Improve error messages for invalid hiccup in emit

src/huff2/core.clj

@@ -118,6 +118,16 @@
 (defmulti emit
   (fn [_append! form _opts] (:key form)))
 
+(defmethod emit :default [_append! form _opts]
+  (throw (ex-info (cond
+                    (keyword? (:value form))
+                    (str "Got a bare keyword " (:value form)
+                         " — wrap it in a vector to make an HTML element, e.g. ["
+                         (:value form) "]")
+                    :else
+                    (str "Unexpected hiccup form: " (pr-str form)))
+                  {:form form})))
+
 (defmethod emit :primative [append! {:keys [value]} _opts]
   (maybe-escape-html append! value))
 
@@ -258,7 +268,14 @@
     (emit append! c opts)))
 
 (defmethod emit :component-node [append! {{{:keys [view-fxn children]} :values} :value} {:keys [parser] :as opts}]
-  (emit append! (parser (apply view-fxn children)) opts))
+  (let [result (apply view-fxn children)
+        parsed (parser result)]
+    (if (= parsed :malli.core/invalid)
+      (throw (ex-info (str "Component " view-fxn " returned invalid hiccup")
+                      {:component view-fxn
+                       :args children
+                       :returned result}))
+      (emit append! parsed opts))))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; Public api
 

test/huff/core2_test.clj

@@ -1,7 +1,7 @@
 (ns huff.core2-test
   (:require [huff2.core :as h]
             [clojure.string :as str]
-            [clojure.test :refer [deftest is]]))
+            [clojure.test :refer [deftest is testing]]))
 
 (deftest class-list-equality-test
   (is (= (h/html [:div]) (h/html [:div])))
@@ -180,3 +180,14 @@
          (str (h/html [:div {:style {:width (-> 10 h/vmin)}}]))))
   (is (= "<div style=\"width:10vmax;\"></div>"
          (str (h/html [:div {:style {:width (-> 10 h/vmax)}}])))))
+
+(deftest component-invalid-hiccup-error-test
+  (testing "component returning invalid hiccup gives clear error"
+    (let [bad-component (fn [] :div)]
+      (is (thrown-with-msg? Exception #"returned invalid hiccup"
+            (h/html [:div [bad-component]])))))
+  (testing "component returning bare keyword mentions the component"
+    (let [bad-component (fn [] :div)
+          ex-data' (try (h/html [:div [bad-component]])
+                        (catch Exception e (ex-data e)))]
+      (is (= :div (:returned ex-data'))))))