Throw error when :render-fn is not a list or symbol (#699)

When called in JVM Clojure.

src/nextjournal/clerk/viewer.cljc

@@ -1555,13 +1555,23 @@
  (.update hasher (goog.crypt/stringToUtf8ByteArray (pr-str x)))
  (.digest hasher))))
 
+(defn validate-viewer! [{:as viewer :keys [render-fn]}]
+ #?(:clj (when (and render-fn (not (or (list? render-fn)
+ (symbol? render-fn))))
+ (throw (ex-info (str "`:render-fn` must to be a quoted form or symbol, got a "
+ (if (fn? render-fn) "function" (type render-fn))
+ " instead.")
+ {:viewer viewer
+ :render-fn-type (type render-fn)})))))
+
 (defn process-viewer [viewer {:nextjournal/keys [render-evaluator]}]
  ;; TODO: drop wrapped-value arg here and handle this elsewhere by
  ;; passing modified viewer stack
  ;; `(clerk/update-viewers viewers {:render-fn #(assoc % :render-evaluator :cherry)})`
  (if-not (map? viewer)
  viewer
- (-> viewer
+ (-> (doto viewer
+ validate-viewer!)
  (cond-> (and (not (:render-evaluator viewer)) render-evaluator)
  (assoc :render-evaluator render-evaluator))
  (dissoc :add-viewers :pred :transform-fn :update-viewers-fn)

test/nextjournal/clerk/viewer_test.clj

@@ -145,7 +145,11 @@
 
  (is (= (v/present (v/with-viewer v/html-viewer [:h1 "ahoi"]))
  (v/present {:nextjournal/value (v/with-viewer v/html-viewer [:h1 "ahoi"])})
- (v/present {:nextjournal/value {:nextjournal/value (v/with-viewer v/html-viewer [:h1 "ahoi"])}})))))
+ (v/present {:nextjournal/value {:nextjournal/value (v/with-viewer v/html-viewer [:h1 "ahoi"])}}))))
+
+ (testing "invalid `:render-fn` throws error"
+ (is (thrown? clojure.lang.ExceptionInfo
+ (v/present (v/with-viewer {:render-fn (fn [x] [:h1 x])} "Mike"))))))
 
 (deftest present-exceptions
  (testing "can represent ex-data in a readable way"