Just had a chat with my Dad, who spent the afternoon tracking down a fence post error in his code, about how folds are dramatically superior to for loops (he doesn't disagree). :-)

Ah, sorry. Yes- the deep connections between folds, monoids/semigroups, and functors is fascinating. And pragmatically useful.

That's not the type foldr has. Given that Endo b = b -> b, that would give my first (and preferred) alternative: (a -> b -> b) -> f a -> b -> b.

I mean, obviously we can't fix it now. But jesus. This probably breaks the top five worst mis-features of Haskell.

You elected him.
You haven't removed him.
You're fuckups.
Own it.

I got a value out of a map. It's null. Does that mean that the key was not in the map, or that the key was in the map, and the value associated with it just happened to be null? Nothing = key not in map, Just Nothing = key in map, value is Nothing.