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.)
Are monads an /advance/, or are they just a language feature that might be nice in some cases, pointless or harmful in others -- like any other feature?
Some monads like the maybe monad would be a dramatic improvement over just defaulting everything to nil everywhere. Likewise error monads would nicely complement the existing CL error mechanism.
You can get some monad behavior in CL now. Bind is fairly easy to write in CL but unit is messy. And CL's lack of automatic currying doesn't help.
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.)