Getting monads into Lisp in a useful way would require the introduction of features that would certainly make it a different language. You may consider this a feature but you'd be arguing with a lot of people who consider it a bug. (I mean that sentence very seriously; no sarcasm, no joke, it is an accurate description of the situation, nor am I implying I would find the idea horrifying... it would simply be a quite significant change to the core type system.)