def is way better than fn. you can read def at least.

Read def? What the point of that? You might say "def" when you see it, but you think of the concept of a function. You should be translating fn to "function" in your head or if you speak it out loud, the same way you say "plus" when you see +. How do you "read" + otherwise? It's the same for every mathematical symbol, since there is no actual way to read them, since they communicate a concept that you most likely translate into your own language in your head.

Previous poster has a small point. Even I am guilty of saying 'f-n' when reading rust code alound. Kotlin is the best - all functions are fun