Am Mittwoch 17 Juni 2009 20:25:08 schrieb Nico Rolle:

Hi everyone. I made parallel versions of my functions projection selection and sort and i wanted to test the difference in runtime. But he only printed 0.00 twice. Maybe hes just too fast? Or is it because of lazy evaluation? or is there a better way to mesure performance in execution time? regards

main = do xs <- readCSV "dataconvert/lineitem.tbl" '|' start <- getCurrentTime let pnp = projection [5] xs let snp = selection (\x -> (x!!0) > (Int 17000)) pnp let sortnp = sort [0] [ASC] snp end <- getCurrentTime putStrLn $ show (end `diffUTCTime` start) start2 <- getCurrentTime let pp = pProjection [5] xs let sp = pSelection (\x -> (x!!0) > (Int 17000)) pp let sortp = pSort [0] [ASC] sp end2 <- getCurrentTime putStrLn $ show (end2 `diffUTCTime` start2) return snp

The let bindings do not cause any computation because of laziness, all they do is bind the names to a thunk that says how to calculate the value if it is needed. Regardless of how long the computation would take, binding the names to a thunk takes only a few nanoseconds. To measure the time needed for the computations, you must force them to be carried out between the two calls to getCurrentTime. Probably inserting print $ last snp before end <- getCurrentTime (and likewise for the second) would be enough (if the sorting doesn't require the values in snp to be fully evaluated, you would have to do more forcing).