Iterating over subclasses of a sealed trait

[adamw]

I'm trying to summon a typeclass instance for each child class of a sealed trait. So far I've got the following code (here simplified):

object TestMacro:
  inline def test[T]: Unit = ${ testImpl[T] }
  def testImpl[T: Type](using Quotes): Expr[Unit] =
    import quotes.reflect.*
    val tpe = TypeRepr.of[T]
    tpe.typeSymbol.children.map { childSymbol =>
      val childTpe = tpe.memberType(childSymbol)
      println("Child TypeRepr: " + childTpe)
      childTpe.asType match
        case '[c] =>
          println("Got Type!")
    }
    '{ () }

The childTpe: TypeRepr is correct, however I get an exception when converting it to a Type and trying to get a handle to the type parameter c (so that later I can do Expr.summon[MyTypeClass[c]]). Using the following invocation:

sealed trait A
case class X(i: Int) extends A

object Test extends App {
  TestMacro.test[A]
}

The output is:

Child type: ClassInfo(ThisType(TypeRef(NoPrefix,module class newschema)), class X, List(TypeRef(ThisType(TypeRef(NoPrefix,module class lang)),class Object), TypeRef(ThisType(TypeRef(NoPrefix,module class newschema)),trait A), TypeRef(TermRef(TermRef(NoPrefix,object _root_),object scala),trait Product), TypeRef(ThisType(TypeRef(NoPrefix,module class io)),trait Serializable)))
[info] assertion failure for class X in package sttp.tapir.newschema <:< c, frozen = false
[error] -- Error: Test.scala:33:16
[error] 33 |  TestMacro.test[A]
[error]    |  ^^^^^^^^^^^^^^^^^
[error]    |Exception occurred while executing macro expansion.
[error]    |java.lang.AssertionError: assertion failed: ClassInfo(ThisType(TypeRef(NoPrefix,module class newschema)), class X, List(TypeRef(ThisType(TypeRef(NoPrefix,module class lang)),class Object), TypeRef(ThisType(TypeRef(NoPrefix,module class newschema)),trait A), TypeRef(TermRef(TermRef(NoPrefix,object _root_),object scala),trait Product), TypeRef(ThisType(TypeRef(NoPrefix,module class io)),trait Serializable)))
[error]    |	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
[error]    |	at dotty.tools.dotc.core.Types$TypeBounds.<init>(Types.scala:5038)
[error]    |	at dotty.tools.dotc.core.Types$RealTypeBounds.<init>(Types.scala:5114)
[error]    |	at dotty.tools.dotc.core.Types$TypeBounds$.apply(Types.scala:5158)
[error]    |	at dotty.tools.dotc.core.Types$TypeBounds.derivedTypeBounds(Types.scala:5046)
[error]    |	at dotty.tools.dotc.core.ConstraintHandling.addOneBound(ConstraintHandling.scala:262)
[error]    |	at dotty.tools.dotc.core.ConstraintHandling.addOneBound$(ConstraintHandling.scala:29)
[error]    |	at dotty.tools.dotc.core.ProperGadtConstraint.addOneBound(GadtConstraint.scala:61)
[error]    |	at dotty.tools.dotc.core.ConstraintHandling.addBoundTransitively(ConstraintHandling.scala:316)
[error]    |	at dotty.tools.dotc.core.ConstraintHandling.addBoundTransitively$(ConstraintHandling.scala:29)
[error]    |	at dotty.tools.dotc.core.ProperGadtConstraint.addBoundTransitively(GadtConstraint.scala:61)
[error]    |	at dotty.tools.dotc.core.ProperGadtConstraint.addBound(GadtConstraint.scala:168)
[error]    |	at dotty.tools.dotc.core.TypeComparer.gadtAddLowerBound(TypeComparer.scala:116)
[error]    |	at dotty.tools.dotc.core.TypeComparer.narrowGADTBounds(TypeComparer.scala:1893)
[error]    |	at dotty.tools.dotc.core.TypeComparer.compareGADT$1(TypeComparer.scala:509)
[error]    |	at dotty.tools.dotc.core.TypeComparer.thirdTryNamed$1(TypeComparer.scala:512)
[error]    |	at dotty.tools.dotc.core.TypeComparer.thirdTry$1(TypeComparer.scala:561)
[error]    |	at dotty.tools.dotc.core.TypeComparer.secondTry$1(TypeComparer.scala:492)
[error]    |	at dotty.tools.dotc.core.TypeComparer.compareNamed$1(TypeComparer.scala:301)
[error]    |	at dotty.tools.dotc.core.TypeComparer.firstTry$1(TypeComparer.scala:307)
[error]    |	at dotty.tools.dotc.core.TypeComparer.recur(TypeComparer.scala:1309)
[error]    |	at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:189)
[error]    |	at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:199)
[error]    |	at dotty.tools.dotc.core.TypeComparer.topLevelSubType(TypeComparer.scala:126)
[error]    |	at dotty.tools.dotc.core.TypeComparer$.topLevelSubType(TypeComparer.scala:2709)
[error]    |	at dotty.tools.dotc.core.Types$Type.$less$colon$less(Types.scala:1040)
[error]    |	at scala.quoted.runtime.impl.QuoteMatcher$.$eq$qmark$eq(QuoteMatcher.scala:336)
[error]    |	at scala.quoted.runtime.impl.QuoteMatcher$.treeMatch(QuoteMatcher.scala:129)
[error]    |	at scala.quoted.runtime.impl.QuotesImpl.scala$quoted$runtime$impl$QuotesImpl$$treeMatch(QuotesImpl.scala:3051)
[error]    |	at scala.quoted.runtime.impl.QuotesImpl$TypeMatch$.unapply(QuotesImpl.scala:3021)

