[Haskell-cafe] slow code

brian briand at aracnet.com
Sun Jun 14 14:44:19 EDT 2009

Haskell Gurus,

I have tried to use profiling to tell me what's going on here, but it  
hasn't helped much, probably because I'm not interpreting the results  

Empirically I have determined that the show's are pretty slow, so an  
alternative to them would be helpful.  I replaced the show's with "",  
and compiled with -O2 and not much improvement.

I need to write _a lot_ of code in this style.  A few words about how  
best to do this would be helpful.  Laziness, infinite lists, uvector ??




import Complex
import System.IO

genData :: Double -> Int -> (Double -> Complex Double) -> ([Double],  
[Complex Double])
genData tstop n f =
     let deltat = tstop / (fromIntegral n)
         t = [ fromIntegral(i) * deltat | i <- [0..n-1]]
       (t, map f t)

main =
     do let (t, y) = genData 100.0E-6 (2 ^ 15) (\x -> x :+ 0.0)
        h <- openFile "data.txt" WriteMode
        mapM_ (\(x, y) ->
                   do hPutStr h (show t)
                      hPutStr h " "
                      hPutStrLn h (show (realPart y)))
                  (zip t y)
        hClose h

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