Implement the of macro

hy/core/macros.hy

@@ -139,6 +139,20 @@ the second form, the second result is inserted into the third form, and so on."
   ret)
 
 
+(defmacro of [base &rest args]
+  "Shorthand for indexing for type annotations.
+
+If only one arguments are given, this expands to just that argument. If two arguments are
+given, it expands to indexing the first argument via the second. Otherwise, the first argument
+is indexed using a tuple of the rest.
+
+E.g. `(of List int)` -> `List[int]`, `(of Dict str str)` -> `Dict[str, str]`."
+  (if
+    (empty? args) base
+    (= (len args) 1) `(get ~base ~@args)
+    `(get ~base (, ~@args))))
+
+
 (defmacro if-not [test not-branch &optional yes-branch]
   "Like `if`, but execute the first branch when the test fails"
   `(if* (not ~test) ~not-branch ~yes-branch))

tests/native_tests/py36_only_tests.hy

@@ -6,7 +6,7 @@
 ;; conftest.py skips this file when running on Python <3.6.
 
 (import [asyncio [get-event-loop sleep]])
-(import [typing [get-type-hints]])
+(import [typing [get-type-hints List Dict]])
 
 
 (defn run-coroutine [coro]
@@ -48,6 +48,11 @@
   (assert (= (get annotations "x") int))
   (assert (= (get annotations "z") bool)))
 
+(defn test-of []
+  (assert (= (of str) str))
+  (assert (= (of List int) (get List int)))
+  (assert (= (of Dict str str) (get Dict (, str str)))))
+
 (defn test-pep-487 []
   (defclass QuestBase []
     (defn --init-subclass-- [cls swallow &kwargs kwargs]