On the lesser topic. There are at least two notable things about `setf' (the first one smaller, the second one bigger). I. It eliminates the need for explicit setters. If you know how to write a form to get something, you automatically know how to write a form to set it. II. Modify macros become available. To illustrate with `incf': (let ((h (make-hash-table))) (incf (gethash 'foo h 0)) (gethash 'foo h)) works and evaluates to 1. (Does not fail trying to increment a non-existent element.) This is noticeably different from the typical operation of `+='. See also `define-modify-macro'. I'll leave the greater topic, mutability and immutability, for another day. _________ Disclosure I am a descendant of barbarians, who settled in the old Roman province of Thracia. Civis serdicensis sum. 🙂 #CommonLisp #Lisp @simon_brooke@mastodon.scot @janneke@todon.nl @spook@pleroma.m68k.church @pascal_costanza@functional.cafe