CODEWITHSUNDEEP
scalagenericsshapeless
joel
joel

Reputation: 480

Scala Shapeless No Implicits Found Case Class Migration Adding New Field

In order to get familiar with shapeless, I did investigate into the examples of the shapeless-guide. I am especially interested in case class migrations. Therefore, I did dive into the case study of the case class migration section of the shapeless-guide.

Unfortunately, the code of the case class migration case study is not compiling as the last conversion where a field is added is failing as no implicits can be found.

I am trying to understand the issue with the code. However, understanding the issue is quite difficult and I failed so far. Hence, i would like to ask whether somebody can give an hint.

I tried also several compilers, i.e. scala 2.12, scala 2.13.

I did run the compiler with the -Xlog-implicits flag which gives several error messages:

[info] Icecream.scala:67:42: shapeless.this.Generic.materialize is not a valid implicit value for shapeless.Generic.Aux[Icecream.IceCreamV1,V] because:
[info] type parameters weren't correctly instantiated outside of the implicit tree: inferred type arguments [String :: Int :: Boolean :: shapeless.HNil,Nothing] do not conform to method materializeCoproduct's type parameter bounds [V <: shapeless.Coproduct,R <: shapeless.Coproduct]
[info]   IceCreamV1("Sundae", 1, true).migrateTo[IceCreamV2c]
[info]                                          ^
[info] Icecream.scala:67:42: shapeless.this.LabelledGeneric.materializeCoproduct is not a valid implicit value for shapeless.LabelledGeneric.Aux[Icecream.IceCreamV1,ARepr] because:
[info] hasMatchingSymbol reported error: could not find implicit value for parameter gen: shapeless.Generic.Aux[Icecream.IceCreamV1,V]
[info]   IceCreamV1("Sundae", 1, true).migrateTo[IceCreamV2c]
[info]                                          ^
[info] Icecream.scala:67:42: shapeless.this.Generic.materialize is not a valid implicit value for shapeless.Generic.Aux[Icecream.IceCreamV2c,V] because:
[info] type parameters weren't correctly instantiated outside of the implicit tree: inferred type arguments [String :: Boolean :: Int :: Int :: shapeless.HNil,Nothing] do not conform to method materializeCoproduct's type parameter bounds [V <: shapeless.Coproduct,R <: shapeless.Coproduct]
[info]   IceCreamV1("Sundae", 1, true).migrateTo[IceCreamV2c]
[info]                                          ^
[info] Icecream.scala:67:42: shapeless.this.LabelledGeneric.materializeCoproduct is not a valid implicit value for shapeless.LabelledGeneric.Aux[Icecream.IceCreamV2c,BRepr] because:
[info] hasMatchingSymbol reported error: could not find implicit value for parameter gen: shapeless.Generic.Aux[Icecream.IceCreamV2c,V]
[info]   IceCreamV1("Sundae", 1, true).migrateTo[IceCreamV2c]
[info]                                          ^
[info] Icecream.scala:67:42: hlist.this.Prepend.hnilPrepend1 is not a valid implicit value for shapeless.ops.hlist.Prepend.Aux[shapeless.labelled.FieldType[Symbol @@ String("numWaffles"),Int] :: shapeless.ops.hlist.ZipWithKeys.hnilZipWithKeys.Out,shapeless.labelled.FieldType[Symbol @@ String("name"),String] :: shapeless.labelled.FieldType[Symbol @@ String("numCherries"),Int] :: shapeless.labelled.FieldType[Symbol @@ String("inCone"),Boolean] :: shapeless.HNil,Unaligned] because:
[info] typing TypeApply reported errors for the implicit tree: type arguments [shapeless.labelled.FieldType[Symbol @@ String("numWaffles"),Int] :: shapeless.ops.hlist.ZipWithKeys.hnilZipWithKeys.Out,shapeless.labelled.FieldType[Symbol @@ String("name"),String] :: shapeless.labelled.FieldType[Symbol @@ String("numCherries"),Int] :: shapeless.labelled.FieldType[Symbol @@ String("inCone"),Boolean] :: shapeless.HNil] do not conform to method hnilPrepend1's type parameter bounds [P <: shapeless.HNil,S <: shapeless.HList]
[info]   IceCreamV1("Sundae", 1, true).migrateTo[IceCreamV2c]
[info]                                          ^
[info] Icecream.scala:67:42: hlist.this.Prepend.hnilPrepend0 is not a valid implicit value for shapeless.ops.hlist.Prepend.Aux[shapeless.labelled.FieldType[Symbol @@ String("numWaffles"),Int] :: shapeless.ops.hlist.ZipWithKeys.hnilZipWithKeys.Out,shapeless.labelled.FieldType[Symbol @@ String("name"),String] :: shapeless.labelled.FieldType[Symbol @@ String("numCherries"),Int] :: shapeless.labelled.FieldType[Symbol @@ String("inCone"),Boolean] :: shapeless.HNil,Unaligned] because:
[info] typing TypeApply reported errors for the implicit tree: type arguments [shapeless.labelled.FieldType[Symbol @@ String("numWaffles"),Int] :: shapeless.ops.hlist.ZipWithKeys.hnilZipWithKeys.Out,shapeless.labelled.FieldType[Symbol @@ String("name"),String] :: shapeless.labelled.FieldType[Symbol @@ String("numCherries"),Int] :: shapeless.labelled.FieldType[Symbol @@ String("inCone"),Boolean] :: shapeless.HNil] do not conform to method hnilPrepend0's type parameter bounds [P <: shapeless.HList,S <: shapeless.HNil]
[info]   IceCreamV1("Sundae", 1, true).migrateTo[IceCreamV2c]
[info]                                          ^
[info] Icecream.scala:67:42: hlist.this.Remove.recurse is not a valid implicit value for shapeless.ops.hlist.Remove.Aux[shapeless.labelled.FieldType[Symbol @@ String("numCherries"),Int] :: shapeless.HNil,shapeless.labelled.FieldType[Symbol @@ String("numCherries"),Int],(shapeless.labelled.FieldType[Symbol @@ String("numCherries"),Int], OutT)] because:
[info] hasMatchingSymbol reported error: could not find implicit value for parameter r: shapeless.ops.hlist.Remove.Aux[shapeless.HNil,shapeless.labelled.FieldType[Symbol @@ String("numCherries"),Int],(shapeless.labelled.FieldType[Symbol @@ String("numCherries"),Int], OutT)]
[info]   IceCreamV1("Sundae", 1, true).migrateTo[IceCreamV2c]
[info]                                          ^
[info] Icecream.scala:67:42: hlist.this.Remove.recurse is not a valid implicit value for shapeless.ops.hlist.Remove.Aux[shapeless.labelled.FieldType[Symbol @@ String("numWaffles"),Int] :: shapeless.HNil,shapeless.labelled.FieldType[Symbol @@ String("numWaffles"),Int],(shapeless.labelled.FieldType[Symbol @@ String("numWaffles"),Int], R)] because:
[info] hasMatchingSymbol reported error: could not find implicit value for parameter r: shapeless.ops.hlist.Remove.Aux[shapeless.HNil,shapeless.labelled.FieldType[Symbol @@ String("numWaffles"),Int],(shapeless.labelled.FieldType[Symbol @@ String("numWaffles"),Int], OutT)]
[info]   IceCreamV1("Sundae", 1, true).migrateTo[IceCreamV2c]

