Converting case class to another recursively structural identical case class - scala

Question

I am trying to use Shapeless to convert case class like these:
case class A(int: Int, str: String)
case class B(a: A, str: String)
case class AnotherA(int: Int, str: String)
case class AnotherB(a: AnotherA, str: String)
Using Generic I can easily convert between A and AnotherA, but not for B and AnotherB because (of course) the a member is of different type.
I think I can convert case class into a nested HList following this example, but how do I convert the nested HList back to case class or there is a more straightforward way to do it ?

Answer 1

Here's a possible implementation of a DeepHUnlister which recursively replaces the sub-Hlists within the generic representation (nested HList) with their respective case class instances:
trait DeepHUnlister[L <: HList, R <: HList] extends (L => R)
object DeepHUnlister {
implicit object hnil extends DeepHUnlister[HNil, HNil] {
def apply(r: HNil) = HNil
}
implicit def headList[H <: HList, T <: HList, HR <: HList, TR <: HList, A](
implicit
gen: Generic.Aux[A, HR],
dhh: Lazy[DeepHUnlister[H, HR]],
dht: Lazy[DeepHUnlister[T, TR]]):
DeepHUnlister[H :: T, A :: TR] =
new DeepHUnlister[H :: T, A :: TR] {
def apply(r: H :: T) = gen.from(dhh.value(r.head)) :: dht.value(r.tail)
}
implicit def headAtomic[H, T <: HList, TR <: HList](
implicit dht: Lazy[DeepHUnlister[T, TR]]):
DeepHUnlister[H :: T, H :: TR] =
new DeepHUnlister[H :: T, H :: TR] {
def apply(r: H :: T) = r.head :: dht.value(r.tail)
}
def apply[T <: HList, R <: HList](implicit du: DeepHUnlister[T, R]):
DeepHUnlister[T, R] = du
}
Example:
case class A(int: Int, str: String)
case class B(a: A, str: String)
case class C(b: B, d: Double)
case class A1(int: Int, str: String)
case class B1(a: A1, str: String)
case class C1(a: B1, d: Double)
type ARepr = Int :: String :: HNil
type BRepr = ARepr :: String :: HNil
type CRepr = BRepr :: Double :: HNil
val c = C(B(A(1, "a"), "b"), 1.0)
val lister = DeepHLister[C :: HNil]
val repr = lister(c :: HNil)
println(repr)
val unlister = DeepHUnlister[CRepr :: HNil, C1 :: HNil]
val c1 = unlister(repr).head
// c1 = C1(B1(A1(1,a),b),1.0)
Maybe it's possible to enhance this solution to avoid passing the representation type as parameter to the unlister.
Update
Here's a version that omits the source type, but unfortunately requires a cast:
trait DepInvFn1[T] {
type In
def apply(i: In): T
}
trait DeepHUnlister[R <: HList] extends DepInvFn1[R] { type In <: HList }
trait DeepHUnlisterLP {
type Aux[L <: HList, R <: HList] = DeepHUnlister[R] { type In = L }
implicit def headAtomic[H, TR <: HList](
implicit dt: Lazy[DeepHUnlister[TR]]):
Aux[H :: dt.value.In, H :: TR] =
new DeepHUnlister[H :: TR] {
type In = H :: dt.value.In
def apply(r: H :: dt.value.In) = r.head :: dt.value(r.tail)
}
}
object DeepHUnlister extends DeepHUnlisterLP {
implicit object hnil extends DeepHUnlister[HNil] {
type In = HNil
def apply(r: HNil) = HNil
}
implicit def headList[HR <: HList, TR <: HList, A](
implicit
gen: Generic.Aux[A, HR],
dh: Lazy[DeepHUnlister[HR]],
dt: Lazy[DeepHUnlister[TR]]):
Aux[dh.value.In :: dt.value.In, A :: TR] =
new DeepHUnlister[A :: TR] {
type In = dh.value.In :: dt.value.In
def apply(r: dh.value.In :: dt.value.In) = gen.from(dh.value(r.head)) :: dt.value(r.tail)
}
def apply[R <: HList](implicit du: DeepHUnlister[R]): DeepHUnlister[R] = du
}
Usage:
val unlister = DeepHUnlister[C1 :: HNil]
val c1 = unlister(repr.asInstanceOf[unlister.In]).head
// c1 = C1(B1(A1(1,a),b),1.0)