Re: Proposal: Control.Concurrent.Async

On 15/06/12 10:07, Simon Marlow wrote:

On 14/06/2012 13:12, Sjoerd Visscher wrote:

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On Thu, 14 Jun 2012 12:42:51 0100, Simon Marlow wrote: On 13/06/2012 22:58, Sjoerd Visscher wrote:

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

More code here: https://gist.github.com/2926572

I'm not sure about this. What you get with the above code is a strange nesting of concurrently calls, whereas what the user might expect is for it to behave like the existing concurrently but on 3-tuples instead of pairs.

Actually, that is what I expected too. So, this is not the way to use concurrently?

concurrently3 m1 m2 m3 = (((v1, v2), v3) -> (v1, v2, v3)) <$> concurrently (concurrently m1 m2) m3

So what bothered me about this is that it makes 4 threads when I would expect 3, and also the asymmetry looks strange. However see below.

For your enjoyment / horror, I present my implementation of Concurrently that uses a single MVar, and exactly one thread per concurrent action. The code is at https://gist.github.com/2938456 . It uses an MVar (IO (Maybe a)), i.e. each worker thread can post a new action that can be done to combine the results; and in the end you hopefully get a value of type a. I'm sure that the code could be improved a lot. Twan