The full code is as following:

  case class IceCreamV1(name: String, numCherries: Int, inCone: Boolean)
  case class IceCreamV2a(name: String, inCone: Boolean)
  case class IceCreamV2b(name: String, inCone: Boolean, numCherries: Int)
  case class IceCreamV2c(name: String, inCone: Boolean, numCherries: Int, numWaffles: Int)

  trait Migration[A, B] {
    def apply(a: A): B
  }

  implicit class MigrationOps[A](a: A) {
    def migrateTo[B](implicit migration: Migration[A, B]): B =
      migration.apply(a)
  }

  def createMonoid[A](zero: A)(add: (A, A) => A): Monoid[A] =
    new Monoid[A] {
      def empty = zero
      def combine(x: A, y: A): A = add(x, y)
    }

  implicit val hnilMonoid: Monoid[HNil] = createMonoid[HNil](HNil)((x, y) => HNil)

  implicit def emptyHList[K <: Symbol, H, T <: HList](
     implicit
     hMonoid: Lazy[Monoid[H]],
     tMonoid: Monoid[T]
   ): Monoid[FieldType[K, H] :: T] =
    createMonoid(field[K](hMonoid.value.empty) :: tMonoid.empty) {
      (x, y) =>
        field[K](hMonoid.value.combine(x.head, y.head)) ::
          tMonoid.combine(x.tail, y.tail)
    }

  implicit def genericMigration[
    A, B, ARepr <: HList, BRepr <: HList,
    Common <: HList, Added <: HList, Unaligned <: HList
  ](
     implicit
     aGen: LabelledGeneric.Aux[A, ARepr],
     bGen: LabelledGeneric.Aux[B, BRepr],
     inter: hlist.Intersection.Aux[ARepr, BRepr, Common],
     diff: hlist.Diff.Aux[BRepr, Common, Added],
     monoid: Monoid[Added],
     prepend: hlist.Prepend.Aux[Added, Common, Unaligned],
     align: hlist.Align[Unaligned, BRepr]
   ): Migration[A, B] =
    new Migration[A, B] {
      def apply(a: A): B = bGen.from(align(prepend(monoid.empty, inter(aGen.to(a)))))
    }

  IceCreamV1("Sundae", 1, true).migrateTo[IceCreamV2a]
  // res14: IceCreamV2a = IceCreamV2a(Sundae,true)

  IceCreamV1("Sundae", 1, true).migrateTo[IceCreamV2b]
  // res15: IceCreamV2b = IceCreamV2b(Sundae,true,1)

  IceCreamV1("Sundae", 1, true).migrateTo[IceCreamV2c] // <- fails as implicit can not be found
  // res16: IceCreamV2c = IceCreamV2c(Sundae,true,1,0)

Upvotes: 1

Views: 205

Answers (1)

Dmytro Mitin
Dmytro Mitin

Reputation: 51658

Make sure that you have an instance of type class Monoid for type Int (i.e. implicit Monoid[Int]). For example check that implicitly[Monoid[Int]] compiles. Migration from IceCreamV1 into IceCreamV2c is adding new Int field so you need empty value for Int (i.e. 0).

So either use cats.Monoid (in fresh Cats you no longer need to add extra imports for instances of type classes since they are defined in companion objects) or if you define your custom Monoid

trait Monoid[A] {
  def empty: A
  def combine(x: A, y: A): A
}

then add

implicit val intMonoid: Monoid[Int] = createMonoid[Int](0)(_ + _)

Upvotes: 1

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