Parsing with clojure.spec

core.clj

(require '[clojure.spec.alpha :as s])

[:h1 {} "0" 1 [:span]]

(s/def :hiccup/form
  (s/or
   :string string?
   :number number?
   :element :hiccup/element))

(s/def :hiccup/element
  (s/cat
   :tag keyword?
   :attrs (s/? map?)
   :children (s/* :hiccup/form)))

(defn parse-hiccup [hiccup]
  (s/conform :hiccup/form hiccup))

(defmulti html first)

(defmethod html :string [[_ v]]
  v)

(defmethod html :number [[_ v]]
  v)

(defmethod html :element [[_ {:keys [tag attrs children]}]]
  (str "<" (name tag) ">"
       (->> (map html children)
            (clojure.string/join ""))
       "</" (name tag) ">"))

(html (parse-hiccup [:h1 {} "Hello" [:span "1" 2]]))