Revert "yieldAsync() (#120)"

This reverts commit 284d6778155fa956a773613b56b32b2522365849.

There is no rough consensus for how this should be implemented and
whether it's needed indeed - a use case that cannot be solved with
a queue or a lock/event should be identified before pursuing this
functionality.

README.md

@@ -242,53 +242,6 @@ other specific exception, in order to avoid catching by mistake
 `CancelledError` (object of `Exception`, used internally to propagate
 cancellation).
 
-### Yielding
-
-If you need to give control back to the event loop, but there's no future you
-need to `await` at that point, you can call `yieldAsync()` - a simple wrapper
-around `sleepAsync()`:
-
-```nim
-template yieldAsync*() =
-  await sleepAsync(0.seconds)
-```
-
-This comes in handy when splitting a blocking task into smaller parts:
-
-```nim
-proc showMsg(after: int) {.async.} =
-  echo "begin showMsg()"
-  await sleepAsync(after.seconds)
-  echo "msg: after = ", after
-
-let maxDuration = 10
-
-proc heavy() {.async.} =
-  echo "begin showMsg(", after, ")"
-  let start = Moment.now()
-  var s: uint64
-  while true:
-    s += 1
-
-    # try commenting this out and see what happens
-    if s mod 1000000 == 0:
-      yieldAsync()
-
-    if (Moment.now() - start).seconds >= (maxDuration + 1):
-      break
-  echo "sum: ", s
-
-proc p1() {.async.} =
-  var futures: seq[Future[void]]
-
-  for after in 1..maxDuration:
-    futures.add(showMsg(after))
-  futures.add(heavy())
-  await allFutures(futures)
-
-waitFor p1()
-```
-
 ## TODO
   * Pipe/Subprocess Transports.
   * Multithreading Stream/Datagram servers

chronos/asyncloop.nim

@@ -891,10 +891,6 @@ proc sleepAsync*(ms: int): Future[void] {.
      inline, deprecated: "Use sleepAsync(Duration)".} =
   result = sleepAsync(ms.milliseconds())
 
-# This can't be an async proc, because its `await` needs to modify the caller.
-template yieldAsync*() =
-  await sleepAsync(0.seconds)
-
 proc withTimeout*[T](fut: Future[T], timeout: Duration): Future[bool] =
   ## Returns a future which will complete once ``fut`` completes or after
   ## ``timeout`` milliseconds has elapsed.

tests/testfut.nim

@@ -969,38 +969,6 @@ suite "Future[T] behavior test suite":
   proc testCancellationRace(): bool =
     waitFor(testCancellationRaceAsync())
 
-  proc testYieldAsync(): Future[bool] {.async.} =
-    proc showMsg(after: int, results: ptr seq[string]) {.async.} =
-      await sleepAsync(after.seconds)
-      results[].add("after " & $after)
-
-    let maxDuration = 2
-
-    proc heavy(results: ptr seq[string]) {.async.} =
-      let start = Moment.now()
-      var s: uint64
-      while true:
-        s += 1
-        if s mod 1000000 == 0:
-          yieldAsync()
-        if (Moment.now() - start).seconds >= (maxDuration + 1):
-          break
-      results[].add("heavy")
-
-    var
-      futures: seq[Future[void]]
-      results: seq[string]
-
-    for after in 1..maxDuration:
-      futures.add(showMsg(after, results.addr))
-    futures.add(heavy(results.addr))
-    await allFutures(futures)
-
-    return results == @["after 1", "after 2", "heavy"]
-
-  proc testYield(): bool =
-    waitFor(testYieldAsync())
-
   test "Async undefined behavior (#7758) test":
     check test1() == true
   test "Immediately completed asynchronous procedure test":
@@ -1054,6 +1022,3 @@ suite "Future[T] behavior test suite":
     check testWithTimeout() == true
   test "Cancellation race test":
     check testCancellationRace() == true
-  test "Yielding test":
-    check testYield() == true
-