Singleton Types for Value Classes

[tzbob]

  class ValueTest(val x: Int) extends AnyVal

  object PathDependentSingleton2 {
    val x = new ValueTest(5)
    val wX = Witness(x)
    type XT = wX.T
  }

This compiles just fine but I have no idea what XT is supposed to be here. x.type is an invalid expression since x is not subclassing AnyRef so I'm not sure if this behavior is intended?

If it is then Shapeless Witness macros add support for singleton types on value classes by treating them as reference types which may lead to un{expected, documented} behavior? An example that could possibly confuse people:

    val x = new ValueTest(5)
    val y = new ValueTest(5)
    val wX = Witness(x)
    val wY = Witness(y)
    implicitly[wX.T =:= wY.T]

[milessabin]

Would you mind taking a look to see if this is still an issue for shapeless-2.2.2?

[tzbob]

On master (2.2.3) it's still odd that the following compiles:

  object PathDependentSingleton3 {
    val x = new ValueTest(5)
    val wX = Witness(x)
  }

Since it's invalid Scala to reference the singleton type of a Value class:

  object PathDependentSingleton3 {
    val x = new ValueTest(5)
    val wX = Witness(x)
    implicitly[wX.T =:= x.type] 
  }

[error]  found   : shapeless.SingletonTypesTests.PathDependentSingleton3.x.type (with underlying type shapeless.ValueTest)
[error]  required: AnyRef
[error] Note that ValueTest extends Any, not AnyRef.
[error] Such types can participate in value classes, but instances
[error] cannot appear in singleton types or in reference comparisons.
[error]     implicitly[wX.T =:= x.type]

I think it's best not to allow Value classes in Witness at all, right? That's what I meant in the original post, this behavior might confuse people unless it's explicitly stated somewhere.

[milessabin]

Agreed. It's just that there were some changes in the Witness materializer in 2.2.2 and it was at least conceivable that this might have come out in the wash.

Thanks for the update :-)

[milessabin]

This decision has been reversed in e70f01e. As of SIP-23 value types do admit singleton types of this form, and the exclusion no longer seems well motivated.