Ah, I am blocked by Gary on Twitter and there is some irony that his Boundaries talk culminates in a Twitter client. (Gary if you're reading this I enjoyed your feed!)

I'm not aware of any good OSS examples, either.

I think React and Redux are in a way an implementation of these concepts. React is a view layer that sends messages and each component is (ideally) a pure function of state, and sometimes its own history. It's so, so easy to unit test the interactions.

Hooks make it a little harder to reason about, and it's unfortunately easy to write hooks that perform IO and you suddenly end up in a miasma of difficult-to-test code where you have to return to using a mocking library to "replace" IO functions with fake versions, and then you're on a slippery slope again toward mixing interaction and mutation in one layer.

"most projects in elixir". Some good open-source examples are oban, hex.pm, papercups.io