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7842 :: Let's Make Async Great Again!

• alexeypetrushin (orginal) [2021-04-23T14:58:58+02:00] view original

  Seems like we have to live with async, let's think if we can improve how it's used. I used Nim async and it looks to me, compared to the experience of Async in JavaScript, there are couple of things could be improved.

  1 Use same await instead of wait_for if async needs to be called in sync function.

  2 No sync IO instead of async proc allowed, mark all non-async IO with syncio and throw error if it's used in async proc. It is ok to block async event loop with CPU, it terrible if it's blocked with sync-IO.

  3 Allow implicit return from async functions.

  4 Allow to return future from async function.

  5 Rename async_check as spawn(Future) or check(Future).

  6 Remove run_forewer, if there's at least one listener registered on the event loop keep process running.

  7 Don't print ugly multi-page async error stack traces, if it can't be made short and clean it's better to not print it at all, as they are usually useless anyway.

  8 with_timeout should throw error or return Future[T], not Future[bool].

  9 Maybe Stop using {.async.} the code is noisy, if function returns Future it's async.

  What do you think? If we have to live with it, let's make it great! :)

  Examples

  None of it works right now:

  a)

  proc a: Future[int] = 10
  Run

  b)

  proc a: Future[int] {.async.} = 10
  Run

  c)

  proc a: Future[int] {.async.} = return 10
  
  proc b: Future[int] {.async.} = a()
  Run

  d)

  proc somefn1: Future[int] {.async.} = return 10
  
  proc somefn2: Future[int] {.async.} = return somefn1()
  Run

  5. How the async error message looks like

  
  nim c -r nodem/greeting_example/user.nim
  /alex/projects/nim/nodem/greeting_example/user.nim(14) user
  /alex/projects/nim/nodem/nodem.nim(359) run
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1935) waitFor
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1627) poll
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1368) runOnce
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(208) processPendingCallbacks
  /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(29) callNimAsyncContinue
  /alex/projects/nim/nodem/nodem/netm.nim(120) callIter
  [[reraised from:
  /alex/projects/nim/nodem/greeting_example/user.nim(14) user
  /alex/projects/nim/nodem/nodem.nim(359) run
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1935) waitFor
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1627) poll
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1368) runOnce
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(208) processPendingCallbacks
  /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(29) call_nimported_functionNimAsyncContinue
  /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(145) call_nimported_functionIter
  /balex/applications/nim-1.4.2/lib/pure/asyncfutures.nim(372) read
  ]]
  [[reraised from:
  /alex/projects/nim/nodem/greeting_example/user.nim(14) user
  /alex/projects/nim/nodem/nodem.nim(359) run
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1935) waitFor
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1627) poll
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1368) runOnce
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(208) processPendingCallbacks
  /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(29) call_nimported_functionNimAsyncContinue
  /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(145) call_nimported_functionIter
  /balex/applications/nim-1.4.2/lib/pure/asyncfutures.nim(372) read
  ]]
  [[reraised from:
  /alex/projects/nim/nodem/greeting_example/user.nim(14) user
  /alex/projects/nim/nodem/nodem.nim(359) run
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1935) waitFor
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1627) poll
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1368) runOnce
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(208) processPendingCallbacks
  /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(29) say_hiNimAsyncContinue
  /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(145) say_hiIter
  /balex/applications/nim-1.4.2/lib/pure/asyncfutures.nim(372) read
  ]]
  [[reraised from:
  /alex/projects/nim/nodem/greeting_example/user.nim(14) user
  /alex/projects/nim/nodem/nodem.nim(359) run
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1935) waitFor
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1627) poll
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1368) runOnce
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(208) processPendingCallbacks
  /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(29) selfNimAsyncContinue
  /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(145) selfIter
  /balex/applications/nim-1.4.2/lib/pure/asyncfutures.nim(372) read
  ]]
  [[reraised from:
  /alex/projects/nim/nodem/greeting_example/user.nim(14) user
  /alex/projects/nim/nodem/nodem.nim(359) run
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1935) waitFor
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1627) poll
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1368) runOnce
  /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(208) processPendingCallbacks
  /balex/applications/nim-1.4.2/lib/pure/asyncfutures.nim(420) asyncCheckCallback
  ]]
  Error: unhandled exception: Timed out, connection closed
  Async traceback:
    /alex/projects/nim/nodem/greeting_example/user.nim(14)         user
    /alex/projects/nim/nodem/nodem.nim(359)                        run
    /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1935) waitFor
    /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1627) poll
    /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1368) runOnce
    /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(208)  processPendingCallbacks
    /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(29)      callNimAsyncContinue
    /alex/projects/nim/nodem/nodem/netm.nim(120)                   callIter
    #[
      /alex/projects/nim/nodem/greeting_example/user.nim(14)         user
      /alex/projects/nim/nodem/nodem.nim(359)                        run
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1935) waitFor
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1627) poll
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1368) runOnce
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(208)  processPendingCallbacks
      /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(29)      call_nimported_functionNimAsyncContinue
      /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(145)     call_nimported_functionIter
      /balex/applications/nim-1.4.2/lib/pure/asyncfutures.nim(372)   read
    ]#
    #[
      /alex/projects/nim/nodem/greeting_example/user.nim(14)         user
      /alex/projects/nim/nodem/nodem.nim(359)                        run
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1935) waitFor
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1627) poll
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1368) runOnce
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(208)  processPendingCallbacks
      /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(29)      call_nimported_functionNimAsyncContinue
      /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(145)     call_nimported_functionIter
      /balex/applications/nim-1.4.2/lib/pure/asyncfutures.nim(372)   read
    ]#
    #[
      /alex/projects/nim/nodem/greeting_example/user.nim(14)         user
      /alex/projects/nim/nodem/nodem.nim(359)                        run
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1935) waitFor
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1627) poll
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1368) runOnce
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(208)  processPendingCallbacks
      /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(29)      say_hiNimAsyncContinue
      /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(145)     say_hiIter
      /balex/applications/nim-1.4.2/lib/pure/asyncfutures.nim(372)   read
    ]#
    #[
      /alex/projects/nim/nodem/greeting_example/user.nim(14)         user
      /alex/projects/nim/nodem/nodem.nim(359)                        run
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1935) waitFor
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1627) poll
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(1368) runOnce
      /balex/applications/nim-1.4.2/lib/pure/asyncdispatch.nim(208)  processPendingCallbacks
      /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(29)      selfNimAsyncContinue
      /balex/applications/nim-1.4.2/lib/pure/asyncmacro.nim(145)     selfIter
      /balex/applications/nim-1.4.2/lib/pure/asyncfutures.nim(372)   read
    ]#
  Exception message: Timed out, connection closed
  Exception type: [Exception]
  Error: execution of an external program failed: '/alex/projects/nim/build/user '
  Run

