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