Properly add yield-from, using python's real "yield from".

The yield-from that existed previously wasn't actually implementing the
full complexity of "yield from":

  http://legacy.python.org/dev/peps/pep-0380/#formal-semantics

... this includes passing along errors, and many other things.

Also removes the yield-from backport macro, since it does not seem
possible at present to conditionally build macros.

Thus, there is no longer yield-from on pre-python-3.3 systems.

Includes updated docs and tests to reflect all this.

docs/language/api.rst

@@ -1227,6 +1227,20 @@ infinite series without consuming infinite amount of memory.
     => (list-comp x [x (take 15 (random-numbers 1 50))])])
     [7, 41, 6, 22, 32, 17, 5, 38, 18, 38, 17, 14, 23, 23, 19]
 
+
+yield-from
+----------
+
+.. versionadded:: 0.9.13
+
+**PYTHON 3.3 AND UP ONLY!**
+
+`yield-from` is used to call a subgenerator.  This is useful if you
+want your coroutine to be able to delegate its processes to another
+coroutine, say if using something fancy like
+`asyncio <http://docs.python.org/3.4/library/asyncio.html>`_.
+
+
 .. _zipwith:
 
 zipwith

hy/compiler.py

@@ -1017,6 +1017,28 @@ class HyASTCompiler(object):
 
         return ret
 
+    @builds("yield_from")
+    @checkargs(max=1)
+    def compile_yield_from_expression(self, expr):
+        if not PY33:
+            raise HyCompileError(
+                "yield-from only supported in python 3.3+!")
+
+        expr.pop(0)
+        ret = Result(contains_yield=True)
+
+        value = None
+        if expr != []:
+            ret += self.compile(expr.pop(0))
+            value = ret.force_expr
+
+        ret += ast.YieldFrom(
+            value=value,
+            lineno=expr.start_line,
+            col_offset=expr.start_column)
+
+        return ret
+
     @builds("import")
     def compile_import_expression(self, expr):
         def _compile_import(expr, module, names=None, importer=ast.Import):

hy/core/macros.hy

@@ -27,8 +27,8 @@
 
 
 (import [hy.models.list [HyList]]
-        [hy.models.symbol [HySymbol]])
-
+        [hy.models.symbol [HySymbol]]
+        [hy._compat [PY33 PY34]])
 
 
 (defmacro for [args &rest body]
@@ -155,12 +155,6 @@
   `(if-not ~test (do ~@body)))
 
 
-(defmacro yield-from [iterable]
-  "Yield all the items from iterable"
-  (let [[x (gensym)]]
-  `(for* [~x ~iterable]
-     (yield ~x))))
-
 (defmacro with-gensyms [args &rest body]
   `(let ~(HyList (map (fn [x] `[~x (gensym '~x)]) args))
     ~@body))

tests/native_tests/native_macros.hy

@@ -1,3 +1,6 @@
+(import [hy._compat [PY33]])
+(import [hy.errors [HyCompileError]])
+
 (defmacro rev [&rest body]
   "Execute the `body` statements in reverse"
   (quasiquote (do (unquote-splice (list (reversed body))))))
@@ -99,11 +102,18 @@
 
 (defn test-yield-from []
   "NATIVE: testing yield from"
-  (defn yield-from-test []
-    (for* [i (range 3)]
-      (yield i))
-    (yield-from [1 2 3]))
-  (assert (= (list (yield-from-test)) [0 1 2 1 2 3])))
+  
+  (try
+   (eval
+    '(do (defn yield-from-test []
+           (for* [i (range 3)]
+             (yield i))
+           (yield-from [1 2 3]))
+         (assert (= (list (yield-from-test)) [0 1 2 1 2 3]))))
+   (catch [e HyCompileError]
+     ;; Yup, this should happen on non-Python3.3 thingies
+     (assert (not PY33)))
+   (else (assert PY33))))
 
 (defn test-if-python2 []
   (import sys)