Allow for with an empty body

NEWS.rst

@@ -39,6 +39,7 @@ New Features
 * `defclass` in Python 3 now supports specifying metaclasses and other
   keyword arguments
 * Added a command-line option `-E` per CPython
+* `while` and `for` are allowed to have empty bodies
 
 Bug Fixes
 ------------------------------

hy/compiler.py

@@ -1820,7 +1820,7 @@ class HyASTCompiler(object):
         ret += node(expression,
                     target=target,
                     iter=ret.force_expr,
-                    body=body.stmts,
+                    body=body.stmts or [asty.Pass(expression)],
                     orelse=orel.stmts)
 
         ret.contains_yield = body.contains_yield

hy/core/macros.hy

@@ -110,15 +110,11 @@ used as the result."
 
 (defn _for [node args body]
   (setv body (list body))
-  (if (empty? body)
-    (macro-error None "`for' requires a body to evaluate"))
-  (setv lst (get body -1))
-  (setv belse (if (and (isinstance lst HyExpression) (= (get lst 0) "else"))
+  (setv belse (if (and body (isinstance (get body -1) HyExpression) (= (get body -1 0) "else"))
                 [(body.pop)]
                 []))
   (if
     (odd? (len args)) (macro-error args "`for' requires an even number of args.")
-    (empty? body)     (macro-error None "`for' requires a body to evaluate")
     (empty? args)     `(do ~@body ~@belse)
     (= (len args) 2)  `(~node [~@args] (do ~@body) ~@belse)
     (do

tests/compilers/test_ast.py

@@ -534,14 +534,6 @@ def test_for_compile_error():
         can_compile("(fn [] (for [x] x))")
     assert excinfo.value.message == "`for' requires an even number of args."
 
-    with pytest.raises(HyTypeError) as excinfo:
-        can_compile("(fn [] (for [x xx]))")
-    assert excinfo.value.message == "`for' requires a body to evaluate"
-
-    with pytest.raises(HyTypeError) as excinfo:
-        can_compile("(fn [] (for [x xx] (else 1)))")
-    assert excinfo.value.message == "`for' requires a body to evaluate"
-
 
 def test_attribute_access():
     """Ensure attribute access compiles correctly"""

tests/native_tests/language.hy

@@ -232,7 +232,15 @@
   (assert (= (list ((fn [] (for [x [[1] [2 3]] y x] (yield y)))))
              (list-comp y [x [[1] [2 3]] y x])))
   (assert (= (list ((fn [] (for [x [[1] [2 3]] y x z (range 5)] (yield z)))))
-             (list-comp z [x [[1] [2 3]] y x z (range 5)]))))
+             (list-comp z [x [[1] [2 3]] y x z (range 5)])))
+
+  (setv l [])
+  (defn f []
+    (for [x [4 9 2]]
+      (.append l (* 10 x))
+      (yield x)))
+  (for [_ (f)])
+  (assert (= l [40 90 20])))
 
 
 (defn test-nasty-for-nesting []