Separate Logger and IO-local context to allow Logger to be a ProFunctor

[hejfelix]

Currently, creating a logger is effectful because we create an IOLocal context.

object DefaultLogger:
  def makeIo(output: Output[IO], outputs: Output[IO]*)(using Clock[IO], Printer, Filter): IO[DefaultLogger[IO]] =
    for given StringLocal[IO] <- ioStringLocal
    yield new DefaultLogger[IO](output, outputs*)
end DefaultLogger

The actual interface requires a local instance to allow adding context to effects:

trait Logger[F[_]]:

  val stringLocal: StringLocal[F]

  def doLog(level: LogLevel, message: String)(using LogInfo): F[Unit]
...
end Logger

This in turn allows us to scope context to an effect value whenever we have a logger in scope:

def program(using Logger[IO]): IO[Unit] = 
  for
    _ <- Logger[IO].debug("This is some debug")
    _ <- Logger[IO].info("HEY!")
    _ <- Logger[IO].warn("I'm warning you")
    _ <- Logger[IO].error("I give up")
  yield ()

import Logger.*
val mainWithContext: IO[Unit] = 
  for
    given Logger[IO]  <- DefaultLogger.makeIo(consoleOutput)
    _                 <- program.withLogContext("trace-id", "4d334544-6462-43fa-b0b1-12846f871573")
    _                 <- Logger[IO].info("Now the context is gone")
  yield ()

This is very convenient and easy to use. There are 2 main drawbacks:

1. The constructor of the logger is now effectful
2. It's difficult to construct a correct instance of Profunctor for this interface

Challenges

If the Logger no longer has a reference to the local context, we place the burden on the user to pass around this context. It's not clear if there's a good solution to avoid this (maybe Odin has solved this?).

Proposal

Simplify the interface to something akin to:

case class LogLine(level: LogLevel, message: String, context: Map[String,String], info: LogInfo)

trait Logger[F[_]]:
  def doLog(line: LogLine): F[Unit]
end Logger

[dimitarg]

maybe Odin has solved this?

https://github.com/valskalla/odin?tab=readme-ov-file#contextual-effects

See

https://github.com/valskalla/odin/blob/db4444fe4efcb5c497d4e23bdf9bbcffd27269c2/core/src/main/scala/io/odin/loggers/ContextualLogger.scala#L55

et al.

So, the solution is just Ask[F, Env], which ends up being satisfied some variation / stack on top of ReaderT for a given user program,

I think (could be mistaken) this means that you cannot (lawfully) log with context in only IO. Personally I don't find this a worthy goal anyways because I find that

• It's easier to reason about ReaderT in the face of concurrency / parallelism / racing / cancellation / streaming than it is to reason about IOLocal;
• The programs I write need to be materialised in ReaderT anyways, because of distributed tracing.

[hejfelix]

It's an interesting approach – but I don't want to force users to go down the road of monad transformers, and we don't currently have a dependency on mtl.

[dimitarg]

Hey,

I was rereading this and, unrelated to the issue here, I was wrong in hinting Ask is a requirement. If you read carefully the documentation I linked, they say at the end

Odin automatically derives required type classes for each type F[_] that has Ask[F, E] defined, or in other words for all the types that allow F[A] => F[E].

If this constraint isn't satisfied, it's required to manually provide an instance for WithContext type class:

/**
  * Resolve context stored in `F[_]` effect
  */
trait WithContext[F[_]] {
  def context: F[Map[String, String]]
}

, and that one should be able to admit an IOLocal implementation, I think.