Tag slider/text-input viewer fns with :var-from-def? meta

Function viewers whose returned viewer map opts out of var-from-def
unwrapping need to advertise that intent upfront, otherwise the
outer code can't know whether to pass the raw var-from-def map or
the deref'd value. Metadata on the fn value lets `apply-viewer-
unwrapping-var-from-def` decide in a single pass.

Also updates the viewer-test case for fn viewers to use meta-based
opt-out instead of the post-hoc return-shape detection.

Author: borkdude

src/nextjournal/clerk/experimental.clj

@@ -33,6 +33,8 @@
   ([!state] (slider {} !state))
   ([opts !state] (viewer/with-viewer (assoc viewer/render-eval-viewer :render-fn (render-slider opts)) !state)))
 
+(alter-var-root #'slider with-meta {:var-from-def? true})
+
 
 (defn render-text-input
   ([] (render-slider {}))
@@ -49,3 +51,5 @@
   ([!state] (text-input {} !state))
   ([opts !state] (viewer/with-viewer (assoc viewer/render-eval-viewer :render-fn (render-text-input opts)) !state)))
 
+(alter-var-root #'text-input with-meta {:var-from-def? true})
+

src/nextjournal/clerk/viewer.cljc

@@ -438,13 +438,15 @@
    :transform-fn (update-val unwrap-var-value)})
 
 (defn apply-viewer-unwrapping-var-from-def
-  "Applies the `viewer` (if set) to the given result `result`. In case
-  the `value` is a `var-from-def?` it will be unwrapped (so that the
-  viewer operates on the def's value) unless the viewer map opts out
-  with a truthy `:var-from-def?`."
+  "Applies the `viewer` (if set) to the given result `result`. By default
+  the `value` of a `var-from-def?` is unwrapped so that the viewer
+  operates on the def's value. To opt out, a viewer map must carry
+  `:var-from-def? true`, or a viewer fn must have `:var-from-def? true`
+  in its metadata (use `with-meta` / `alter-var-root`)."
   [{:as result :nextjournal/keys [value viewer]}]
   (if viewer
-    (let [opts-out? (and (map? viewer) (:var-from-def? viewer))
+    (let [opts-out? (or (and (map? viewer) (:var-from-def? viewer))
+                        (and (ifn? viewer) (:var-from-def? (meta viewer))))
           value' (cond-> value
                    (and (var-from-def? value) (not opts-out?))
                    unwrap-var-value)

test/nextjournal/clerk/viewer_test.clj

@@ -238,7 +238,16 @@
     (is (= [1 2 3]
            (apply+get-value {:nextjournal/value {:nextjournal.clerk/var-from-def #'my-test-var
                                                  :nextjournal.clerk/var-snapshot [1 2 3]}
-                             :nextjournal/viewer v/row})))))
+                             :nextjournal/viewer v/row}))))
+
+  (testing "function viewer tagged with `:var-from-def? true` metadata receives the raw var-from-def (e.g. cx/slider)"
+    (let [viewer-fn (with-meta (fn [v] (v/with-viewer v/html-viewer v))
+                               {:var-from-def? true})]
+      (is (= {:nextjournal.clerk/var-from-def #'my-test-var
+              :nextjournal.clerk/var-snapshot [:h1 "hi"]}
+             (apply+get-value {:nextjournal/value {:nextjournal.clerk/var-from-def #'my-test-var
+                                                   :nextjournal.clerk/var-snapshot [:h1 "hi"]}
+                               :nextjournal/viewer viewer-fn}))))))
 
 
 (deftest resolve-aliases