Fix for mapcat, issue #1556

Corrected mapcat to actually concat its results rather than merging.
Added warning to docstring thta this might not be ideal.

Added flatmap to cover map+merge case.

src/main/clojure/rx/lang/clojure/core.clj

@@ -2,7 +2,7 @@
   (:refer-clojure :exclude [concat cons count cycle
                             distinct do drop drop-while
                             empty every?
-                            filter first future
+                            filter first future flatmap
                             group-by
                             interleave interpose into iterate
                             keep keep-indexed
@@ -616,17 +616,46 @@
   return an Observable. The resulting observables are concatentated together
   into one observable.
 
+  WARNING: This operator, like clojure.core/mapcat, preserves ordering of the
+  generated Observables. In an asynchronous context, this may cause unintended
+  blocking. Try flatmap instead.
+
   If multiple Observables are given, the arguments to f are the first item from
   each observable, then the second item, etc.
 
   See:
     clojure.core/mapcat
-    rx.Observable/flatMap
+    flatmap
+    rx.Observable/concatMap
   "
   [f & xs]
   (if (clojure.core/next xs)
     (mapcat* f (seq->o xs))
     ; use built-in flatMap for single-arg case
+    (.concatMap ^Observable (clojure.core/first xs) (iop/fn* f))))
+
+(defn ^Observable flatmap*
+  "Same as multi-arg flatmap, but input is an Observable of Observables.
+
+  See:
+    flatmap
+  "
+  [f ^Observable xs]
+  (->> xs
+       (map* f)
+       (merge*)))
+
+(defn ^Observable flatmap
+  "Like mapcat, but the Observables produced by f are merged rather than concatenated.
+  This behavior is preferable in asynchronous contexts where order is not important.
+
+  See:
+    mapcat
+    rx.Observable/flatMap
+  "
+  [f & xs]
+  (if (clojure.core/next xs)
+    (flatmap* f (seq->o xs))
     (.flatMap ^Observable (clojure.core/first xs) (iop/fn* f))))
 
 (defn map-indexed

src/test/clojure/rx/lang/clojure/core_test.clj

@@ -277,8 +277,8 @@
   (let [xs [{:k :a :v 1} {:k :b :v 2} {:k :a :v 3} {:k :c :v 4}]]
     (testing "with just a key-fn"
       (is (= [[:a {:k :a :v 1}]
-              [:b {:k :b :v 2}]
               [:a {:k :a :v 3}]
+              [:b {:k :b :v 2}]
               [:c {:k :c :v 4}]]
              (->> xs
                   (rx/seq->o)
@@ -452,6 +452,44 @@
                                  (rx/seq->o as)
                                  (rx/seq->o bs)))))))
 
+(deftest test-flatmap
+  (let [f  (fn [v] [v (* v v)])
+        xs (range 10)]
+    (is (= (mapcat f xs)
+           (b/into [] (rx/flatmap (comp rx/seq->o f) (rx/seq->o xs))))))
+
+  ; group-by is a good way to test merge behavior without truly async code
+  ; here the :a and :b observables are interleaved when merged
+  (let [xs [{:k :a :v 1} {:k :b :v 2} {:k :a :v 3} {:k :c :v 4}]]
+    (is (= [[:a {:k :a :v 1}]
+            [:b {:k :b :v 2}]
+            [:a {:k :a :v 3}]
+            [:c {:k :c :v 4}]]
+           (->> xs
+                (rx/seq->o)
+                (rx/group-by :k)
+                (rx/flatmap (fn [[k vo :as me]]
+                              (is (instance? clojure.lang.MapEntry me))
+                              (rx/map #(vector k %) vo)))
+                (b/into [])))))
+
+  ; still looking for a simple demo of merging for the multi-arg case
+  ; Here, because ys is "inline", the interleaving is removed. sigh.
+  (let [xs [{:k :a :v 1} {:k :b :v 2} {:k :a :v 3} {:k :c :v 4}]
+        ys [:ay :by :cy]]
+    (is (= [[:a {:k :a :v 1} :ay]
+            [:a {:k :a :v 3} :ay]
+            [:b {:k :b :v 2} :by]
+            [:c {:k :c :v 4} :cy]]
+           (->> (rx/flatmap (fn [[k vo :as me] y]
+                              (is (instance? clojure.lang.MapEntry me))
+                              (rx/map #(vector k % y) vo))
+                            (->> xs
+                                 rx/seq->o
+                                 (rx/group-by :k))
+                            (rx/seq->o ys))
+                (b/into []))))))
+
 (deftest test-next
   (let [in [:q :r :s :t :u]]
     (is (= (next in) (b/into [] (rx/next (rx/seq->o in)))))))