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What does the warning - which is suppressed by #nowarn "40" - actually mean?
f#async

The following code snippet generates a warning that can be suppressed by #nowarn "40".

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type Contoller() =
  let start =
    MailboxProcessor.Start
      (fun (inbox: MailboxProcessor<ControllerMsg>) ->
        let rec loop =
          async {
                        let! msg = inbox.Receive()
                        match msg with
                        | End _ -> ()
                        | Connected c ->
                            // do some thing specific
                            return! loop  //nowarn "40" warning here
                     }
                loop)

Here is the text of the warning:

This and other recursive references to the object(s) being defined will be checked for initialization-soundness at runtime through the use of a delayed reference. This is because you are defining one or more recursive objects, rather than recursive functions. This warning may be suppressed by using '#nowarn "40"' or '--nowarn:40'.

Can someone please explain what this means? If we suppress this warning, I assume there is some performance impact but don't know if it is significant.

.

You typically get this warning when defining a recursive value that refers to itself (e.g. inside some delayed lambda or workflow):

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let rec x = ... refer to x ...

and the compiler inserts a check to ensure that the value has been initialized before it is used. To get a sense of the injected runtime check, try something that fails, e.g.

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let rec x = (fun () -> x + 1)()  // exception at runtime
By on 6/4/2010 11:53 AM ()

thanks - i have seen code use something like 'loop()' instead of just 'loop' to define the recursive function and that does not cause this warning

By on 6/5/2010 4:02 PM ()

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