CLJS-359: Support metadata on functions.

src/cljs/cljs/core.cljs

@@ -128,6 +128,9 @@
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;; core protocols ;;;;;;;;;;;;;
 
+(defprotocol Fn
+  "Marker protocol")
+
 (defprotocol IFn
   (-invoke
     [this]
@@ -456,6 +459,22 @@
   (-hash [o]
     (if (identical? o true) 1 0)))
 
+(extend-type function
+  Fn
+
+  IMeta
+  (-meta [_] nil)
+
+  IWithMeta
+  (-with-meta [f meta]
+    (with-meta
+      (reify
+        Fn
+        IFn
+        (-invoke [_ & args]
+          (apply f args)))
+      meta)))
+
 (extend-type default
   IHash
   (-hash [o]
@@ -1026,7 +1045,7 @@ reduces them without incurring seq initialization"
   (goog/isNumber n))
 
 (defn ^boolean fn? [f]
-  (goog/isFunction f))
+  (or ^boolean (goog/isFunction f) (satisfies? Fn f)))
 
 (defn ^boolean ifn? [f]
   (or (fn? f) (satisfies? IFn f)))

test/cljs/cljs/core_test.cljs

@@ -400,6 +400,16 @@
     (assert (nil? (f :foo))))
   (assert (nil? (array-seq (array 1) 1)))
 
+  ;; Functions with metadata
+  (let [f (fn [x] (* x 2))
+        m {:foo "bar"}
+        mf (with-meta f m)]
+    (assert (nil? (meta f)))
+    (assert (fn? mf))
+    (assert (= 4 (mf 2)))
+    (assert (= 4 (apply mf [2])))
+    (assert (= (meta mf) m)))
+
   (let [a (atom 0)]
     (assert (= 0 (deref a)))
     (assert (= 1 (swap! a inc)))