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Created July 16, 2022 07:53
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Sharing Builder Code by Some Scala Tricks
case class BarDBConfig(connectionString: String) extends DBConfig
object BarDBConfig extends DBConfig.Builder[BarDBConfig]("bar")
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
case None => None
// 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")
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] =
val barDBConfig: Option[BarDBConfig] =
fooDBConfig.isDefined // true
barDBConfig.isDefined // true
val printConfigs: ZIO[FooDBConfig & BarDBConfig, Nothing, Unit] =
for {
fooDBConfig <- ZIO.environment[FooDBConfig]
barDBConfig <- ZIO.environment[BarDBConfig]
} yield {
// 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)
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