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

[Artem Pelenitsyn]

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 <mail at nh2.me> 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/Control/Concurrent/Async.hs#L21-L143
>
> > 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
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