scalafunctional-programmingscala-catscats-effecttagless-final

Elegant way to change either to error with tagless final

I often do things like:

import cats.effect.Sync
import cats.implicits._

case class User(name: String)
case object Error extends Exception

def validate[F[_]: Sync](name: String): F[Either[Error, User]] = Sync[F].pure(User(name).asRight)

def doSomething[F[_]: Sync]: F[User] = for {
   maybeUser <- validate("Name")
   user <- maybeUser.fold(Sync[F].raiseError[User](_), Sync[F].pure(_))
} yield user

In nutshell, it means if Either is left then use raiseError, if it's right just return value.

Is there a more convenient way to "unwrap" Either?

Solution

  • Use liftTo from the cats Either syntax: maybeUser.liftTo[F].

    You can also use rethrow, from the cats MonadError syntax, directly on F[Either[Error, User]]:

    def doSomething[F[_]: Sync]: F[User] = validate[F]("Name").rethrow

    Note that you don't actually need Sync - MonadError[*[_], Throwable] is sufficient.