#protip this is not the Clojure you're looking for...

failture.clj

(deftype Event (U :new-release))

(defn probability :- Float
  "Return the probability of some `Event` to occur. IRL this should be some
  real and complicated calculus instead of a simply constant"
  [event :- Event :> Float]
  0.95)

(defn descriptive-propability
  "Describe the probability of a given `Event` to occur, for human readers."
  [event :- Event] :- String
  (match (probability event)
    1.00 "certain"
    0.00 "impossible"
    (range 0.00 0.25) "very unlikely"
    (range 0.25 0.50) "unlikely"
    (range 0.50 0.75) "likely"
    (range 0.75 1.00) "very likely"
    _ (unreachable!)))

(defn -main []
  (println! (descriptive-propability :new-release)))