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Zio-delegate has been merged into Zio-Macros. This repo will no longer be maintained.

This package defines an annotation and a typeclass that simplify working with mixins and proxies in scala.


Add this dependency to your build.sbt

"dev.zio" %% "zio-delegate" % "0.0.5"

If using a scala version < 2.13 you'll also need to add the macro paradise compiler plugin.

compilerPlugin(("org.scalamacros" % "paradise"  % "2.1.1") cross CrossVersion.full)

If using scala 2.13 you need to add the macro annotation compiler option.


@delegate annotation

This annotation can only be used on a constructur parameter in a class definition. This will do a number of things to the resulting class definitions:

  • The class will additionally extend any traits extended by the annotated member.
import zio.delegate._

trait Foo {
  def foo: Int = 4
object FooImpl extends Foo

class Bar(@delegate f: Foo)
val b: Foo = new Bar(FooImpl)
  • Any methods on the resulting type of the defined class that are also defined on the annotated member will be forwarded to the member unless a definition exists in the body of the class.
import zio.delegate._

trait Foo {
  def foo: Int
abstract class Foo1 extends Foo {
  def foo = 4
  def foo1: Int

class Bar(@delegate f: Foo)
println(new Bar(new Foo {}).foo) // 4

class Bar1(@delegate f: Foo) {
  def foo = 3
println(new Bar1(new Foo {}).foo) // 3

// classes have to be explicitly extended. Forwarders will still
// be automatically generated though.
class Bar2(@delegate f: Foo1) exends Foo1
println(new Bar1(new Foo1 { def foo1 = 3 }).foo1) // 3
  • The behavior of the annotation can be customized with three options
    class delegate(verbose: Boolean = false, forwardObjectMethods: Boolean = false, generateTraits: Boolean = true)
    • verbose: The generated class will be reported during compilation. This is very useful for debugging behavior of the annotation or getting a feel for the generated code.
    • forwardObjectMethods: controls whether methods defined on Object and similiar classes should be forwarded. The list of methods affected by this is currently:
    • generateTraits: Whether the class should be adopted to automatically extend any traits defined on the annotated member. If set to false only methods of traits / classes that are explicitly extended will be forwarded.

Mix Typeclass

An instance of

trait Mix[A, B] {

  def mix(a: A, b: B): A with B


provides evidence that an instance of B can be mixed into an instance of A. A macro is defined that can derive an instance of Mix for any two types if the first is a nonfinal class or a trait and the second one is a trait. It can be used like this

class Foo {
  def foo: Int = 2
trait Bar {
  def bar: Int
def withBar[A](a: A)(implicit ev: Mix[A, Bar]): A with Bar = {
  ev.mix(a, new Bar { def bar = 2 })
withBar[Foo](new Foo()).bar // 2

Definitions in the second type will override implementations in the first type.

ZIO Modules

One of the primary motivations for writing this library was more comfortable incremental building of ZIO environment cakes. A possible way of doing this is:

import zio.delegate._
import zio.blocking.Blocking
import zio.clock.Clock

trait Sys extends Serializable {
  def sys: Sys.Service[Any]
object Sys {
  trait Service[R] extends Serializable

  trait Live extends Sys { self: Clock with Blocking =>
    def sys = new Service[Any] {}

  def withSys[A <: Clock with Blocking](a: A)(implicit ev: A Mix Sys): A with Sys = {
    class SysInstance(@delegate underlying: Clock with Blocking) extends Live
    ev.mix(a, new SysInstance(a))


This is heavily inspired by adamw/scala-macro-aop and b-studios/MixinComposition. Make sure to check out the projects!

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