[Haskell-cafe] Questions about concurrency and laziness

[Tony Garcia]

I see, so I might put a thunk in the MVar but the IO action that I wanted
to do in parallel would be executed when I get to the putMVar anyway.

OK, thank you!
Jose.

On Thu, 11 Jun 2015 at 12:44 Roman Cheplyaka <roma at ro-che.info> wrote:

> You may be confusing execution of IO actions with evaluation.
>
> Looking at the source of getURL, its result probably *will* be a thunk:
> one that computes "rspBody rsp" when evaluated.
>
> But why would that bother you?
>
> The IO effects of getURL, though, are a completely different beast.
> Unless you practice some dirty magic or use lazy IO, sequenced IO
> actions are executed in order. So, by the time execution reaches
> putMVar, the effects of getURL (ie, downloading) will have finished.
>
>
> On 11/06/15 14:35, Tony Garcia wrote:
> > Hi,
> >
> > First of all, apologies if this is not the right place to ask this
> question.
> >
> > I'm reading the book "Parallel and concurrent programming in Haskell"
> > and I'm having some issues reasoning about the impact of laziness.
> >
> > Looking at the first example in chapter 8 [1]
> >
> > main = do
> >   m1 <- newEmptyMVar
> >   m2 <- newEmptyMVar
> >
> >   forkIO $ do
> >     r <- getURL "http://www.wikipedia.org/wiki/Shovel"
> >     putMVar m1 r
> >
> >   forkIO $ do
> >     r <- getURL "http://www.wikipedia.org/wiki/Spade"
> >     putMVar m2 r
> >
> >   r1 <- takeMVar m1
> >   r2 <- takeMVar m2
> >   print (B.length r1, B.length r2)
> >
> > I don't understand why this function is not just putting an unevaluated
> > thunk in the MVar. Well, I assume that getURL is an eager function, but
> > looking at its code [2] or at the documentation of Network.Browser [3] I
> > don't see why...
> >
> > Am I looking at this wrong? Is there any rule of thumb to be used in
> > these cases? I have the impression that it's really easy to end up
> > creating thunks in parallel threads and evaluating them in the main
> one...
> >
> > Thanks,
> > Jose.
> >
> > [1] http://chimera.labs.oreilly.com/books/1230000000929/ch08.html
> > [2] https://github.com/simonmar/parconc-examples/blob/master/GetURL.hs
> > [3]
> >
> http://hackage.haskell.org/package/HTTP-4000.0.8/docs/Network-Browser.html
> >
> >
> >
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> >
>
>
>
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