Look into type tagging and value classes

[dwijnand]

See the conversation in the typelevel/scala Gitter room on October 11, 2016 8:58 PM.

[AlecZorab]

Welcome to Scala 2.11.8 (OpenJDK 64-Bit Server VM, Java 1.8.0_91).
Type in expressions for evaluation. Or try :help.

scala> import shapeless._
import shapeless._

scala> import tag._
import tag._

scala> case class A(i:Int) extends AnyVal
defined class A

scala> trait Bar
defined trait Bar

scala> val a = A(1)
a: A = A(1)

scala> tag[Bar](a)
res0: shapeless.tag.@@[A,Bar] = A(1)

scala> Array(res0)
java.lang.ClassCastException: [I cannot be cast to [Ljava.lang.Object;
  ... 42 elided

scala> Array(tag[Bar](A(1)))
java.lang.ClassCastException: [I cannot be cast to [Ljava.lang.Object;
  ... 42 elided

scala> trait Baz extends Any
defined trait Baz

scala> Array(tag[Baz](A(1)))
java.lang.ClassCastException: [I cannot be cast to [Ljava.lang.Object;
  ... 42 elided

scala> val z = tag[Baz](A(1))
z: shapeless.tag.@@[A,Baz] = A(1)

scala> Array(z)
java.lang.ClassCastException: [I cannot be cast to [Ljava.lang.Object;
  ... 42 elided

We'd probably expect the Baz case to work.

[dwijnand]

It works for Vector:

Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_102).
Type in expressions for evaluation. Or try :help.

scala> :pa
// Entering paste mode (ctrl-D to finish)

type @@[T, Tag] = T with Tag

trait Tagger[Tag] { def apply[A](a: A): A @@ Tag = a.asInstanceOf[A @@ Tag] }

def tag[Tag]: Tagger[Tag] = new Tagger[Tag] {}

case class Foo(x: Int) extends AnyVal

trait Bar extends Any


// Exiting paste mode, now interpreting.

defined type alias $at$at
defined trait Tagger
tag: [Tag]=> Tagger[Tag]
defined class Foo
defined trait Bar

scala> Vector(tag[Bar](Foo(1)), tag[Bar](Foo(2)))
res0: scala.collection.immutable.Vector[@@[Foo,Bar]] = Vector(1, 2)

[AlecZorab]

https://github.com/milessabin/shapeless/blob/ab081796c183530efdd8b29dab8fee1fee7c61f9/core/src/main/scala/shapeless/typeoperators.scala

Tagged uses a slightly different merchanism. This appears to work:

Welcome to Scala 2.11.8 (OpenJDK 64-Bit Server VM, Java 1.8.0_91).
Type in expressions for evaluation. Or try :help.

scala> object tag {
     |   def apply[U] = new Tagger[U]
     | 
     |   trait Tagged[U] extends Any
     |   type @@[+T, U] = T with Tagged[U]
     | 
     |   class Tagger[U] {
     |     def apply[T](t : T) : T @@ U = t.asInstanceOf[T @@ U]
     |   }
     | }
defined object tag

scala> case class A(i:Int) extends AnyVal
defined class A

scala> val a = A(1)
a: A = A(1)

scala> trait Bar
defined trait Bar

scala> trait Baz extends Any
defined trait Baz

scala> Array(tag[Baz](a))
res0: Array[tag.@@[A,Baz]] = Array(1)

scala> Array(tag[Bar](a))
res1: Array[tag.@@[A,Bar]] = Array(1)

Though I've no idea how to check if those are Arrays of Int or Object underneath

edit:

scala> :t res1
Array[tag.@@[A,Bar]]

scala> res1.getClass
res2: Class[_ <: Array[tag.@@[A,Bar]]] = class [I

So even when the trait we're tagging with doesn't extend Any, it stays as an int

[dwijnand]

Yes, looks like Tagged needs to be a universal trait to not explode at runtime when the type T is a value class.

[AlecZorab]

Any idea what the story is with universal traits and binary compatibility? I'm not sure whether we can change that in 2.3.X or not

[dwijnand]

I agree with your suspicion - I would assume it's not a compatible change and would want to see proof that it actually is :D

[milessabin]

The tag is supposed to be erased, so I think there's a fair chance we can get away with this change ... could someone give it a try and attempt to confirm via MiMa?

[milessabin]

FTR, the issue I was remembering was #44. It looks as though I didn't consider the possibility of having Tagged extend Any, and thought that the only way to proceed would be to revert #20.

[lJoublanc]

Would this extend to shapeless.labelled.field? I'm seeing similar issues when trying to create records. Posted on gitter https://gitter.im/milessabin/shapeless?at=5ae890ccb37eab7d045bef1e