what I have learned from this in this exercice
The result is this code :
import shapeless._
object GenericV3 {
trait CsvEncoder[A] {
def width: Int
def encode(value: A): List[String]
}
def createEncoder[A](cols: Int)(func: A => List[String]): CsvEncoder[A] =
new CsvEncoder[A] {
val width = cols
def encode(value: A): List[String] =
func(value)
}
implicit val stringEncoder: CsvEncoder[String] =
createEncoder(1)(str => List(str))
implicit val intEncoder: CsvEncoder[Int] =
createEncoder(1)(num => List(num.toString))
implicit val booleanEncoder: CsvEncoder[Boolean] =
createEncoder(1)(bool => List(if(bool) "cone" else "glass"))
implicit val doubleEncoder: CsvEncoder[Double] =
createEncoder(1)(d => List(d.toString))
import shapeless.{HList, HNil, ::}
implicit val hnilEncoder: CsvEncoder[HNil] =
createEncoder(0)(hnil => Nil)
implicit def hlistEncoder[H, T <: HList](
implicit
hEncoder: Lazy[CsvEncoder[H]],
tEncoder: CsvEncoder[T]
): CsvEncoder[H :: T] =
createEncoder(hEncoder.value.width + tEncoder.width) {
case h :: t =>
hEncoder.value.encode(h) ++ tEncoder.encode(t)
}
import shapeless.{Coproduct, CNil, :+:, Inl, Inr}
implicit val cnilEncoder: CsvEncoder[CNil] =
createEncoder(0) { cnil =>
throw new Exception("The impossible has happened!")
}
implicit def coproductEncoder[H, T <: Coproduct](
implicit
hEncoder: Lazy[CsvEncoder[H]],
tEncoder: CsvEncoder[T]
): CsvEncoder[H :+: T] =
createEncoder(hEncoder.value.width + tEncoder.width) {
case Inl(h) => hEncoder.value.encode(h) ++ List.fill(tEncoder.width)("")
case Inr(t) => List.fill(hEncoder.value.width)("") ++ tEncoder.encode(t)
}
import shapeless.Generic
implicit def genericEncoder[A, R](
implicit
gen: Generic.Aux[A, R],
lEncoder: Lazy[CsvEncoder[R]]
): CsvEncoder[A] =
createEncoder(lEncoder.value.width) { value =>
lEncoder.value.encode(gen.to(value))
}
def writeCsv[A](values: List[A])(implicit encoder: CsvEncoder[A]): String =
values.map(encoder.encode).map(_.mkString(",")).mkString("\n")
}
when calling GenericV3.writeCsv with a sealed case class that has a case containing the sealed case class itself.
the example from the guide :
sealed trait Tree[A]
final case class Branch[A](left: Tree[A], right: Tree[A]) extends Tree[A]
final case class Leaf[A](value: A) extends Tree[A]
I got a StackOverflowError at runtime.
Exemple
GenericV3.writeCsv[Tree[String]](List(Branch(Leaf("toto"),Leaf("titi"))))
The error is produced because I call hEncoder.value.width before matching Inl and Inr.
I tried to read how it's done in other libs.
And I have seen this which looks the same.
So I think I missed something.
ps : I would like to thank to the author of this guide it's very helpful