The first final tagless approach

Question

I am trying to use the first time tagless final pattern in Scala and struggling a bit.

I have the following algebras definition:

trait DbSetting[F[_]] {
  def read(url: String, user: String, pw: String): F[DbParameter]
}

trait Environment[F[_]] {
  def get(v: String) : F[Option[String]]
}

and the interpreters implementation:

final case class DbParameter(url: String, user: String, pw: String)

sealed trait DbError extends NoStackTrace
case object DbSettingError extends DbError


sealed trait DbError extends NoStackTrace
case object DbSettingError extends DbError


// Environment implementation
object Environment {

  def apply[F[_]](implicit ev: Environment[F]): ev.type = ev

  def impl[F[_]: Sync]: Environment[F] = new Environment[F] {
    override def get(v: String): F[Option[String]] =
      Sync[F].delay(sys.env.get(v))
  }
}

// DbSetting implementation
class DbSystemEnvironment[F[_] : MonadError[*, Throwable]] private(env: Environment[F])
  extends DbSetting[F] {
  override def read(url: String, user: String, pw: String): F[DbParameter] = env.get(url)

}

What I am trying to do is, to compose Environment into DbSystemEnvironment. The problem here is, I can not get the value out of env.get(url) because, I do not know anything about F in DbSystemEnvironment, except it is a MonadError. How to get the value out of env.get(url)::F[Option[String]]?

In addition, if env.get(url) returns Nothing in read function, then it should return MonadError.

Why do you mean by "getting the value of env.get(url)"? You want to .map it, .handle errors or something else? Because with algebras you have defined for F (Sync/MonadError), you can lift value, map/flatMap it handle, mapN, etc, but not turn F[A] into A (that would require Comonad). — Mateusz Kubuszok
Tangential, but I would look at ZIO (zio.dev) as an alternative to tagless final. I personally find it easier to deal with. — Eric

paulpdaniels paulpdaniels · Accepted Answer · 2020-06-17T16:18:55

If I understand your question correctly you are asking how to extract the returned value from calling env.get(url) and then map it to a F[DbParameter].

Since you have MonadError for your F[_] you should be able to map the result pretty readily by doing something like this:

import cats.syntax.flatMap._
import cats.syntax.applicative._
import cats.syntax.applicativeError._

override def read(url: String, user: String, pw: String): F[DbParameter] = 
  env.get(url).flatMap {
    case Some(ev) => DbParameter(ev, user, pw).pure[F]
    case None     => (new Exception("No environment parameter found!")).raiseError[F, DbParameter]
  }

The first final tagless approach

1 Answers