a slight modification to compile it :
change:
where sumFile = sum . map read . L.words
to :
where sumFile = sum . map (read . L.unpack) . L.words
but it's actually _slower_ than the non-bytestring version.
i did a little test, three versions of the same script and manufactured
meself a ~50 MB file containing 1M of ints 0-65535. and replaced the sum
with length for obvious reasons.
module Main where
import qualified Data.ByteString.Lazy.Char8 as L
main1 = do
contents <- L.getContents
print (sumFile contents)
where sumFile = length . map L.readInt . L.words
main2 = do
contents <- getContents
print (sumFile contents)
where sumFile = length . map (read :: String -> Int) . words
main3 = do
contents <- L.getContents
print (sumFile contents)
where sumFile = length . map ((read :: String -> Int) .
L.unpack) . L.words
time main3 < nums
real 0m22.421s
user 0m0.031s
sys 0m0.000s
time main2 < nums
real 0m14.296s
user 0m0.015s
sys 0m0.016s
time main1 < nums
real 0m22.078s
user 0m0.015s
sys 0m0.015s
i expected the conversions (L.unpack in main3) to kill the performance a
little, but not to make it nearly two times as slow.
and i certainly did not expect that even the version using the bytestring
readInt to be as slow ...
did I do something wrong ?
On Tue, Oct 7, 2008 at 4:06 AM, Mike Coleman
Hi,
I could use a little help. I was looking through the Real World Haskell book and came across a trivial program for summing numbers in a file. They mentioned that that implementation was very slow, as it's based on String's, so I thought I'd try my hand at converting it to use lazy ByteString's. I've made some progress, but now I'm a little stuck because that module doesn't seem to have a 'read' method.
There's a readInt method, which I guess I could use, but it returns a Maybe, and I don't see how I can easily strip that off.
So:
1. Is there an easy way to strip off the Maybe that would allow an equivalently concise definition for sumFile? I can probably figure out how to do it with pattern matching and a separate function--I'm just wondering if there's a more concise way.
2. Why doesn't ByteString implement 'read'? Is it just that this function (like 'input' in Python) isn't really very useful for real programs?
3. Why doesn't ByteString implement 'readDouble', etc.? That is, why are Int and Integer treated specially? Do I not need readDouble?
Thanks, Mike
-- lazy version (doesn't compile)
-- file: ch08/SumFile.hs
import qualified Data.ByteString.Lazy.Char8 as L
main = do contents <- L.getContents print (sumFile contents) where sumFile = sum . map read . L.words _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe