programs-as-values VS natchez

Go straight to the "programs as values" paradigm and its main data type; IO - I have a small repo with concrete examples and resources here: https://github.com/pslcorp/programs-as-values note however it is not intended as a tutorial on its own since I use it as support for my talks, but the linked resources are helpful.

Finally, while I get the point / joke, I just want to show how easy is to write something like a high-low priority scheduler using IO just because we can manipulate programs as values: https://github.com/pslcorp/programs-as-values/blob/main/src/main/scala/example/priority/HighLowPriorityRunner.scala

Regarding real-world uses of this concept, I used it today to put natchez tracing in my http4s project. While the project is still small, I was shocked at the lack of invasiveness of this approach compared to usage of Kleisli to achieve the same effect.

It's never going to work with a ThreadLocal-based implementation because Future (and other Scala async stuff like cats.effect.IO) hop from thread to thread in an unpredictable way. The only way to do it is to jettison the magical "current span" thing and pass the span explicitly. You can hide this in the plumbing if you program with abstract effects (this is what Natchez Trace does) but with Future I think you're going to have to pass everything around in the open.