inits

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.


Spencer Janssen