Add new API

- a noop middleware
   - a comp-middleware

src/re_frame/core.cljs

@@ -16,6 +16,7 @@
 
 
 
+(def noop        middleware/noop)
 (def pure        middleware/pure)
 (def debug       middleware/debug)
 (def undoable    middleware/undoable)
@@ -34,3 +35,21 @@
     (register-handler id pure handler))
   ([id middleware handler]
     (register-handler id [pure middleware] handler)))
+
+
+(defn comp-middleware
+  "Given a vector of middleware, filter out any nils, and use comp to compose.
+  v can  be vectors of vectors and will be flattended before use.
+  For convienience, if v turns out to already be a function, just return it."
+  [v]
+  (if (ifn? v)     ;; noop if v is a fucntion
+    v
+    (do
+      (assert  (or (vector? v) (list v))
+               (str "re-frame:  compose expects a vector, got: " v))
+      (let [v (remove nil? (flatten v))]
+        (cond
+          (empty? v)       noop
+          (= 1 (count v))  (first v)
+          :else            (apply comp v))))))
+