Add Type Alias For Intersection Types On Scala 2

[adamgfraser]

Resolves #4844. Adds a type alias to allow & to be used on both Scala 2 and Scala 3.

[regiskuckaertz]

Bloody hell that was fast 🕺 I refrained from going one step further because of the dotty compat but this would be nicer:

type &[A, B] = Has[A] with Has[B]

What do you think?

I thought about this after @kitlangton 's presentation at zio world. I think it's a great idea to get rid of the type alias boilerplate but Has[_] is 5 characters long and life is too short 😄

[kitlangton]

Bloody hell that was fast 🕺 I refrained from going one step further because of the dotty compat but this would be nicer:

type &[A, B] = Has[A] with Has[B]

What do you think?

I thought about this after @kitlangton 's presentation at zio world. I think it's a great idea to get rid of the type alias boilerplate but Has[_] is 5 characters long and life is too short 😄

The following works in Dotty, but there's not much we can do in Scala 2.

type ++[A,B] <: Has[_] = (A,B) match {
  case (Has[_], Has[_]) => A & B & Has[_]
  case (_, Has[_]) => Has[A] & B
  case (Has[_], _) => A & Has[B]
  case (_,_) => Has[A] & Has[B] 
}

val layer = ZLayer.fromAuto[Console ++ String ++ Has[Int] ++ Boolean](layer, boolLayer, Console.live) 

[adamgfraser]

@regiskuckaertz I think one of our lessons from ZIO 1.0 has been that hiding the Has data type doesn't pay off in the long term. It seems really nice to avoid writing Has everywhere, which is why we created the type aliases for the services. However, it creates user confusion about which date types are Has and which ones are not and leads to errors when users accidentally created nested Has data types so I think it is best to stick to the simple definition of intersection types that is the same across Scala versions.

[adamgfraser]

That is really interesting regarding the match types.

[kitlangton]

While I agree it could be nice to have something like that in Scala 2:

type ++[A,B] = Has[A] with Has[B]

The problem is when you use it to join more than two types, the left hand side is now a Has and will become doubly wrapped, so you'd actually need:

type ++[A,B] = A with Has[B]

But then it's a bit weird to use, as you have to explicitly wrap the first type—or add some second operator that also wraps the lhs. Either way, not too great. Probably best for individuals to implement it themselves in their own libraries if they really want that, until we can figure out something more elegant (like that match type approach).

[regiskuckaertz]

type ++[A,B] <: Has[_] = (A,B) match {
  case (Has[_], Has[_]) => A & B & Has[_]
  case (_, Has[_]) => Has[A] & B
  case (Has[_], _) => A & Has[B]
  case (_,_) => Has[A] & Has[B] 
}

@kitlangton hmm, I see, at that point ++ is not a type alias but a type constructor, like *: for inductive tuples. Then the problem disappears.

@adamgfraser good point, I think this is wise.