On Sun, Mar 15, 2009 at 7:35 PM, Will Ness
which is then just rewritable as:
myCycle xs = ys where ys = foldr (:) ys xs
or (arriving at the same result)
myCycle xs = foldr (:) (myCycle xs) xs
Note that, because 'ys' only has to be calculated once, GHC makes the most efficient code for the former one. In the latter 'myCycle xs' has to be calculated each time 'xs' runs empty. Here are some efficiency tests: (All programs are compiled with GHC with -O2. I excluded the first run of each program because of possible initialisation overhead.) ------------------------------------------------------------ module Main where import System.Environment (getArgs) myCycle1 xs = ys where ys = foldr (:) ys xs myCycle2 xs = foldr (:) (myCycle2 xs) xs myCycle3 xs = ys where ys = xs ++ ys main = do ns:ms:_ <- getArgs print $ sum $ take (read ns) (myCycle1 [1..read ms]) ------------------------------------------------------------ $ for i in 1 2 3 4 5 6; do time ./cycle1 30000000 1000; done; 15015000000 real 0m4.854s user 0m4.801s sys 0m0.017s 15015000000 real 0m3.921s user 0m3.870s sys 0m0.016s 15015000000 real 0m3.861s user 0m3.806s sys 0m0.023s 15015000000 real 0m3.857s user 0m3.806s sys 0m0.018s 15015000000 real 0m4.382s user 0m4.331s sys 0m0.022s 15015000000 real 0m3.976s user 0m3.923s sys 0m0.020s (3.870 + 3.806 + 3.806 + 4.331 + 3.923) / 5 = 3.9471999999999996 ------------------------------------------------------------ $ for i in 1 2 3 4 5 6; do time ./cycle2 30000000 1000; done; 15015000000 real 0m4.675s user 0m4.629s sys 0m0.016s 15015000000 real 0m5.076s user 0m5.024s sys 0m0.022s 15015000000 real 0m4.735s user 0m4.687s sys 0m0.015s 15015000000 real 0m4.727s user 0m4.676s sys 0m0.021s 15015000000 real 0m4.677s user 0m4.632s sys 0m0.018s 15015000000 real 0m5.023s user 0m4.971s sys 0m0.016s (5.024 + 4.687 + 4.676 + 4.632 + 4.971) / 5 = 4.798 ------------------------------------------------------------ $ for i in 1 2 3 4 5 6; do time ./cycle3 30000000 1000; done; 15015000000 real 0m4.150s user 0m4.102s sys 0m0.018s 15015000000 real 0m3.823s user 0m3.784s sys 0m0.014s 15015000000 real 0m4.918s user 0m4.863s sys 0m0.021s 15015000000 real 0m3.807s user 0m3.769s sys 0m0.015s 15015000000 real 0m4.129s user 0m4.082s sys 0m0.016s 15015000000 real 0m4.420s user 0m4.366s sys 0m0.021s (3.784 + 4.863 + 3.769 + 4.082 + 4.366) / 5 = 4.1728000000000005 ------------------------------------------------------------ Summary: cycle1: 3.94 cycle2: 4.79 cycle3: 4.17 regards, Bas