[Haskell-cafe] Why Haskell?

Bulat Ziganshin bulat.ziganshin at gmail.com
Sun Jul 23 08:15:28 EDT 2006

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

Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com

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