Self modifying code in the sense of changing one instruction to another at runtime (as was commonly done in assembler in the 1960s) is not generally possible wi

Self modifying code in the sense of changing one instruction to another at runtime (as was commonly done in assembler in the 1960s) is not generally possible with modern Common Lisps mostly because modern operating systems don't allow it. And that's a good thing because such code would be hopelessly insecure and impossible to reason about. But if you mean "compile a new function called X and replace the old X at runtime", that's easy in Common Lisp. It's not commonly done unless you're explicitly writing some kind of compiler. What is commonly done is to create a lexical closure at compile time and change its bound values at runtime. IOW changing the private data of a function at runtime is more generally useful than changing its instructions. What's most common is to write lisp programs that emit lisp source code and compile it at compile time (but usually not run time). Such programs are called macros.