Multimethod schemas

multischema.clj

(def Apple
  {::type (s/eq :apple)
   :color (s/enum :red :yellow :green)})

(def Orange
  {::type (s/eq :orange)
   :color (s/eq :orange)})

(def Fruit
  (s/conditional
   #(= (::type %) :apple)  Apple
   #(= (::type %) :orange) Orange))

(defmulti fruit-schema
  ::type)

(defmethod fruit-schema :apple  [_]
  {::type (s/eq :apple)
   :color (s/enum :red :yellow :green)})

(defmethod fruit-schema :orange [_]
  {::type (s/eq :orange)
   :color (s/eq :orange)})

(defrecord MultiSchema [multifn]
  s/Schema)

(defn multi-schema [^clojure.lang.MultiFn multifn]
  (reify
    s/Schema
    (spec [this]
      (schema.spec.variant/variant-spec
       ;; precondition
       (partial get-method multifn)
       ;; options
       (for [[dispatch-value f] (methods multifn)]
         {:guard  #(= ((.dispatchFn multifn) %) dispatch-value)
          :schema (f nil)})
       ;; error message function
       (fn [_] (s/explain this))))
    (explain [_]
      (cons 'one-of (keys (methods multifn))))))

(def MultiFruit (multi-schema fruit-schema))

(s/check MultiFruit {::type :apple, :color :yellow})
;; -> nil

(s/check MultiFruit {::type :apple})
;; -> {:color missing-required-key}

(s/check MultiFruit {::type :strawberry})
;; -> (not (one-of :orange :apple))

(defmethod fruit-schema :banana [_]
  {::type (s/eq :banana)
   :ripe? s/Bool})