[Haskell-cafe] Re[2]: Why Haskell?

[SevenThunders]

Bulat Ziganshin-2 wrote:
> 
> Hello Matthew,
> 
> Sunday, July 23, 2006, 10:35:41 AM, you wrote:
> 
>  >>     sequence $ [ reffill b s | s <- [0..(fi temits)-1], b <- [0..(fi
>> nc)-1]]
> 
>> 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
>                                return (x:xs)
>                                
> sequence_ [] = return ()
> sequence_ (action:actions) = do action
>                                 sequence_ actions
> 
> 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
> 
> 
> Thanks Bulat, that was concise and explains it well.  Now the question is
> what do I do if I do have data building up in the monad.  ie if I want to
> generate a large list of doubles in the IO monad? Moreover those doubles
> depend in some way on data wrapped up or generated in the monad. It seems
> that building a list of IO actions in advance is a bad idea.  
> 
> Perhaps the implementaton with the tail call will work in this case if an
> extra accumulator argument is added.
> 
> -- 
> Best regards,
>  Bulat                            mailto:Bulat.Ziganshin at gmail.com
> 
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