If you really badly want types, you'd slap something like this below it: (s/def ::future atom?)) (s/def ::task-sender async/chan?) (s/def ::task

If you really badly want types, you'd slap something like this below it: (s/def ::future atom?)) (s/def ::task-sender async/chan?) (s/def ::task (s/keys :req-un [::future ::task-sender])) (s/fdef create-task :ret ::task) (stest/instrument `create-task) But then I don't think you'd reach for something like Clojure if static typing is something that you require.