[Haskell-cafe] Re: Can't seem to get `par` working appropriately with lists

[Ben Franksen]

Luke Andrew wrote:
>     main = do print $ zipWith (+) (fiblist2 37 `par` fiblist1 37)
> (fiblist2 37)
> 
> compilation & testing:
> 
>     luke at Sonata:~/mcls$ ghc -O2 -threaded --make test2
> 
>     luke at Sonata:~/mcls$ time ./test2 +RTS -N1
>     [2,2,4,6,10,16,26,42...    ...405774,18454930,29860704,48315634]
> 
>     real    0m15.294s
>     user    0m15.196s
>     sys     0m0.013s
> 
>     luke at Sonata:~/mcls$ time ./test2 +RTS -N2
>     [2,2,4,6,10,16,26,42...    ...405774,18454930,29860704,48315634]
> 
>     real    0m15.268s
>     user    0m15.169s
>     sys     0m0.013s

This is due to lazyness: 'fiblist2 37' does not evaluate the whole resulting
list, so the thread spawned by 'par' will almost immediately return.

Take a look at Control.Parallel.Strategies, the 'parList' combinator is
probably what you need here. To get a feel what this 'strategies' stuff is
all about, it is a good idea to read (or at least skim over) the
accompanying paper, just google for "Algorithm + Strategy = Parallelism".

Cheers
Ben