Add tail threading (->>)

hy/core/bootstrap.py

@@ -94,6 +94,20 @@ def threading_macro(tree):
     return ret
 
 
+@macro("_>>")
+def threading_tail_macro(tree):
+    tree.pop(0)
+    ret = tree.pop(0)
+    for node in tree:
+        if not isinstance(node, HyExpression):
+            nnode = HyExpression([node])
+            nnode.replace(node)
+            node = nnode
+        node.append(ret)
+        ret = node
+    return ret
+
+
 @macro("car")
 @macro("first")
 def first_macro(tree):

tests/native_tests/language.hy

@@ -235,6 +235,12 @@
              ["X" "B" "C" "D"])))
 
 
+(defn test-tail-threading []
+  "NATIVE: test tail threading macro"
+  (assert (= (.join ", " (* 10 ["foo"]))
+             (->> ["foo"] (* 10) (.join ", ")))))
+
+
 (defn test-threading-two []
   "NATIVE: test threading macro"
   (assert (= (-> "a b c d" .upper (.replace "A" "X") .split)