Validate :render-fn in process-viewer

src/nextjournal/clerk/viewer.cljc

@@ -1555,13 +1555,22 @@
              (.update hasher (goog.crypt/stringToUtf8ByteArray (pr-str x)))
              (.digest hasher))))
 
+(defn validate-viewer! [{:as viewer :keys [render-fn]}]
+  (when (and render-fn (not (or (list? render-fn)
+                                (symbol? render-fn))))
+    (throw (ex-info (format "`:render-fn` must to be a quoted form or symbol, got a %s instead." (if (fn? render-fn)
+                                                                                                   "function" (type render-fn)))
+                    {: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"