Scala-3: Use Opaque Types.

[diesalbla]

Scala-3 comes with its own zero-cost Newtype implementation, as Opaque Types. This is an improvement over existing case classes, or Newtype encodings we are using.

In this commit, we bifurcate the IdT wrapper in two implementations: the existing one goes to the Scala-2.x branch, a new one with newtype is used on the new.

#3835

[rossabaker]

The goal is compelling, but this is 3500 extra lines of code, and maintaining parity across Scala 2 and Scala 3 will be exceedingly difficult. I wonder if there's a less duplicative encoding of this?

[diesalbla]

The goal is compelling, but this is 3500 extra lines of code, and maintaining parity across Scala 2 and Scala 3 will be exceedingly difficult. I wonder if there's a less duplicative encoding of this?

I doubt we can avoid duplication, since we are writing in two languages. The goal is not just compellingPlus, the sooner we start using features of Dotty, the sooner we can find the minefields and bugs on the language... as I just found some.

Update: I have converted the OptionT case class into an opaque type. Of course, with opaque types all methods are extension methods.

• One feature of extension methods is that we can group methods that use a same type-class constraint, such as "Functor[F]" or "Monad[F]".
• One difficulty is that the compiler seems unable to recognise references to other methods, in the same collective, as using the same using Functor[F] (for example).
• It also seems unable to tell apart a map on the F from a map on the OptionT[F, A].

Currently, this is crashing the Scala 3 compiler. scala/scala3#11499