  P.S.

  Eventually though, I hope Nim deprecate async and use better approaches, async is too low level and designed for machines, not humans, working with async is like writting C by hands.

---

• Araq (orginal) [2021-04-23T15:11:44+02:00] view original

  The future lies in https://github.com/disruptek/cps and https://github.com/mratsim/weave . I've said it many times, if CPS requires compiler support to be really usable, so be it. Please join these efforts!

---

• PMunch (orginal) [2021-04-23T15:19:12+02:00] view original

  The thing with Nim async is that it is just a library just like any other. All the async stuff is just clever macro transformations into closure iterators and IO polling. As such it needs to live within the limits of that system, such as the {.async.} pragma which is what actually transforms the procedure body into something that would return a future. There are of course things that could be made more ergonomic, and I'm all for that, but some of the things you mention here is simply limitations of the macro system that we can't work around without baking the system into the compiler.

---

• alexeypetrushin (orginal) [2021-04-23T15:25:05+02:00] view original

  The future lies in https://github.com/disruptek/cps and https://github.com/mratsim/weave . I've said it many times, if CPS requires compiler support to be really usable, so be it. Please join these efforts!

  Yes, these are long-term strategic goals. My suggestions are more like short-term tactical improvements :)

---

• Araq (orginal) [2021-04-23T16:59:29+02:00] view original

  To clarify my remarks:

  Async means lots of things:

  • The event loop mechanism.
  • The async macro transformation which maps async to Nim's closure iterators.
  • The Future[T] design.

  CPS is an alternative to our closure iterators and all the rest could remain the same or be improved independently.

---

• xigoi (orginal) [2021-04-23T22:36:45+02:00] view original

  I thought Weave was for parallelism, not for concurrency?

---

• dom96 (orginal) [2021-04-24T22:24:46+02:00] view original

  Thanks for writing up this list!

  I agree with a lot of the items you've listed. In particular I think the following should be prioritised:

  5 Don't print multi-page async error stack traces, if it can't be made short and clean maybe it's better to trim it somehow.

  6 No sync IO instead of async proc allowed, mark all non-async IO with syncio and throw error if it's used in async proc. It is ok to block async event loop with CPU, it's terrible if it's blocked with sync-IO.

  If you have the time I'm always happy to help answer any questions about how to approach these and review any PRs for them. I'd love to see these improvements :)

---

• hugosenario (orginal) [2021-04-30T08:23:44+02:00] view original

  @alexeypetrushin did you researched for effect handlers?

  Good: remove de async pollution of procedural codes parts

  Bad: now you have not only blue/red function you all color spectrum

  Not sure how much far from continuations it is.

  I'd bet it is implemented using continuations but with name focused on developer interface instead of how is implemented.

  sources:

  https://www.stephendiehl.com/posts/exotic03.html

  https://koka-lang.github.io/koka/doc/book.html#why-handlers

  I'm almost sure that I saw some other content about this with some code like above

  Syntax approximation: Used raise, catch and continue to denote keywords and more or less how this flow work.

  type async = effect
  
  proc itWillCallBlockingStuff(): int =
      # ffi lib developers
      # we thrown some effect
      let value = raise async, calling_C_BlockingStuff()
      return value + 1
  
  proc usualOldStyleCode(): int =
      # it doesn't need to be polluted with async syntax
      # Hope most usual case
      let value = itWillCallBlockingStuff()
      return value + 1
  
  proc parallelizeStuff(): int =
      # only if you need some kind o optimization
      let effect1 = catch async, usualOldStyleStuff()
      let effect2 = catch async, usualOldStyleStuff()
      let values = raise async, all(effect1, effect2)
      # let value = raise async, first(effect1, effect2)
      return sum(values) + 1
  
  when isMainModule:
      # won't compile, unhandled effect: 'async' ...
      echo parallelizeStuff()
      
      # sync runner
      let process = catchAll async, parallelizeStuff()
      for v in process:
          let r = run(v)
          continue v, r
      echo process.result()
      
      # parallel implementation
      let process = catchAll async, parallelizeStuff()
      parallel:
          for v in process:
              let r = spawn run(v)
              continue v, r
      echo process.result()
  Run

---