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9134 :: Async proves to be a bit tricky

• archnim (orginal) [2022-05-02T15:18:55+02:00] view original

  Hello world. I found out that, inside an async proc returning a Future[T], the type of the result is not Future[T], but T. For example, I can't write result.complete(54), but result = 54.

  Now here is a simple code:

  import asyncdispatch
  
  proc cb(fut: Future[int]) =
    if fut.failed:
      echo "😭😭😭"
    else:
      echo "😀😀😀"
  
  proc myProc(): Future[int] {.async.} =
    # Many ops here...
    return 43
  
  var fut = myProc()
  fut.callback = cb
  Run

  How can I say, inside myProc that, if a certain condition is met, then the returned Future must fail ? I can't write result.fail.

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• PMunch (orginal) [2022-05-02T15:23:05+02:00] view original

  You simply throw an exception. Here is a modify version of your code which prints the crying faces:

  import asyncdispatch
  
  proc cb(fut: Future[int]) =
    if fut.failed:
      echo "😭😭😭"
    else:
      echo "😀😀😀"
  
  proc myProc(): Future[int] {.async.} =
    # Many ops here...
    return 43
  
  var fut = myProc()
  fut.callback = cb
  Run

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• archnim (orginal) [2022-05-02T15:33:26+02:00] view original

  OK. Thank you. Please, why can I not handle the result as a Future inside the proc ?

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• PMunch (orginal) [2022-05-02T18:13:05+02:00] view original

  That's because of the way that the async macro rewrites the procedure into a closure iterator and a procedure that registers said closure iterator with the async runtime. It is possible to write your own async procedures and deal with all this yourself, although this isn't really documented anywhere.. Why exactly do you want to mess with the Future object yourself?

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• archnim (orginal) [2022-05-02T18:33:27+02:00] view original

  I just find it more intuitive that the result inside the proc has the same type than outside. But as you said, that would just be a messy complication. Thanks for the good explanation.

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