Implement xmap2Either and Sequence for Config

[afsalthaj]

Possibly include a xmap2Either in config data type, that is a form of invariant applicative functor that handle errors.

In Config type,

  def xmap2Either[B, C](that: Config[B])(f: (A, B) => Either[ReadError, C])(g: C => Either[WriteError, (A, B)]): Config[C] = {
    (self |@| that).apply[(A, B)]((a, b) => (a, b), t => Some((t._1, t._2))).mapEither(b => f(b._1, b._2))(g)
  }

This allows us to implement the following in the companion object

 def sequence[A](list: List[Config[A]]): Config[List[A]] = {
    list.foldLeft(Pure(Nil: List[A]): Config[List[A]])((a, b) => b. xmap2Either(a)((aa, bb) => Right( aa :: bb))(t => Right((t.head, t.tail))))
  }

// where Pure is

  final case class Pure[A](a: A) extends Config[A]

This enables user parse complicated groupings in the env. Although, not a convincing example below, it is essential to make config traversable and allow users to do such things.

final case class Variables(variable1: Int, variable2: Option[Int])

  val listOfConfig: List[Config[Variables]] =
    (0 to 3).toList.map(t => (int(s"${t}_VARIABLE1") |@| opt(int(s"${t}_VARIABLE2")))(Variables.apply, Variables.unapply))

  val configOfList: Config[List[Variables]] =
    Config.sequence(listOfConfig)

  val map = mapSource(
    Map(
      "0_VARIABLE1" -> "1",
      "0_VARIABLE2" -> "2",
      "1_VARIABLE1" -> "3",
      "1_VARIABLE2" -> "4",
      "2_VARIABLE1" -> "5",
      "2_VARIABLE2" -> "6"
      "3_VARIABLE1" -> "7",
    )
  )


val result = read(configOfList).run.provide(map)
// Result: List(Variables(7,None), Variables(5,Some(6)), Variables(3,Some(4)), Variables(1,Some(2)))

val written = write(configOfList).run.provide(result._2)

// Result:
 Map(
  0_VARIABLE1 -> 1,
  0_VARIABLE2 -> 2, 
  1_VARIABLE1 -> 3, 
  1_VARIABLE2 -> 4,
  2_VARIABLE1 -> 5, 
  2_VARIABLE2 -> 6, 
  3_VARIABLE1 -> 7
)

[afsalthaj]

@wi101 @jdegoes Requesting your comments on this :)

[afsalthaj]

Implemented in https://github.com/zio/zio-config/pull/44/files