Is there a better (and working :) ) way to achieve the above goal?

[bishabosha]

I think this is a bug in memberType, if I modify the macro a bit then we see that one of the children is ClassInfo, which is not part of the Quotes API:

import quoted.*

object TestMacro:
  inline def test[T]: List[String] = ${ testImpl[T] }
  def testImpl[T: Type](using Quotes): Expr[List[String]] =
    import quotes.reflect.*
    val tpe = TypeRepr.of[T]
    val children = tpe.typeSymbol.children.map { childSymbol =>
      val childTpe = tpe.memberType(childSymbol)
      "Child TypeRepr: " + childTpe
      // childTpe.asType match
      //   case '[c] =>
      //     println("Got Type!")
    }
    Expr(children)

resulting in:

List(Child TypeRepr: ClassInfo(ThisType(TypeRef(NoPrefix,module class <empty>)), class X, List(TypeRef(ThisType(TypeRef(NoPrefix,module class lang)),class Object), TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),trait A), TypeRef(TermRef(TermRef(NoPrefix,object _root_),object scala),trait Product), TypeRef(ThisType(TypeRef(NoPrefix,module class io)),trait Serializable))))

[bishabosha]

@adamw typeRef its available in the nightly I mentioned

[adamw]

Ah it's in a nightly build. I was hoping for something available in a stable release so that I can use it in a library :). So if I'm using 3.1.2 there's no way to get the child's TypeRepr?

[bishabosha]

you can also try in stable releases val childTpe = TypeIdent(childSymbol.owner).tpe.memberType(childSymbol), it still has the ClassInfo bug though - might be better to use val childTpe = TypeIdent(childSymbol), however this would be the type constructor for generic classes

[adamw]

@bishabosha thank you! seems to work for my use-case. Specifically, I do:

tpe.typeSymbol.children.map { childSymbol =>
  val childTpe = TypeIdent(childSymbol).tpe
  childTpe.asType match
    case '[c] =>
      val childSchema: Expr[Schema[c]] = ...
      ...
}

[adamw]

Hm this fails for case objects though ... and as you write generic classes will be a problem since I suppose I won't get the proper type parameters?

Don't mirrors do something similar - it seems to work when summoning them. But maybe that's some internal machinery which isn't exposed.