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Inconsistent behavior when returning instances of an anonymous class from a macro #6992

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scabug opened this issue Jan 18, 2013 · 6 comments
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@scabug
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@scabug scabug commented Jan 18, 2013

In Scala 2.10.0 you can write a macro that will define an anonymous class with a type member and return an instance that will be statically typed as a structural type with that type member:

object MacroExample extends ReflectionUtils {
  import scala.language.experimental.macros
  import scala.reflect.macros.Context

  def foo(name: String): Any = macro foo_impl
  def foo_impl(c: Context)(name: c.Expr[String]) = {
    import c.universe._

    val Literal(Constant(lit: String)) = name.tree
    val anon = newTypeName(c.fresh)

    c.Expr(Block(
      ClassDef(
        Modifiers(Flag.FINAL), anon, Nil, Template(
          Nil, emptyValDef, List(
            constructor(c.universe),
            TypeDef(Modifiers(), newTypeName(lit), Nil, TypeTree(typeOf[Int]))
          )
        )
      ),
      Apply(Select(New(Ident(anon)), nme.CONSTRUCTOR), Nil)
    ))
  }
}

(Where ReflectionUtils just defines a constructor method for convenience.)

And now:

scala> MacroExample.foo("T")
res0: AnyRef{type T = Int} = $1$$1@7da533f6

But if we try the same thing with a method instead of a type member, it doesn't work:

def bar(name: String): Any = macro bar_impl
def bar_impl(c: Context)(name: c.Expr[String]) = {
  import c.universe._

  val Literal(Constant(lit: String)) = name.tree
  val anon = newTypeName(c.fresh)

  c.Expr(Block(
    ClassDef(
      Modifiers(Flag.FINAL), anon, Nil, Template(
        Nil, emptyValDef, List(
          constructor(c.universe),
          DefDef(
            Modifiers(), newTermName(lit), Nil, Nil, TypeTree(),
            c.literal(42).tree
          )
        )
      )
    ),
    Apply(Select(New(Ident(anon)), nme.CONSTRUCTOR), Nil)
  ))
}

We just don't end up with a structural type:

scala> MacroExample.bar("test")
res1: AnyRef = $1$$1@da12492

But for some reason we can add an extra class that extends the anonymous class and instantiate that instead:

def baz(name: String): Any = macro baz_impl
def baz_impl(c: Context)(name: c.Expr[String]) = {
  import c.universe._

  val Literal(Constant(lit: String)) = name.tree
  val anon = newTypeName(c.fresh)
  val wrapper = newTypeName(c.fresh)

  c.Expr(Block(
    ClassDef(
      Modifiers(), anon, Nil, Template(
        Nil, emptyValDef, List(
          constructor(c.universe),
          DefDef(
            Modifiers(), newTermName(lit), Nil, Nil, TypeTree(),
            c.literal(42).tree
          )
        )
      )
    ),
    ClassDef(
      Modifiers(Flag.FINAL), wrapper, Nil,
      Template(Ident(anon) :: Nil, emptyValDef, constructor(c.universe) :: Nil)
    ),
    Apply(Select(New(Ident(wrapper)), nme.CONSTRUCTOR), Nil)
  ))
}

And it works:

scala> MacroExample.baz("test")
res0: AnyRef{def test: Int} = $2$$1@6663f834

See this Stack Overflow question for more discussion.

@scabug
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@scabug scabug commented Jan 18, 2013

Imported From: https://issues.scala-lang.org/browse/SI-6992?orig=1
Reporter: Travis Brown (travisbrown)
Affected Versions: 2.10.0

@scabug
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@scabug scabug commented Dec 8, 2013

@xeno-by said:
Related: #8048

@scabug
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@scabug scabug commented Dec 8, 2013

@scabug
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@scabug scabug commented Jan 14, 2014

@adriaanm said:
was this fixed by the PR? I see no mention of the ticket number there.

@scabug
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@scabug scabug commented Jan 14, 2014

@xeno-by said:
Yes, it was. Sorry, my oversight.

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