Add -union (thanks to @Fuco1)

Fixes #4

README.md

@@ -41,6 +41,7 @@ Or you can just dump `dash.el` in your load path somewhere.
 * [-interpose](#-interpose-sep-list) `(sep list)`
 * [-interleave](#-interleave-rest-lists) `(&rest lists)`
 * [-first](#-first-pred-list) `(pred list)`
+* [-union](#-union-list-list) `(list list2)`
 * [-difference](#-difference-list-list) `(list list2)`
 * [-intersection](#-intersection-list-list) `(list list2)`
 * [-distinct](#-distinct-list) `(list)`
@@ -395,6 +396,18 @@ To get the first item in the list no questions asked, use `car`.
 (--first (> it 2) '(1 2 3)) ;; => 3
 ```
 
+### -union `(list list2)`
+
+Return a new list containing the elements of `list1` and elements of `list2` that are not in `list1`.
+The test for equality is done with `equal`,
+or with `-compare-fn` if that's non-nil.
+
+```cl
+(-union '(1 2 3) '(3 4 5)) ;; => '(1 2 3 4 5)
+(-union '(1 2 3 4) '()) ;; => '(1 2 3 4)
+(-union '(1 1 2 2) '(3 2 1)) ;; => '(1 1 2 2 3)
+```
+
 ### -difference `(list list2)`
 
 Return a new list with only the members of `list` that are not in `list2`.

dash.el

@@ -472,7 +472,16 @@ or with `-compare-fn' if that's non-nil.
 
 Alias: `-uniq'"
   (let (result)
-    (--each list (when (not (-contains? result it)) (!cons it result)))
+    (--each list (unless (-contains? result it) (!cons it result)))
+    (nreverse result)))
+
+(defun -union (list list2)
+  "Return a new list containing the elements of LIST1 and elements of LIST2 that are not in LIST1.
+The test for equality is done with `equal',
+or with `-compare-fn' if that's non-nil."
+  (let (result)
+    (--each list (!cons it result))
+    (--each list2 (unless (-contains? result it) (!cons it result)))
     (nreverse result)))
 
 (defalias '-uniq '-distinct)

examples.el

@@ -161,6 +161,11 @@
   (-first 'even? '(1 3 5)) => nil
   (--first (> it 2) '(1 2 3)) => 3)
 
+(defexamples -union
+  (-union '(1 2 3) '(3 4 5))  => '(1 2 3 4 5)
+  (-union '(1 2 3 4) '())  => '(1 2 3 4)
+  (-union '(1 1 2 2) '(3 2 1))  => '(1 1 2 2 3))
+
 (defexamples -difference
   (-difference '() '()) => '()
   (-difference '(1 2 3) '(4 5 6)) => '(1 2 3)