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

[Niklas Hambüchen]

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