I list Common Lisp & its behavior in Appendix C at the end of the article. Multiple-return-value is definitely better than just returning NIL, but I still disli

I list Common Lisp & its behavior in Appendix C at the end of the article. Multiple-return-value is definitely better than just returning NIL, but I still dislike it; it doesn't actively prevent you from confusing NIL with absence, so "the obvious way to do it" can still be the wrong way. A good CL programmer won't fall into this trap, but the article is in large part about how some languages/interfaces default you into good programming by requiring you to distinguish success from failure, and others don't. Another way of seeing the same thing is: Type-theoretically speaking, MRV is using a product type (two values) to emulate a sum type (one value of two possible forms); why not just use a sum type?