Make Tag object nth-friendly

Author: igrishaev

src/malli/core.cljc

@@ -153,7 +153,21 @@
 #?(:cljs (defn- pr-writer-schema [obj writer opts]
            (-pr-writer (-form ^Schema obj) writer opts)))
 
-(defrecord Tag [key value])
+(defrecord Tag [key value]
+  clojure.lang.Indexed
+  (nth [_ i]
+    (case (long i)
+      0 key
+      1 value
+      (let [message "wrong tag index"]
+        (throw
+         #?(:clj (new IndexOutOfBoundsException message)
+            :cljs (new js/Error message))))))
+  (nth [_ i not-found]
+    (case (long i)
+      0 key
+      1 value
+      not-found)))
 
 (defn tag
   "A tagged value, used eg. for results of `parse` for `:orn` schemas."

test/malli/core_test.cljc

@@ -1236,6 +1236,20 @@
 
       (is (true? (m/validate (over-the-wire schema) valid1)))
 
+      (testing "tag nth support"
+        (let [tag (m/parse schema valid1)
+              [k v x] tag]
+          (is (= :sized k))
+          (is (= valid1 v))
+          (is (nil? x))
+          (try
+            (nth tag 99)
+            (is false "should throw")
+            (catch #?(:clj IndexOutOfBoundsException
+                      :cljs js/Error) e
+              (is (= "wrong tag index"
+                     (ex-message e)))))))
+
       (testing "ast"
         (is (= {:type :multi,
                 :keys {:sized {:order 0,