timtoorop at quicknet.nl
Sat Apr 8 10:35:05 EDT 2006
Tim Toorop wrote:
> Sebastian Sylvan wrote:
>> On 4/7/06, Spencer Janssen <spencerjanssen at gmail.com> wrote:
>>> Earlier today on the #haskell IRC channel, Tim Toorop (bolrod on
>>> #haskell) pointed out that Data.List.inits is rather slow, and
>>> proposed an alternative. After some collabrative tweaking, we came up
>>> with the following:
>>>> inits xs =  : (zipWith take [1..] $ map (const xs) xs)
>>> This function seems to perform significantly better. For example, the
>>> program below takes about 15 seconds with the old inits, and only 3
>>> seconds with the new version (tested with GHC 6.4.1 and -O2).
>>>> main = print $ sum $ map sum $ inits [1..7000]
>>> As this version performs much better and will work as a drop in
>>> replacement, I suggest that it be included in the hierarchical
>> That's quite a bit faster on my machine as well. I think the following
>> slight variation may be a bit clearer, though:
>> inits xs =  : zipWith take [1..length xs] (repeat xs)
>> Sebastian Sylvan
>> UIN: 44640862
>> Libraries mailing list
>> Libraries at haskell.org
> The length xs wont work on infinite lists.
> So you can't do :
> take 100 (length (inits [1..]))
> Tim Toorop
> Libraries mailing list
> Libraries at haskell.org
Hmm.. that won't work anyway.
It should have been: take 100 (inits [1..]) , ofcourse.
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