I can't make a Mappable interface, and have my classes implement map(f). Because map(f) will necessarily return Mappable, not my class itself. So no method chaining for me.

Also null. Yeah I know it's contentious. People don't want to let go of it. Since learning to hate null, I've also lost any nuance in my ability to explain why it's bad. Because I know longer see it as 'subtly-bad' or 'might lead to bugs'. It's just plain, on-the-surface-wrong. One might as well have named it 'wrong' rather than 'null'.

'Null' is the thing which it isn't. I can write business logic that says every Person has a Name. Once you admit null into the mix, I can no longer make that simplest of statements. My autocomplete now lies to me, because person may or may not implement the method .name().

"But how will I half-arse instantiate a Person? I don't have a Name, yet I want to tell the computer I have a Person?" It makes me happy that you can't.

"I wrote a function that promises to return a Person. I was unable to return a Person. How can I tell the computer I'm returning a Person even though I'm not?" Glad that you can't.