Scala Shapeless - Iterating/reading each item of Generic.Repr or converting it to HList

Question

I'm learning shapeless and referring a tutorial from here. Which says,

Generic is a simple way to convert case class and product types (like tuples) to HList, and vice-versa:

import shapeless.Generic

case class UserWithAge(name: String, age: Int)
val gen = Generic[UserWithAge]
val u = UserWithAge("Julien", 30)

val h = gen.to(u)

Now if I print h, I will get Julien :: 30 :: HNil. But, I'm unable to read each element from h such as h.head , h.tail will not work and there aren't any methods available in h. Here, h is type of gen.Repr and I couldn't figure out a way to convert it into HList either. So, how can I read each element from h?

Answer 1

In this case the type of gen.to(u) is gen.Repr, which if you look at the type of gen actually expends to String :: Int :: HNil, so it's already a HList!

scala> import shapeless.Generic
import shapeless.Generic

scala> case class UserWithAge(name: String, age: Int)
defined class UserWithAge

scala> val gen = Generic[UserWithAge]
gen: shapeless.Generic[UserWithAge]{type Repr = shapeless.::[String,shapeless.::[Int,shapeless.HNil]]} = anon$macro$3$1@4ff329b8

scala> val u = UserWithAge("Julien", 30)
u: UserWithAge = UserWithAge(Julien,30)

scala> val h = gen.to(u)
h: gen.Repr = Julien :: 30 :: HNil

scala> h.head
res0: String = Julien

scala> h.tail
res1: shapeless.::[Int,shapeless.HNil] = 30 :: HNil

In the general case, the Repr type of a Generic will be either a HList or a Coproduct. For examples of how to generically program over these, see shapeless-type-class-derivation-2015-demo.