the parts of Guix in which packages are defined are quite pure, even using monads for some things
Monads have nothing to do with purity. In fact, one of the most infamous usages of them is Haskell’s IO monad which is probably the most impure construct in the entire language.
it is really not too different from the Nix language.
Hm, I can’t help but think that a lisp dialect can never really be similar to any language except another lisp.