Re: [Haskell-cafe] Help with async library and killing child processes

Is there a way to get the result of "either of three calls to getUrl" as opposed to "all three calls"? 'Cause that's what Mike seems to want.

That's what the Alternative instance of Concurrently is for. The result of `doA <|> doB` is the result of the argument that finished first. (<|>) = race. firstResult <- runConcurrently (Concurrently (getURL "url1") <|> Concurrently (getURL "url2") <|> Concurrently (getURL "url3")) or firstResult <- runConcurrently $ asum $ map (Concurrently . getURL) [ "url1", "url2", "url3" ] On Fri, Mar 20, 2020 at 9:44 AM Artem Pelenitsyn wrote:

Hey Niklas,

Thanks for your explanation -- it was useful to read.

Is there a way to get the result of "either of three calls to getUrl" as opposed to "all three calls"? 'Cause that's what Mike seems to want.

-- Best regards, Artem

On Thu, 19 Mar 2020 at 21:38, Niklas Hambüchen wrote:

...

Hey Mike,

...

f <- async getPages cancel f

you haven't read the PR I linked carefully enough yet :)

It essentially says to never use the `async` function, as that is almost never what you want

Full docs, with what you need to read highlighted:

https://github.com/nh2/async/blob/1d96dc555b70a4c5aba07f15d0c7895545eb8582/C...

...

Am I guaranteed the the getURL() calls will definitely have either finished, or cancel?

For

do (page1, page2, page3) <- runConcurrently $ (,,) <$> Concurrently (getURL "url1") <*> Concurrently (getURL "url2") <*> Concurrently (getURL "url3")

you are guaranteed that when `runConcurrently` returns, all 3 downloads have finished successfully.

Only the `instance Alternative Concurrently` (that is, when you use <|>) is implemented in terms of `race`.

You are using <*>, as implemented in the `instance Applicative Concurrently`, which uses `concurrently`, which waits for both of two actions.

Your code in

...

main = do f <- async getPages cancel f

does not make much sense: Here you're starting a thread that would go off do something, but then you immediately cancel that thread, so it won't achieve anything.

You do not need to wrap things into additional `async`s. You can directly do, for example:

main :: IO () main = do (page1, page2, page3) <- runConcurrently $ (,,) <$> Concurrently (getURL "url1") <*> Concurrently (getURL "url2") <*> Concurrently (getURL "url3") putStrLn "Here are the page contents:" putStrLn page1 putStrLn page2 putStrLn page3

Hope that helps!

Niklas _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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