添加日志记录需要更改方法签名 Cats-Effect？

Question

I'm learning the whole Cats-Effect FP framework, and am wondering about how to best implement logging. I'm struggling with finding an elegant way to use logging in methods that return non-side-effect types, particularly class constructors. For example:

case class Limit(name: String, value: Int)

object Limit {
  def max(name: String): Limit = Limit(name, Int.MaxValue)
}

Now, if I want to add logging to the construction, which is obviously a side-effect, I would have to change the method signature to something like:

object Limit {
  def max[F[_] : Sync](name: String): F[Limit] = {
    for {
      _ <- Slf4jLogger.getLogger[F].debug("Creating max limit.")
    } yield {
      Limit(name, Int.MaxValue)
    }
  }
}

Such a change breaks code compatibility, and now every place that uses Limit.max needs to be refactored to deal with the type signatures, mapping and/or call .unsafeRun . Quite a radical change for something as small as adding a log line.

Is there some way to do this more elegantly? Do I just drop log4cats and use the regular, unsafe SLF4J? Or do I just need to accept that in PF/Cats-Effect world it's safer to just return F[..] from everything everywhere, no matter how small or trivial the method is?

Answer 1

I think it would be better to use log4cats library. If you use cats-effect, probably your Main class returns IO[Exit] and almost whole your logic wouldn't require any unsafe calls.

But at the same time I saw a lot of projects that uses just a plain logging without wrapping logs inside of IO. It depends on you and/or your team/project. I personally prefer FP way.

I left an example of using log4cats here:

implicit def logger[F[_]: Sync]: Logger[F] = Slf4jLogger.getLogger[F]

case class Limit(name: String, value: Int)
  
object Limit {
  def max[F[_] : Sync: Logger](name: String): F[Limit] = {
    for {
      _ <- Logger[F].info("Creating max limit.")
    } yield {
      Limit(name, Int.MaxValue)
    }
  }
}

build.sbt file:

libraryDependencies ++= Seq(
  "org.typelevel" %% "log4cats-core"    % "2.5.0",  // Only if you want to Support Any Backend
  "org.typelevel" %% "log4cats-slf4j"   % "2.5.0",  // Direct Slf4j Support - Recommended
  // and some backend such as logback or smth
)