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scalascala-catscats-effect

How to attach effects to the F in Resource[F, A] without using A?

Given a Resource[F, A], what is the most idiomatic way to attach effects (e.g. metrics, logging errors) to F without using A (i.e. .use)?

I can easily do that with a F[A] but I depend on an API that specifically returns a Resource[F, A] to make sure that A will be released correctly.

Solution

  • If I understand the question correctly, I'd use Resource.liftF and <*. For example:

    import cats.effect.{ IO, Resource }
import cats.syntax.apply._
import scala.io.Source

val src = Resource.fromAutoCloseable(IO(Source.fromFile("build.sbt")))
val log = IO(println("opening file"))

val loggedSrc = src <* Resource.liftF(log)

    And then:

    scala> val program = loggedSrc.use(s => IO(println(s.mkString)))
program: cats.effect.IO[Unit] = IO$201584418

scala> program.unsafeRunSync
opening file
scalaVersion := "2.12.8"

scalacOptions += "-Ypartial-unification"

libraryDependencies += "org.typelevel" %% "cats-effect" % "1.2.0"

    In general you'd have the same approach you'd use in F, but lifted into Resource[F, _] with Resource.liftF.