when-seq: conditionally apply operation on sequence in threading macro

when-seq.clj

(defn when-seq
  "If condition is true, perform seq operation f on xs with args; otherwise return f.

  Intended for use in the thread first macro. For example:

  (-> []
      (when-seq x conj x))

  will be [:foo] if x is :foo, [] if x is nil"
  [xs condition f & args]
  (if condition
    (apply f xs args)
    xs))

clojure.core/cond-> is probably more useful