In -> and ->>, don't modify the arguments

NEWS.rst

@@ -25,6 +25,7 @@ New Features
 Bug Fixes
 ------------------------------
 * Fix `(return)` so it works correctly to exit a Python 2 generator
+* Fixed a case where `->` and `->>` duplicated an argument
 
 Misc. Improvements
 ----------------------------

hy/core/macros.hy

@@ -142,17 +142,16 @@ in order of the given pairs."
   (_for 'for/a* args body))
 
 
-(defmacro -> [head &rest rest]
+(defmacro -> [head &rest args]
   "Thread `head` first through the `rest` of the forms.
 
 The result of the first threaded form is inserted into the first position of
 the second form, the second result is inserted into the third form, and so on."
   (setv ret head)
-  (for* [node rest]
-    (if (not (isinstance node HyExpression))
-      (setv node `(~node)))
-    (.insert node 1 ret)
-    (setv ret node))
+  (for* [node args]
+    (setv ret (if (isinstance node HyExpression)
+      `(~(first node) ~ret ~@(rest node))
+      `(~node ~ret))))
   ret)
 
 
@@ -168,17 +167,16 @@ the second form, the second result is inserted into the third form, and so on."
      ~@(map build-form expressions)
      ~f))
 
-(defmacro ->> [head &rest rest]
+(defmacro ->> [head &rest args]
   "Thread `head` last through the `rest` of the forms.
 
 The result of the first threaded form is inserted into the last position of
 the second form, the second result is inserted into the third form, and so on."
   (setv ret head)
-  (for* [node rest]
-    (if (not (isinstance node HyExpression))
-      (setv node `(~node)))
-    (.append node ret)
-    (setv ret node))
+  (for* [node args]
+    (setv ret (if (isinstance node HyExpression)
+      `(~@node ~ret)
+      `(~node ~ret))))
   ret)
 
 

tests/native_tests/language.hy

@@ -742,6 +742,18 @@
   (assert (= (.join ", " (* 10 ["foo"]))
              (->> ["foo"] (* 10) (.join ", ")))))
 
+(defn test-threading-in-macro []
+  ; https://github.com/hylang/hy/issues/1537
+  ; The macros need to be defined in another file or else the bug
+  ; isn't visible in cb72a8c155ac4ef8e16afc63ffa80c1d5abb68a7
+  (require tests.resources.macros)
+
+  (tests.resources.macros.thread-set-ab)
+  (assert (= ab 2))
+
+  (tests.resources.macros.threadtail-set-cd)
+  (assert (= cd 5)))
+
 
 (defn test-threading-two []
   "NATIVE: test threading macro"

tests/resources/macros.hy

@@ -0,0 +1,9 @@
+(defmacro thread-set-ab []
+  (defn f [&rest args] (.join "" (+ (, "a") args)))
+  (setv variable (HySymbol (-> "b" (f))))
+  `(setv ~variable 2))
+
+(defmacro threadtail-set-cd []
+  (defn f [&rest args] (.join "" (+ (, "c") args)))
+  (setv variable (HySymbol (->> "d" (f))))
+  `(setv ~variable 5))