[Haskell-cafe] Questions about concurrency and laziness

[Roman Cheplyaka]

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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