Ref as monad transformer 

There’s something difference about the datatypes like Ref and Resource: 

abstract class Ref[F[_], A] extends RefSource[F, A] with RefSink[F, A] {
  /**
   * Modifies the current value using the supplied update function. If another modification
   * occurs between the time the current value is read and subsequently updated, the modification
   * is retried using the new value. Hence, `f` may be invoked multiple times.
   *
   * Satisfies:
   *   `r.update(_ => a) == r.set(a)`
   */
  def update(f: A => A): F[Unit]

  def modify[B](f: A => (A, B)): F[B]

  ....
}

In addition to the type parameter A like Option would take, Ref is also parameterized by F[_]. 

scala> :k -v cats.effect.Ref
cats.effect.Ref's kind is X[F[A1],A2]
(* -> *) -> * -> *
This is a type constructor that takes type constructor(s): a higher-kinded type.

These are monad transformers that take the effects type F as parameter. We can actually put another F like SyncIO: 

import cats._, cats.syntax.all._
import cats.effect.{ IO, Ref, SyncIO }

lazy val program: SyncIO[Int] = for {
  r <- Ref[SyncIO].of(0)
  x <- r.get
} yield x

Resource as monad transformer 

We could probably make resource parametric on F[_]: 

import cats.effect.{ IO, MonadCancel, Resource, Sync }
import java.io.BufferedReader
import java.nio.charset.{ Charset, StandardCharsets }
import java.nio.file.{ Files, Path, Paths }

def bufferedReader[F[_]: Sync](path: Path, charset: Charset): Resource[F, BufferedReader] =
  Resource.fromAutoCloseable(Sync[F].blocking {
    Files.newBufferedReader(path, charset)
  })

lazy val r0: Resource[SyncIO, BufferedReader] = bufferedReader[SyncIO](Paths.get("/tmp/foo"), StandardCharsets.UTF_8)

On most of the monad transformers there is mapK(...) method that takes FunctionK to transform it to another G[_]. If we can define a ~> from one effect type to another, we can transform the resource, which is radical: 

lazy val toIO = λ[SyncIO ~> IO](si => IO.blocking { si.unsafeRunSync() })

lazy val r1: Resource[IO, BufferedReader] = r0.mapK(toIO)

That’s it for today.

herding cats — Ref as monad transformer