On Tue, 2009-12-15 at 09:52 -0800, Johann Höchtl wrote:
Hello,
I'm still to Haskell, and after I read through http://users.aber.ac.uk/afc/stricthaskell.html#seq
I thought, that these tow fragments after term rewriting are really the same:
myLength :: [a] -> Integer myLength xs = len xs 0 where len [] l = l len (x:xs) l = l `seq` len xs (l+1)
main = print $ myLength [1..10000000]
-- vs.
myLength :: [a] -> Integer myLength xs = len xs 0 where len [] l = l len (x:xs) l = len xs $! (l+1)
main = print $ myLength [1..10000000]
main = print $ myLength [1..10000000]
But the first expression evaluates more then twice as fast as the second one. Tested on GHC 6.10.4 and Windows XP, dual core (for what it's worth)
It's on http://moonpatio.com/fastcgi/hpaste.fcgi/view?id=5321#a5321 btw.
I can't see the difference, especially as $! is expressed in terms of seq
The second one is IMGO: myLength :: [a] -> Integer myLength xs = len xs 0 where len [] l = l len (x:xs) l = let l' = l+1 in l' `seq` len xs l' So in thew first + is not forced to be evaluated. My results (ghc 6.12.1, Core 2 Duo 2.8 GHz, Linux 2.6.32, Gentoo): Not Optimized & not compiled: First: 12.47 secs, 1530911440 bytes Second: 17.40 secs, 1929614816 bytes Optimized & compiled: First: 1.24 secs, 966280832 bytes Second: 1.11 secs, 966277152 bytes Repeating gave similar results - first being better w/out optimalization as 1.2:1.7 and second being better with optimalizations (-O). Why the first one is better unoptimalized? Regards