CODEWITHSUNDEEP
scalamonadsscalazmonad-transformers
dcastro
dcastro

Reputation: 68640

Creating a WriterT[F, W, A] from a Writer[W, A]

In Scalaz, is there an easy way to convert an instance of Writer[W, A] (which is an alias for WriterT[Id, W, A]) to WriterT[F, W, A]?

I'm looking for something similar to the optionT function combined with point, but for writers instead:

E.g.

// Similar situation, but using OptionT
val opt:  Option[String] = Some("log")
val optT: OptionT[IO, String] = optionT(opt.point[IO])

// Case in hand, using WriterT
val w:  Writer[String, Unit] = "log".tell
val wt: WriterT[IO, String, Unit] = ???

// Similar scenario
val w:  Writer[String, Int] = 3.set("log")
val wt: WriterT[IO, String, Int] = ???

Upvotes: 2

Views: 320

Answers (1)

Travis Brown
Travis Brown

Reputation: 139038

Some monadic types have lift methods that help with this kind of operation:

import scalaz._, Scalaz._, effect.IO

val stateIO: StateT[IO, Int, Unit] = put(10).lift[IO]

Writer doesn't, but you can use the Hoist instance for WriterT to accomplish the same thing:

type StringWriter[F[_], A] = WriterT[F, String, A]

def fromId[F[_]: Applicative]: Id ~> F = new (Id ~> F) {
  def apply[A](a: A) = a.point[F]
}

val w:  Writer[String, Unit] = "log".tell
val wt: WriterT[IO, String, Unit] = Hoist[StringWriter].hoist(fromId[IO]).apply(w)

This isn't terribly convenient, but that's something you have to get used to when working with monad transformers.

Upvotes: 5

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