Support async for and async with constructs using macros (#1226)

Fixes #1179
Fixes #1181

Author: chrisrink10

CHANGELOG.md

@@ -5,6 +5,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+ * Added `afor` and `awith` macros to support async Python interop (#1179, #1181)
+
 ### Changed
  * Single arity functions can be tagged with `^:allow-unsafe-names` to preserve their parameter names (#1212)
 

src/basilisp/core.lpy

@@ -4085,6 +4085,63 @@
        (finally
          (println (* 1000 (- (perf-counter) start#)) "msecs")))))
 
+;;;;;;;;;;;;;;;;;;
+;; Async Macros ;;
+;;;;;;;;;;;;;;;;;;
+
+(defmacro afor
+  "Repeatedly execute ``body`` while the binding name is repeatedly rebound to
+  successive values from the asynchronous iterable.
+
+  .. warning::
+
+     The ``afor`` macro may only be used in an asynchronous function context."
+  [binding & body]
+  (if (operator/ne 2 (count binding))
+    (throw
+     (ex-info "bindings take the form [name iter]"
+              {:bindings binding}))
+    nil)
+  (let [bound-name (first binding)
+        iter       (second binding)]
+    `(let [it# (. ~iter ~'__aiter__)]
+       (loop [val# nil]
+         (try
+           (let [~bound-name (await (. it# ~'__anext__))
+                 result#     (do ~@body)]
+             (recur result#))
+           (catch python/StopAsyncIteration _
+             val#))))))
+
+(defmacro awith
+  "Evaluate ``body`` within a ``try`` / ``except`` expression, binding the named
+  expressions as per Python's async context manager protocol spec (Python's
+  ``async with`` blocks).
+
+  .. warning::
+
+     The ``awith`` macro may only be used in an asynchronous function context."
+  [bindings & body]
+  (let [binding (first bindings)
+        expr    (second bindings)]
+    `(let [obj#        ~expr
+           ~binding    (await (. obj# ~'__aenter__))
+           hit-except# (volatile! false)]
+       (try
+         (let [res# ~@(if (nthnext bindings 2)
+                        [(concat
+                          (list 'awith (vec (nthrest bindings 2)))
+                          body)]
+                        (list (concat '(do) body)))]
+           res#)
+         (catch python/Exception e#
+           (vreset! hit-except# true)
+           (when-not (await (. obj# (~'__aexit__ (python/type e#) e# (.- e# ~'__traceback__))))
+             (throw e#)))
+         (finally
+           (when-not @hit-except#
+             (await (. obj# (~'__aexit__ nil nil nil)))))))))
+
 ;;;;;;;;;;;;;;;;;;;;;;
 ;; Threading Macros ;;
 ;;;;;;;;;;;;;;;;;;;;;;

tests/basilisp/test_core_async_macros.lpy

@@ -0,0 +1,37 @@
+(ns tests.basilisp.test-core-async-macros
+  (:import asyncio contextlib)
+  (:require
+   [basilisp.test :refer [deftest is are testing]]))
+
+(defn async-to-sync
+  [f & args]
+  (let [loop (asyncio/new-event-loop)]
+    (asyncio/set-event-loop loop)
+    (.run-until-complete loop (apply f args))))
+
+(deftest awith-test
+  (testing "base case"
+    (let [get-val    (contextlib/asynccontextmanager
+                      (fn ^:async get-val
+                        []
+                        (yield :async-val)))
+          val-ctxmgr (fn ^:async yield-val
+                       []
+                       (awith [v (get-val)]
+                              v))]
+      (is (= :async-val (async-to-sync val-ctxmgr))))))
+
+(deftest afor-test
+  (testing "base case"
+    (let [get-vals (fn ^:async get-vals
+                     []
+                     (dotimes [n 5]
+                       (yield n)))
+          val-loop (fn ^:async val-loop
+                     []
+                     (let [a   (atom [])
+                           res (afor [v (get-vals)]
+                                     (swap! a conj v)
+                                     v)]
+                       [@a res]))]
+      (is (= [[0 1 2 3 4] 4] (async-to-sync val-loop))))))