Okay so the important things seem to be this comment

https://social.treehouse.systems/@dysfun/117441025783295712

the second screenshot (the terminal shot) attached to this post

and this comment

https://mastodon.social/@porglezomp/117441075637749928

I still don't understand it, but I think I'm closer to understanding it than I have been ever in the last 25 years. Just gotta think about those type signatures. And in the meantime I don't *have* to understand it, I can just use do notation and worry later about which Math Things my code corresponds to.