I know Matthew personally, and he's like that all the time. Very laid-back, and also very go-getter. Difficult combination, but he pulls it off well. Racket's

I know Matthew personally, and he's like that all the time. Very laid-back, and also very go-getter. Difficult combination, but he pulls it off well. Racket's libraries are designed to encourage doing everything with immutable data, though there are plenty of mutable types if you need them. (And of course you can always use boxes.) I don't know enough about Clojure to make a comparison, but I can say that working with immutable data structures in Racket is easy.