Sharing Builder Code by Some Scala Tricks

BarDBConfig.scala

case class BarDBConfig(connectionString: String) extends DBConfig

object BarDBConfig extends DBConfig.Builder[BarDBConfig]("bar")

DBConfig.scala

trait DBConfig(val connectionString: String)

object DBConfig {
  // 1. `Builder` extends `Function` type `String => C`
  abstract class Builder[C <: DBConfig](val prefix: String) extends (String => C) {
    // Some generic build logic
    def build(map: Map[String, String]): Option[C] =
      map.get(prefix) match {
        case Some(connectionString) =>
         // Calling `apply` because it exists due to `Builder` extending `Function` type
         Some(apply(connectionString))

        case None => None
      }
  }
}

FooDBConfig.scala

// 2. This is a case class so it generates an `apply` method conforming to `String => FooDBConfig` type.
case class FooDBConfig(connectionString: String) extends DBConfig

// 3. Generated `apply` happens to match the `apply` method defined in `Function` type `Builder` extends.
// That's why the `apply` that `Function` defines is implemented by the `apply` that case class generates.
object FooDBConfig extends DBConfig.Builder[FooDBConfig]("foo")

Test.scala

val dbConfigMap: Map[String, String] = Map(
  "foo" -> "fooConnectionString",
  "bar" -> "barConnectionString"
)

// 4. Since correct implementation of `apply` is provided by case class, this works!
val fooDBConfig: Option[FooDBConfig] = FooDBConfig.build(dbConfigMap)
val barDBConfig: Option[BarDBConfig] = BarDBConfig.build(dbConfigMap)
fooDBConfig.isDefined // true
barDBConfig.isDefined // true

val printConfigs: ZIO[FooDBConfig & BarDBConfig, Nothing, Unit] =
  for {
    fooDBConfig <- ZIO.environment[FooDBConfig]
    barDBConfig <- ZIO.environment[BarDBConfig]
  } yield {
    println(fooDBConfig)
    println(barDBConfig)
  }

// 5. This makes it easy to work in some `ZIO` environment that needs both DB configurations
// because they are different types but built the same way easily!
printConfigs.provide(fooDBConfig.get, barDBConfig.get)