Yeah, it's extremely difficult. Everything interacts with everything else. A recent snag I've run into recently with my language: I want to have mutable strings and lists but that complicates their use as hash table keys.

I'm so used to the C mindset of just modifying everything in place. It wasn't until I actually tried this that I understood why so many languages just copy everything. I researched how other languages dealt with this problem and many only allow immutable objects as hash keys. Most interesting was Ruby which allows mutable objects as keys, warns programmers that modifying keys can invalidate the hash table and provides a rehashing method to fix it.

https://ruby-doc.org/core/Hash.html

> Modifying a Hash key while it is in use damages the hash’s index.

> You can repair the hash index using method rehash

> A String key is always safe

> That’s because an unfrozen String passed as a key will be replaced by a duplicated and frozen String

I like Ruby a lot.