Just to make sure I understand, what do you mean by "output" in this context?

stdout, ie *out*

Ah, okay. Given that *out* is global, mutable, and thread-unsafe, we are unlikely to try to support that. You can, of course, use with-out-str if you need to capture output side-effects from an impure function...

Certainly possible, but a little intrusive. It might also be nice to see both output and return value.