I saw a fun debugging issue the other day on r/lisp that almost made me want to walk away from lisp for good.

You have a function (defun my-val () "a string")

Referentially transparent, easy to reason about right? Wrong. Lisp strings are mutable, and everything is a reference. So if you do something like this

(replace (my-val) "b" :start1 0 :end1 1)

(my-val) => "b string"

I appreciate that CL doesn't force everything to be immutable, but it seems like at least strings would benefit from being treated as immutable values.