Shapeless and annotations - scala

Question

I would like to have some function applied to fields in a case class, that are annotated with MyAnnotation. The idea is to transform type T into its generic representation, extract annotations, zip, fold right (or left) to reconstruct a generic representation and finally get back to type T. I followed the answer provided here and this gist.
I'm using scala 2.11.12 and shapeless 2.3.3.
Hereafter is my code:
import shapeless._
import shapeless.ops.hlist._
case class MyAnnotation(func: String) extends scala.annotation.StaticAnnotation
trait Modifier[T] {
def modify(t: T): T
}
object Modifier {
def apply[A: Modifier]: Modifier[A] = implicitly[Modifier[A]]
def create[T](func: T => T): Modifier[T] = new Modifier[T] { override def modify(t: T): T = func(t) }
private def id[T](t: T) = t
implicit val stringModifier: Modifier[String] = create(id)
implicit val booleanModifier: Modifier[Boolean] = create(id)
implicit val byteModifier: Modifier[Byte] = create(id)
implicit val charModifier: Modifier[Char] = create(id)
implicit val doubleModifier: Modifier[Double] = create(id)
implicit val floatModifier: Modifier[Float] = create(id)
implicit val intModifier: Modifier[Int] = create(id)
implicit val longModifier: Modifier[Long] = create(id)
implicit val shortModifier: Modifier[Short] = create(id)
implicit val hnilModifier: Modifier[HNil] = create(id)
implicit def hlistModifier[H, T <: HList, AL <: HList](
implicit
hser: Lazy[Modifier[H]],
tser: Modifier[T]
): Modifier[H :: T] = new Modifier[H :: T] {
override def modify(ht: H :: T): H :: T = {
ht match {
case h :: t =>
hser.value.modify(h) :: tser.modify(t)
}
}
}
implicit val cnilModifier: Modifier[CNil] = create(id)
implicit def coproductModifier[L, R <: Coproduct](
implicit
lser: Lazy[Modifier[L]],
rser: Modifier[R]
): Modifier[L :+: R] = new Modifier[L :+: R] {
override def modify(t: L :+: R): L :+: R = t match {
case Inl(l) => Inl(lser.value.modify(l))
case Inr(r) => Inr(rser.modify(r))
}
}
object Collector extends Poly2 {
implicit def myCase[ACC <: HList, E] = at[(E, Option[MyAnnotation]), ACC] {
case ((e, None), acc) => e :: acc
case ((e, Some(MyAnnotation(func))), acc) => {
println(func)
e :: acc
}
}
}
implicit def genericModifier[T, HL <: HList, AL <: HList, ZL <: HList](
implicit
gen: Generic.Aux[T, HL],
ser: Lazy[Modifier[HL]],
annots: Annotations.Aux[MyAnnotation, T, AL],
zip: Zip.Aux[HL :: AL :: HNil, ZL],
rightFolder: RightFolder[ZL, HNil.type, Collector.type]
): Modifier[T] = new Modifier[T] {
override def modify(t: T): T = {
val generic = gen.to(t)
println(generic)
val annotations = annots()
println(annotations)
val zipped = zip(generic :: annotations :: HNil)
println(zipped)
val modified = zipped.foldRight(HNil)(Collector)
println(modified)
val typed = gen.from(generic) // temporary
typed
}
}
}
The code above compiles. However, when instanciating a Modifier in a test:
case class Test(a: String, #MyAnnotation("sha1") b: String)
val test = Test("A", "B")
val modifier: Modifier[Test] = implicitly
the test file does not compile and give the following error:
[error] ambiguous implicit values:
[error] both value StringCanBuildFrom in object Predef of type =>
scala.collection.generic.CanBuildFrom[String,Char,String]
[error] and method $conforms in object Predef of type [A]=> <:<[A,A]
[error] match expected type T
[error] val ser1: Modifier[Test] = implicitly
The problem seems to come from the right folder definition: when removing rightFolder from the list of implicits in genericModifier, then it works:
implicit def genericModifier[T, HL <: HList, AL <: HList, ZL <: HList](
implicit
gen: Generic.Aux[T, HL],
ser: Lazy[Modifier[HL]],
annots: Annotations.Aux[MyAnnotation, T, AL],
zip: Zip.Aux[HL :: AL :: HNil, ZL]/*,
rightFolder: RightFolder[ZL, HNil.type, Collector.type]*/
): Modifier[T] = new Modifier[T] {
override def modify(t: T): T = {
val generic = gen.to(t)
println(generic)
val annotations = annots()
println(annotations)
val zipped = zip(generic :: annotations :: HNil)
println(zipped)
/*val modified = zipped.foldRight(HNil)(Collector)
println(modified)*/
val typed = gen.from(generic) // temporary
typed
}
}
What is wrong?

Answer 1

There are several mistakes in your code:
defining Poly just for Option is too rough (pattern matching is performed at runtime and compiler should know definitions for Some and None at compile time)
HNil should be instead of HNil.type and HNil : HNil instead of HNil (types HNil and HNil.type are different)
compiler doesn't know that RightFolder actually returns the original HList type, so you should use RightFolder.Aux type.
Correct code is
import shapeless.ops.hlist.{RightFolder, Zip}
import shapeless.{::, Annotations, Generic, HList, HNil, Lazy, Poly2}
import scala.annotation.StaticAnnotation
object App {
case class MyAnnotation(func: String) extends StaticAnnotation
object Collector extends Poly2 {
// implicit def myCase[ACC <: HList, E] = at[(E, Option[PII]), ACC] {
// case ((e, None), acc) => e :: acc
// case ((e, Some(MyAnnotation(func))), acc) => {
// println(func)
// e :: acc
// }
// }
implicit def someCase[ACC <: HList, E]: Case.Aux[(E, Some[MyAnnotation]), ACC, E :: ACC] = at {
case ((e, Some(MyAnnotation(func))), acc) =>
println(func)
e :: acc
}
implicit def noneCase[ACC <: HList, E]: Case.Aux[(E, None.type), ACC, E :: ACC] = at {
case ((e, None), acc) => e :: acc
}
}
trait Modifier[T] {
def modify(t: T): T
}
implicit def hListModifier[HL <: HList]: Modifier[HL] = identity(_)
// added as an example, you should replace this with your Modifier for HList
implicit def genericModifier[T, HL <: HList, AL <: HList, ZL <: HList](implicit
gen: Generic.Aux[T, HL],
ser: Lazy[Modifier[HL]],
annots: Annotations.Aux[MyAnnotation, T, AL],
zip: Zip.Aux[HL :: AL :: HNil, ZL],
rightFolder: RightFolder.Aux[ZL, HNil/*.type*/, Collector.type, HL /*added*/]
): Modifier[T] = new Modifier[T] {
override def modify(t: T): T = {
val generic = gen.to(t)
println(generic)
val annotations = annots()
println(annotations)
val zipped = zip(generic :: annotations :: HNil)
println(zipped)
val modified = zipped.foldRight(HNil : HNil /*added*/)(Collector)
println(modified)
val typed = gen.from(modified)
typed
}
}
case class Test(a: String, #MyAnnotation("sha1") b: String)
val test = Test("A", "B")
val modifier: Modifier[Test] = implicitly[Modifier[Test]]
def main(args: Array[String]): Unit = {
val test1 = modifier.modify(test) // prints "sha1"
println(test1) // Test(A,B)
}
}