Re: inits

Sebastian Sylvan wrote:
> On 4/7/06, Spencer Janssen <spencerjanssen@xxxxxxxxx> 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
> > libraries.
> >
> >     
>
> 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
> +46(0)736-818655
> UIN: 44640862
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>   
The length xs wont work on infinite lists.
So you can't do :
  take 100 (length (inits [1..]))

--
Tim Toorop