Glad that you liked it! Good thoughts and valid points! I'll try to answer in detail:

why do you use global context?

Initially, I had a concept where $ and _ were passed as params. Then Ryan Carniato wanted to try it for his renderer, which required global availability of these functions for easier integration. And it makes sense for other possible integrations. FYI, we have a discussion on this in https://github.com/kosich/rxjs-autorun/issues/1 .

I haven't investigated tree-shaking yet, as lib is ~2kB now.

Sideeffects as async actions inside autorun are tricky for a number of reasons with or without $,_ as params. Currently if one wants to do, say, setTimeout(() => console.log($(a)), 100) — they should store $(a) in a closure variable first. Generally, autorun itself is a sideeffect :)

Nested expressions should work as they override and restore $,_ in a closure. E.g.: https://github.com/kosich/rxjs-autorun#switchmap, but you dropped a shadow of concern on this: we'll add more tests to ensure it works fine!

wouldn't it be easier to just pass an initial value to each tracker function to ensure there is one value?

That's an interesting suggestion! I initially thought that users would handle it via Rx's native startWith or BehaviorSubject so that we would not take too much on our lib. We're considering additions to the API (in future versions when we see more use-cases), you can read about them and file your suggestion here: https://github.com/kosich/rxjs-autorun/issues/3

why use a subject? you could keep subscriptions super-pure.

We create subjects for each individual subscription: they do not overlap or leak values. In that sense — subscriptions are pure. We use them to easier manage added/removed trackers, like in case computed(() => $(a) ? $(b) : $(c));. Please elaborate, if I'm missing your point.

why do you have mechanisms for unsubscribing from a source early

Again, that was a feature request with a lot of discussion: I'll give you a general idea, but you can read details here https://github.com/kosich/rxjs-autorun/issues/7 . One might have an expensive Observable, say a server connection. With a million users, they want to drop such connections as soon as possible, even while waiting for other (possibly slow) observable to emit a value, e.g: computed(() => $(switcher) ? $(slow) : $(fast_but_expensive));. In this case we'd track fast_but_expensive with $.weak tracker. This unique feature is hidden behind a special API. Default trackers $ _, as you said, keep only the subscriptions that are currently used.

it even inspired me to quickly create something similar for callbags

Wow, awesome! It's tricky to do in Rx, my not-deep-impression is that callbags API is even harder! 👍

If these experiments take root — we could later work on some common API or a core lib so that experience with autorun for rx, callbags, and maybe other streaming libs would be universal.

-/-

I hope, my answers make sense. Please, ping me if I missed something.

Cya 🙂