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

[Artem Pelenitsyn]

That makes sense. Thanks a lot, Patrick!

On Thu, Mar 19, 2020, 11:01 PM Patrick Chilton <chpatrick at gmail.com> wrote:

> 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 <a.pelenitsyn at gmail.com>
> 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 <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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>>
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