Is metadata a good idea here?

metadata-example.clj

(ns my-lib.validation
  "Validation functions"
  (:refer-clojure :rename {keyword to-keyword}))

(defn keyword
  "If key not nil, is value a keyword?"
  [k v]
  {:error "value must be a keyword when key is given"}
  (if (nil? k) true
    (keyword? v)))

; later, a `validate` function uses the :error metadata