[Haskell-cafe] Why Haskell?
bulat.ziganshin at gmail.com
Sun Jul 23 08:15:28 EDT 2006
Sunday, July 23, 2006, 10:35:41 AM, you wrote:
>> sequence $ [ reffill b s | s <- [0..(fi temits)-1], b <- [0..(fi
> Now thats interesting. I can see that this function is more appropriate
> since I do not need to retrieve data from the IO monad,
> but what I don't understand is why it's actually faster. I will give it
> a try and test it on a large set to see if things change.
let's see at their (possible) definitions:
sequence  = return 
sequence (action:actions) = do x <- action
xs <- sequence actions
sequence_  = return ()
sequence_ (action:actions) = do action
sequence_ is TAIL-RECURSIVE function. moreover, when it's inlined, the
result is what just all the action in list are sequentially performed
without creating any intermediate data structures. so, using sequence_
in above example is equivalent to implementing this cycle by hand.
but when you use sequence, result of each action is saved and list
of all results is built. this list requires 12 bytes per element, so
you got 600 mb of garbage and much slower execution
Bulat mailto:Bulat.Ziganshin at gmail.com
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