bridge_errors.scala

/*
 * My application is splitted in several modules, possibly independently built and
 * distributed.
 * I have a Base error trait for the whole application.
 *
 * Each module has its own business error defined by a module base error trait which
 * extends BaseError.
 *
 * I want to have a common framework to manage bridging errors between modules so
 * that a module can use the other ones and subsume error type to its own, especially
 * in the canonical exemple:
 * 
 * ```
 * for {
 *   a <- module1
 *   b <- module2
 * } yield {
 *   ...
 * } : error of kind module 3
 * ```
 * 
 * The bridging could be done with a "chain" error kind that encapsulated one error module
 * in an other: `ErrorModule2.Chain("some explanation", errorModule1)`
 * 
 * Finally, I want minimum boilerplate for that. How is it usually done?
 */
object errors {
  trait BaseError {
    def msg: String

  }

  trait BaseChainError[E <: BaseError] extends BaseError {
    def cause: E
    def hint: String
    def msg = s"${hint}; cause was: ${cause.getClass.getSimpleName}: ${cause.msg}"
  }
}

object TestImplicits {
  import scalaz.zio._
  import scalaz.zio.syntax._

  object module1 {
    import com.normation.errors._
    sealed trait M_1_Error extends BaseError
    object M_1_Error {
      final case class BusinessError1(msg: String) extends M_1_Error
      final case class Chained[E <: BaseError](hint: String, cause: E) extends M_1_Error with BaseChainError[E]
    }

    object M_1_Result {
       // implicits ?
    }

    object service1 {
      def doStuff(param: String): IO[M_1_Error, String] = param.succeed
    }
  }

  object module2 {
    import com.normation.errors._
    sealed trait M_2_Error extends BaseError
    object M_2_Error {
      final case class MissingImportantStuff(msg: String) extends M_2_Error
      final case class Chained[E <: BaseError](hint: String, cause: E) extends M_2_Error with BaseChainError[E]
    }
    object M_2_Result {
      // implicits
    }
    object service2 {
      def doStuff(param: Int): IO[M_2_Error, Int] = param.succeed
    }
  }

  object testModule {
    import module1._
    import module2._

    import com.normation.errors._
    sealed trait M_3_Error extends BaseError
    object M_3_Error {
      final case class Oups(msg: String) extends M_3_Error
      final case class Chained[E <: BaseError](hint: String, cause: E) extends M_3_Error with BaseChainError[E]
    }
    object M_3_Result {
      // implicits ?
    }

    import module1.M_1_Result._
    import module2.M_2_Result._
    import M_3_Result._

    /*
     * I would like all of that to be possible, with the minimum boilerplate,
     * and the maximum homogeneity between modules
     */
    object service {

      // need error adpatation 1 toward 2
      def test0(a: String): IO[M_2_Error, Int] = service1.doStuff(a)

      // need error adpatation 1 toward 3
      def test1(a: String): IO[M_3_Error, Int] = service1.doStuff(a)

      // need error adpatation 1 and 2 toward 3
      def test2(a: String, b: Int): IO[M_3_Error, (String, Int)] = {
        for {
          x <- service1.doStuff(a)
          y <- service2.doStuff(b)
        } yield {
          (x, y)
        }
      }
    }
  }
}