Hi, I’m looking for advice on how to do logging in...

# functional

[dnowak]

Hi, I’m looking for advice on how to do logging in functional code.
Functional -> no side effects -> no logs ;-).
Our code is base on Arrow’s IO, so I would appreciate some hints for logging in this context.

[raulraja]

@dnowak you can also make it poart of your effects by exposing it as a type class:

interface Logger<F> {
  val A: Async<F>
  fun info(msg: String): Kind<F, Unit>
}

object IOLogger : Logger<ForIO> {
  override val A = IO.async()
  override fun info(msg: String): IO<Unit> = IO.effect { log(msg) }
}

//...other runtimes like Rx, etc will also be able to provide an impl for it

[Gopal S Akshintala]

@dnowak you may refer one of @raulraja’s repo for how Logger is defined and implemented: https://github.com/47degrees/arrow-architecture/blob/784082cb3e86f4b4e9b338a0179aa7ffbf50dc1f/src/main/kotlin/algebras/GistApiDataSource.kt#L25

[raulraja]

A complete library that integrates with slf4j too asn example in scala https://github.com/ChristopherDavenport/log4cats

[raulraja]

It also observes F for polymorphism and abstracts what runtime it targets

[dnowak]

Thank you for all the responses. Right now flatTap is all what I need.