Prelude Newtypes lack implicit coercion

[googley42]

Prelude Newtypes lack of implicit coercion causes issues with interop with Newtypes from other libraries.

So in some of our services at work we have > 60 Newtypes which have to span libraries eg Smithy4s Newtypes and Prelude Newtypes, and for the transformations we use Chimney.

The smith4s newtypes have a Wrap/Unwrap typeclass called Bijection which is shown below

trait Bijection[A, B] extends Function[A, B] { outer =>
  def to(a: A): B
  def from(b: B): A
}

This type class is used to define implicit conversions via Chimney transformations.

Just about all the other Newtype libraries have some kind of coercion, however Prelude Newtypes do not - the team had provide a cake style hack to achieve this coercion ie:

  trait BijectionSupport[A] { self: zio.prelude.Newtype[A] =>
    
    implicit final val zioNewTypeSmithyBijection: Bijection[A, Type] =
      new Bijection[A, Type] {
        def to(a: A): Type = self.apply(a) 
        def from(b: Type): A = self.unwrap(b)
      }
  }

// usage of BijectionSupport
type AccountId = AccountId.Type
object AccountId extends Newtype[String] with BijectionSupport[String]

cc @jdegoes