From bfc71e4a60d0e8801039a0d20e7e7a06a70fc476 Mon Sep 17 00:00:00 2001
From: "Paul R. Tagliamonte" <paultag@debian.org>
Date: Sun, 7 Apr 2013 15:05:30 -0400
Subject: [PATCH] Add tail threading (->>)

---
 hy/core/bootstrap.py           | 14 ++++++++++++++
 tests/native_tests/language.hy |  6 ++++++
 2 files changed, 20 insertions(+)

diff --git a/hy/core/bootstrap.py b/hy/core/bootstrap.py
index e77fb02..50efc66 100644
--- a/hy/core/bootstrap.py
+++ b/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):
diff --git a/tests/native_tests/language.hy b/tests/native_tests/language.hy
index d41d20b..1044c41 100644
--- a/tests/native_tests/language.hy
+++ b/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)