Async proves to be a bit tricky

[archnim]

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`.