How to debug a macro annotation? - scala

Question

Background
I'm trying to add a #Prisms annotation to Monocle that will work as follows. Given:
#Prisms sealed trait Foo
case object A extends Foo
case object B extends Foo
it will generate a companion object for Foo:
object Foo {
val a = monocle.macros.GenPrism.apply[Foo, A]
val b = monocle.macros.GenPrism.apply[Foo, B]
}
(or if the companion object already exists, it will add those methods to it).
The macro relies on directKnownSubclasses to find A and B, so there will be a note in the Monocle readme recommending that people use Typelevel Scala. I've configured Monocle to use TLS on my branch.
First attempt
I tried using TypeNames to represent the parent type Foo and child type (A or B):
def findSubclasses(typeName: TypeName): Set[TypeName] = {
// Note: disable macros to prevent stack overflow caused by infinite typing loop!
val tpe = c.typecheck(Ident(typeName), mode = c.TYPEmode, silent = true, withMacrosDisabled = true)
tpe.symbol.asClass.knownDirectSubclasses.map(_.asType.name)
}
def prisms(parentTypename: TypeName, childNames: Set[TypeName]): Set[Tree] = childNames.map { childName =>
val prismName = TermName(prefix + childName.decodedName.toString.toLowerCase)
q"""val $prismName = monocle.macros.GenPrism.apply[$parentTypename, $childName]"""
}
This generates what looks like reasonable code:
{
sealed trait Foo;
object Foo {
val a = monocle.macros.GenPrism.apply[Foo, A];
val b = monocle.macros.GenPrism.apply[Foo, B]
};
()
}
But it doesn't seem to work:
[error] /Users/chris/code/Monocle/test/shared/src/test/scala/other/PrismsAnnotationSpec.scala:5: not found: type A
[error] #monocle.macros.Prisms sealed trait Foo
[error] ^
[error] one error found
Second attempt
I also tried using TypeSymbols instead of TypeNames:
def prisms(parentTypename: TypeName, childSymbols: Set[TypeSymbol]): Set[Tree] = {
val parentSymbol: TypeSymbol =
c.typecheck(Ident(parentTypename), mode = c.TYPEmode, silent = true, withMacrosDisabled = true)
.symbol
.asType
childSymbols.map { childSymbol =>
val prismName = TermName(prefix + childSymbol.name.decodedName.toString.toLowerCase)
q"""val $prismName = monocle.macros.GenPrism.apply[$parentSymbol, $childSymbol]"""
}
}
But this gives me the following error, so I'm guessing the generated tree is invalid:
[error] /Users/chris/code/Monocle/test/shared/src/test/scala/other/PrismsAnnotationSpec.scala:5: macro annotation could not be expanded (the most common reason for that is that you need to enable the macro paradise plugin; another possibility is that you try to use macro annotation in the same compilation run that defines it)
[error] #monocle.macros.Prisms sealed trait Foo
[error] ^
[error] one error found
Third attempt
I also tried turning the TypeSymbols into Types:
def prisms(parentTypename: TypeName, childSymbols: Set[TypeSymbol]): Set[Tree] = {
val parentSymbol: TypeSymbol =
c.typecheck(Ident(parentTypename), mode = c.TYPEmode, silent = true, withMacrosDisabled = true)
.symbol
.asType
childSymbols.map { childSymbol =>
val prismName = TermName(prefix + childSymbol.name.decodedName.toString.toLowerCase)
q"""val $prismName = monocle.macros.GenPrism.apply[${parentSymbol.toType}, ${childSymbol.toType}]"""
}
}
But I get the same error, saying the annotation could not be expanded.
Now I'm out of ideas.
How to reproduce
The complete code is available on this branch: https://github.com/cb372/Monocle/tree/sealed-trait-macro-annotation.
First attempt
Second attempt
Third attempt
The macro is used in test/shared/src/test/scala/other/PrismsAnnotationSpec.scala. To compile it, run sbt test:compile.