How to handle an ADT (sealed trait) with ZIO config

Question

How can I add a Configuration Description manually for an Algebraic Data Type with ZIO Conf.

In the examples I found an example on how to handle ADTs with Magnolia.

Is this also possible when adding manually a Description of the Configuration?

Here an example:

sealed trait Dance
final case class A(any: Person)   extends Dance
final case class B(body: Height)  extends Dance

final case class Person(name: String, age: Option[Int])
final case class Height(height: Long)

With Magnolia:

val danceConfig = description[Dance]

Manually:

val danceConfig = ???

Answer 1

It is verbose as you would expect. But there are different ways of doing it, and it's a matter of preference.

We tried to be bit more verbose than required in both these options for better understanding

Option 1:

  val personConfig =
    (string("name") |@| int("age").optional)(Person.apply, Person.unapply)

  val heightConfig =
    long("height").xmap(Height)(_.height)

  val aConfig = nested("any")(personConfig).xmap(A)(_.any)
  val bConfig = nested("body")(heightConfig).xmap(B)(_.body)
  val cConfig = boolean("can").xmap(C)(_.can)
  val dConfig = string("dance").xmap(D)(_.dance)

  val danceConfig =
    aConfig
      .orElseEither(bConfig)
      .orElseEither(cConfig)
      .orElseEither(dConfig)
      .xmap({
        case Right(value) => value: Dance
        case Left(value) =>
          value match {
            case Right(value) => value: Dance
            case Left(value) =>
              value match {
                case Right(value) => value: Dance
                case Left(value)  => value: Dance
              }
          }
      })({
            case d @ D(_) => Right(d)
            case c @ C(_) => Left(Right(c))
            case b @ B(_) => Left(Left(Right(b)))
            case a @ A(_) => Left(Left(Left(a)))
          }
      )

A bit convoluted during the write side, but it is all type driven.

Option 2

   val personConfig =
    (string("name") |@| int("age").optional)(Person.apply, Person.unapply)

  val heightConfig =
    long("height").xmap(Height)(_.height)

  val aConfig = nested("any")(personConfig).xmap(A)(_.any)
  val bConfig = nested("body")(heightConfig).xmap(B)(_.body)
  val cConfig = boolean("can").xmap(C)(_.can)
  val dConfig = string("dance").xmap(D)(_.dance)

  val aConfigAsDance =
    aConfig.xmapEither(a => Right(a: Dance))({
      case a: A => Right(a)
      case _    => Left("unable to write back") 
    })

  val bConfigAsDance =
    bConfig.xmapEither(a => Right(a: Dance))({
      case a: B => Right(a)
      case _    => Left("unsable to write back")
    })

  val cConfigAsDance =
    cConfig.xmapEither(a => Right(a: Dance))({
      case a: C => Right(a)
      case _    => Left("unsable to write back")
    })

  val dConigAsDance =
    dConfig.xmapEither(a => Right(a: Dance))({
      case a: D => Right(a)
      case _    => Left("unsable to write back")
    })

  val danceConfig =
    aConfigAsDance.orElse(bConfigAsDance).orElse(cConfigAsDance).orElse(dConigAsDance)

You would already note, during the write part (second arg to xmapEither) we make sure it is the correct type. Example: In aConfigAsDance, it is unsafe to assume it can only be A and do an asInstanceOf.

With xmapeither we are able to write safe and pure code and we followed it.

In future, zio-config will come up with some helper functions to deal with Either. This is because ZIO-Config philosophy is to provide as less magical interface to the user as possible, while you can still use zio-config-magnolia to shorten them to just one line, which is

val danceConfig = description[Dance]

Good to have this example back in zio-config if you are interested. Thanks a lot for this question, and hope the answer is helpful.