There’s a lot of info on cats-effect these days, but a lot of it is about concrete use-cases. Yet there’s not much for those who know the basics but don’t feel confident enough with the library to build full-fledged applications. I’ve seen a couple of questions, however, which can be well generalized and are more complex and interesting than examples provided in documentation. So, I decided to turn them into exercises.

If you know how to do FP in Scala, and a bit about cats and cats-effect, the initial solution shouldn’t take more than an hour for you to arrive at. If you struggle to find a solution, there will be a couple of hints. And for those who want to dive deeper, there are bonus assignments which require additional generalization and/or refactoring.

Currently, there are just two. I plan to slowly build up the list as I solve more interesting problems, mine or others’.

Cats-effect 1.0.0-RC2 is out, and among various improvements, it brought to us some goodies in cats.effect.concurrent, exported from fs2 and Monix with a number of changes:

• Ref - pure mutable reference
• Deferred - a purely functional alternative to Promise
• Semaphore - an access control tool
• MVar - a mutable location that can be empty, useful as synchronization and communication channel

This post will focus on Ref and to show what interesting techniques it enables.

Problem:

I want to figure out which log entries belong to which request.

Also, being a lazy person, I want to get away with as little work as possible. In particular that means:

• I don’t want to pass extra function parameters / implicits (solutions like Logger.takingImplicit of scala-logging won’t cut)
• I don’t want to pollute my domain signatures with something like ReaderT[Task, RequestId, A] instead of plain Tasks.
• I don’t want to manually insert request ID to every call to log function. I want to set it once and be done with it.

Java SLF4J API already support all this functionality in form of MDC. However, the existing implementations use a ThreadLocal variable, which doesn’t work if you’re not creating a separate thread per each request - which I do not.

Solution: