How to terminate loop in async block?

Question

I'm use Tomas' BlockingQueueAgent and created an F# console program.

https://github.com/tpetricek/FSharp.AsyncExtensions/blob/master/src/Agents/BlockingQueueAgent.fs

I have the following code. However, the program never end. How to exit the loop in the consumer?

let producer() = 
    let addLinks = async {
        for url in links do
            do! ag.AsyncAdd(Some (url))
            printfn "Producing %s" url }
    async { do! addLinks
            do! ag.AsyncAdd(None) }

let consumer() = async {
    while true do 
        let! item = ag.AsyncGet()
        match item with 
        | Some (url) ->
            printfn "Consuming  %s" url
            ....
        | None -> 
            printfn "Done" } // How to exit the loop from here?

producer() |> Async.Start
consumer() |> Async.RunSynchronously

Use recursion instead of a loop.

ildjarn
– ildjarn

2013-10-28 21:15:05 +00:00
Commented Oct 28, 2013 at 21:15 — ildjarn
– ildjarn, Commented Oct 28, 2013 at 21:15

kvb · Accepted Answer · 2013-10-28 21:18:09Z

4

As ildjarn suggests, use recursion instead of a loop:

let rec consumer() = async {
    let! item = ag.AsyncGet()
    match item with
    | Some(url) ->
        printfn "Consuming %s" url
        ...
        return! consumer() // recursive call only in this case
    | None -> 
        printfn "Done" }

answered Oct 28, 2013 at 21:18

kvb

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2 Comments

ca9163d9 Over a year ago

Curiously, is there any way to do it without using recursive function?

kvb Over a year ago

@dc7a9163d9 - sure - use a ref cell's value as the guard condition for the while loop and set it to false when you hit a None.

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