Fix #770: unwrap var-from-def before applying the viewer

When a `def` has an explicit viewer like `^{::clerk/viewer clerk/row}`,
the viewer should see the def's value, not the internal
`{:var-from-def ... :var-snapshot ...}` map Clerk adds around it.
Before, that map was passed to function viewers, which either crashed
(#770) or rendered the raw map. Now the value is unwrapped first,
unless the viewer opts out with `:var-from-def? true` (as
`render-eval-viewer` does).

Author: borkdude

src/nextjournal/clerk/viewer.cljc

@@ -429,27 +429,30 @@
 (defn var-from-def? [x]
   (var? (get-safe x :nextjournal.clerk/var-from-def)))
 
+(def unwrap-var-value (some-fn :nextjournal.clerk/var-snapshot
+                               (comp deref :nextjournal.clerk/var-from-def)))
+
 (def var-from-def-viewer
   {:name `var-from-def-viewer
    :pred var-from-def?
-   :transform-fn (update-val (some-fn :nextjournal.clerk/var-snapshot
-                                      (comp deref :nextjournal.clerk/var-from-def)))})
+   :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 unless the
-  viewer opts out with a truthy `:nextjournal.clerk/var-from-def`."
+  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?`."
   [{:as result :nextjournal/keys [value viewer]}]
   (if viewer
-    (let [value+viewer (if (or (var? viewer) (fn? viewer))
-                         (viewer value)
-                         {:nextjournal/value value
-                          :nextjournal/viewer (normalize-viewer viewer)})
-          {unwrap-var :transform-fn var-from-def? :pred} var-from-def-viewer]
-      (assoc result :nextjournal/value (cond-> value+viewer
-                                         (and (var-from-def? value)
-                                              (-> value+viewer ->viewer :var-from-def? not))
-                                         unwrap-var)))
+    (let [opts-out? (and (map? viewer) (:var-from-def? viewer))
+          value' (cond-> value
+                   (and (var-from-def? value) (not opts-out?))
+                   unwrap-var-value)
+          value+viewer (if (or (var? viewer) (fn? viewer))
+                         (viewer value')
+                         {:nextjournal/value value'
+                          :nextjournal/viewer (normalize-viewer viewer)})]
+      (assoc result :nextjournal/value value+viewer))
     result))
 
 #?(:clj

test/nextjournal/clerk/viewer_test.clj

@@ -212,19 +212,33 @@
     (is (= [{:render-fn 'foo}] (v/get-viewers 'nextjournal.clerk.viewer-test.random-ns-name)))))
 
 (def my-test-var [:h1 "hi"])
+(def my-test-var2 1)
+
+(def apply+get-value #(-> % v/apply-viewer-unwrapping-var-from-def :nextjournal/value :nextjournal/value))
 
 (deftest apply-viewer-unwrapping-var-from-def
-  (let [apply+get-value #(-> % v/apply-viewer-unwrapping-var-from-def :nextjournal/value :nextjournal/value)]
-    (testing "unwraps var when viewer doens't opt out"
-      (is (= my-test-var
-             (apply+get-value {:nextjournal/value [:h1 "hi"]                                      :nextjournal/viewer v/html})
-             (apply+get-value {:nextjournal/value {:nextjournal.clerk/var-from-def #'my-test-var} :nextjournal/viewer v/html})
-             (apply+get-value {:nextjournal/value {:nextjournal.clerk/var-from-def #'my-test-var} :nextjournal/viewer v/html-viewer}))))
-
-    (testing "leaves var wrapped when viewer opts out"
-      (is (= {:nextjournal.clerk/var-from-def #'my-test-var}
-             (apply+get-value {:nextjournal/value {:nextjournal.clerk/var-from-def #'my-test-var}
-                               :nextjournal/viewer (assoc v/html-viewer :var-from-def? true)}))))))
+  (testing "unwraps var when viewer doens't opt out"
+    (is (= my-test-var
+           (apply+get-value {:nextjournal/value [:h1 "hi"]                                      :nextjournal/viewer v/html})
+           (apply+get-value {:nextjournal/value {:nextjournal.clerk/var-from-def #'my-test-var} :nextjournal/viewer v/html})
+           (apply+get-value {:nextjournal/value {:nextjournal.clerk/var-from-def #'my-test-var} :nextjournal/viewer v/html-viewer}))))
+
+  (testing "leaves var wrapped when viewer opts out"
+    (is (= {:nextjournal.clerk/var-from-def #'my-test-var}
+           (apply+get-value {:nextjournal/value {:nextjournal.clerk/var-from-def #'my-test-var}
+                             :nextjournal/viewer (assoc v/html-viewer :var-from-def? true)}))))
+
+  (testing "function viewer receives the deref'd value, not the var-from-def wrapper (fixes #770)"
+    (is (= [my-test-var2]
+           (apply+get-value {:nextjournal/value {:nextjournal.clerk/var-from-def #'my-test-var2
+                                                 :nextjournal.clerk/var-snapshot my-test-var2}
+                             :nextjournal/viewer v/row}))))
+
+  (testing "function viewer on a seq-valued def spreads across row cells"
+    (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})))))
 
 
 (deftest resolve-aliases