On Thu, 14 Jun 2012 12:42:51 0100, Simon Marlow wrote: On 13/06/2012 22:58, Sjoerd Visscher wrote:  > > (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 Would this be the correct implementation then: concurrently3 m1 m2 m3 =   withAsync m1 $ \a1 ->   withAsync m2 $ \a2 ->   withAsync m3 $ \a3 ->   wait3 a1 a2 a3 And can I then write wait3 as wait3 a1 a2 a3 = (((v1, v2), v3) -> (v1, v2, v3)) <$> do   withAsync (waitBoth a1 a2) $ \a12 -> waitBoth a12 a3 or does it have to be: wait3 a1 a2 a3 =   atomically $ do     v1 <- waitSTM a1 `orElse` (waitSTM a2 >> waitSTM a3 >> retry)     v2 <- waitSTM a2 `orElse` (waitSTM a3 >> retry)     v3 <- waitSTM a3     return (v1, v2, v3) If it is this complicated, then (properly written) Applicative and Alternative instances are certainly useful.

I like the idea of generalising to arbitrary Traversable structures though. Maybe we could provide something like

doConcurrently :: Traversable t => t (IO a) -> IO (t a)

I haven't tried to write it, but it looks like it ought to be possible.

You'll need an Applicative instance for this anyway. -- Sjoerd Visscher https://github.com/sjoerdvisscher/blog