That&#x27;s a very HN comment.  Completely wrong but delivered with strong convictions.<p>Lisp doesn&#x27;t actually exist.  Lisp is just axioms.  You run an implementation of Lisp allowing various operations.  Some flavors actually have (practically) <i>zero</i> mutability.  You just create functions and cons cells and the rest of the world gets built on top of those.<p>But, pure functional systems are useless (to be purely functional would mean you can&#x27;t even do I&#x2F;O... your program would just heat up the computer, which, in a way, is its own form of output, so you can&#x27;t ever be <i>fully</i> functional).<p>Every functional language gets non-functional at some point for data processing, bootstrapping, loading&#x2F;saving programs, and the best functional languages even have optional mutability constructs for performance (see: <i>!</i>).  You can&#x27;t create the most performant data structures possible if all your operations require path copying, but mutability should be reserved for internal high performance library creation and avoided in business logic&#x2F;application level code to make systems easier to reason about.