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Ciris
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package ciris

import ciris.api._
import ciris.api.syntax._

/**
  * [[ConfigValue]] represents the value part of a [[ConfigEntry]],
  * without any other details like the key, key type, or original
  * source value. [[ConfigEntry]] extends [[ConfigValue]], but
  * you can also create one with [[ConfigValue#apply]].
  *
  * @tparam F the context in which the value exists
  * @tparam V the type of the value
  */
abstract class ConfigValue[F[_]: Apply, V] {
  def value: F[Either[ConfigError, V]]

  /**
    * If the value of this [[ConfigValue]] is unavailable, tries to
    * use the value of that [[ConfigValue]], accumulating errors if
    * both values are unavailable.
    *
    * @param that the [[ConfigValue]] to use if this value is unavailable
    * @tparam A the value type of that [[ConfigValue]]
    * @return a new [[ConfigValue]]
    * @example {{{
    * scala> val combined =
    *      |  ConfigEntry[String, Int]("key", ConfigKeyType.Environment, Left(ConfigError("error1"))).
    *      |    orElse(ConfigEntry[String, Int]("key2", ConfigKeyType.Property, Left(ConfigError("error2"))))
    * combined: ConfigValue[api.Id, Int] = ConfigValue(Left(Combined(ConfigError(error1), ConfigError(error2))))
    *
    * scala> combined.value.left.map(_.message).toString
    * res0: String = Left(error1, error2)
    *
    * scala> ConfigEntry[String, Int]("key", ConfigKeyType.Environment, Left(ConfigError("error1"))).
    *      |   orElse(ConfigEntry("key2", ConfigKeyType.Property, Right(123)))
    * res1: ConfigValue[api.Id, Int] = ConfigValue(Right(123))
    * }}}
    */
  final def orElse[A >: V](that: ConfigValue[F, A]): ConfigValue[F, A] =
    ConfigValue.applyF[F, A] {
      (this.value product that.value).map {
        case (Right(v), _)                      => Right(v)
        case (Left(_), Right(a))                => Right(a)
        case (Left(thisError), Left(thatError)) => Left(thisError combine thatError)
      }
    }

  private[ciris] final def append[A](next: ConfigValue[F, A]): ConfigValue2[F, V, A] = {
    new ConfigValue2((this.value product next.value).map {
      case (Right(v), Right(a))         => Right((v, a))
      case (Left(error1), Right(_))     => Left(ConfigErrors(error1))
      case (Right(_), Left(error2))     => Left(ConfigErrors(error2))
      case (Left(error1), Left(error2)) => Left(error1 append error2)
    })
  }
}

object ConfigValue {

  /**
    * Creates a new [[ConfigValue]] from the specified value.
    *
    * @param value the value or an error
    * @tparam V the type of the value
    * @return a new [[ConfigValue]]
    */
  def apply[V](value: Either[ConfigError, V]): ConfigValue[Id, V] =
    ConfigValue.applyF[Id, V](value)

  /**
    * Creates a new [[ConfigValue]] from the specified value,
    * wrapped in context `F`, which can be [[api.Id]] if no
    * context is desired. [[ConfigValue#apply]] also exists
    * for the case when `F` is [[api.Id]].
    *
    * @param value the value or an error, in context `F`
    * @tparam F the context in which the value exists
    * @tparam V the type of the value
    * @return a new [[ConfigValue]]
    */
  def applyF[F[_]: Apply, V](value: F[Either[ConfigError, V]]): ConfigValue[F, V] = {
    val theValue = value
    new ConfigValue[F, V] {
      override def value: F[Either[ConfigError, V]] = theValue
      override def toString: String = s"ConfigValue($value)"
    }
  }
}