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MailboxProcessor and let!
f#compilerasync

Why is it that this works:

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let interactiveFunction maxI f f_zero =
	MailboxProcessor.Start(fun inbox ->
		let rec loop x i =
			async { 
			 let! tryMsg =
					if i < maxI then
						async {
								let! msg = inbox.TryReceive(timeout=0)
								return msg
						}
					else
						async {
								let! msg = inbox.Receive()
								return Some(msg)
						}

				match tryMsg with
						| None -> return! loop (f x) (i+1)
						| Some msg -> ()
				}

		loop f_zero 0)

But, this does not:

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let interactiveFunction maxI f f_zero =
	MailboxProcessor.Start(fun inbox ->
		let rec loop x i =
			async {
				let! tryMsg =
					if i < maxI then
						inbox.TryReceive(timeout=0)
					else
						let! msg = inbox.Receive()
						Some(msg)

				match tryMsg with
				| None -> return! loop (f x) (i+1)
				| Some msg -> ()
				}
		loop f_zero 0)

That copy-paste is tough to read. But basically, the compiler is complaining about ``let!'' inside of an if-else. I'm a little confused why that is so.

.

The reason is that the right-hand-side of a "let! ident=" is a normal expression, not a computation expression, so as in your original code, you need to introduce another async{} block to get back into an async workflow.

See below (third example) for I think maybe the best way to write it (similar to your first example).

Start a reply to this message with the 'Quote' button to see how to format code well on hubfs.

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// your orig
let interactiveFunction maxI f f_zero =
    MailboxProcessor.Start(fun inbox ->
    let rec loop x i =
        async { 
            let! tryMsg =
                if i < maxI then
                    async {
                        let! msg = inbox.TryReceive(timeout=0)
                        return msg
                    }
                else
                    async {
                        let! msg = inbox.Receive()
                        return Some(msg)
                    }
            match tryMsg with
            | None -> return! loop (f x) (i+1)
            | Some msg -> ()
        }
    loop f_zero 0) 

// your orig attempt
let interactiveFunction maxI f f_zero =
    MailboxProcessor.Start(fun inbox ->
    let rec loop x i =
        async {
            let! tryMsg =
                if i < maxI then
                    inbox.TryReceive(timeout=0)
                else
                    let! msg = inbox.Receive()  // does not compile
                    Some(msg)
            match tryMsg with
            | None -> return! loop (f x) (i+1)
            | Some msg -> ()
        }
    loop f_zero 0) 

// best?
let interactiveFunction maxI f f_zero =
    MailboxProcessor.Start(fun inbox ->
    let rec loop x i =
        async { 
            let! tryMsg =
                if i < maxI then
                    inbox.TryReceive(timeout=0)
                else
                    async {
                        let! msg = inbox.Receive()
                        return Some(msg)
                    }
            match tryMsg with
            | None -> return! loop (f x) (i+1)
            | Some msg -> ()
        }
    loop f_zero 0)
By on 7/20/2009 3:37 PM ()

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