Should release have access to the acquired resource?

trait AcquireRelease[-R, +E] extends Scoped[R, E] {
  type A
  final def scoped(implicit trace: Trace): ZIO[R & Scope, E, Any] =
    ZIO.acquireRelease(acquire)(release)
  def acquire: ZIO[R, E, A]
  def release(a: A): ZIO[R, Nothing, Any]
}

How about just AcquireRelease[-R, +E, A]? A not being covariant is a bit inconsistent with the other datatypes but it's simpler.

Here's the revised example with the new design:

import zio._
import java.io.File

def acquireLockFile(path: String): ZIO[Any, Throwable, File] = ???
def deleteFile(file: File): ZIO[Any, Throwable, Unit] = ???

class ASingletonService(lockFilePath: String) extends ZLayer.Derive.AcquireRelease[Any, Throwable, File] {

  override def acquire: ZIO[Any, Throwable, File] =
     acquireLockFile(lockFilePath)

  override def release(lockFile: File): ZIO[Any, Nothing, Any] =
     deleteFile(lockFile).ignore
}

This could probably be moved into the Derive companion object since like Scoped and AcquireRelease it really only has meaning within the context of derive.

I thought about it too, but unlike Scoped sometimes it might be used like Default[A].layer without ZLayer.derive. What do you think?

Besides the possible independent use case, putting Default inside of Derive can imply that the layer derivation process is aware of Default.

I tend to think there is not a use of Default independent of derive. Theoretically the concept of a default value could make sense in other contexts but practically there is nothing else that uses it and the fact that a default value could itself by constructed from a layer makes it very specific to the derivation.

I don't understand your comment about the derivation process being aware of default. The derivation process is aware of default in that if you construct something that has like a Ref where are going to look for a default value for the value inside the Ref. So the default values are intimately related to derivation and in fact derivation is the reason for their existence.

Agreed. I mean when we put Default in Derive it will reveal that relationship more explicitly. I'll move it into Derive.

Documentation could be updated here to reflect the changes we have made.

Updated Scaladoc.

Same here.

Updated Scaladoc.

Moved under ZLayer.Derive.

Improved doc about Scoped with an example.