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wman | 7 Oct 08:34

Re: newbie questions (read, etc., with Data.ByteString.Lazy.Char8)

as usual, i forgot to use the magical "reply to all" and under the impression I'm still talking to the list had bothered dons personally (heresy/sacrilege/deathwish , i know)
to rectify it a bit at least, i'm posting a summary, in hopes someone find it useful.
----------------------------
-- this doesn't work - space leak no matter which optimizations you use, with -O2 it crashes in 5 secs instead in 75 ;-)
import qualified Data.ByteString.Lazy.Char8 as S
main = print . go 0 =<< S.getContents
  where
    go n s = case S.readInt s of
               Nothing     -> n
               Just (k,t)  -> go (n+k) (S.tail t)

-- slightly stricter version, notice those ! before parameters in go declaration and the fancy -XBangPatterns option to ghc which it requires ;-)
-- let dons_bstr.hs =
import qualified Data.ByteString.Lazy.Char8 as S
main = print . go 0 =<< S.getContents
  where
    go !n !s = case S.readInt s of
               Nothing     -> n
               Just (k,t)  -> go (n+k) (S.tail t)

ghc -XBangPatterns --make dons_bstr.hs
time dons_bstr < nums
real    1m8.686s
user    0m0.015s
sys     0m0.031s

ghc -XBangPatterns -Onot -fstrictness --make dons_bstr.hs
time dons_bstr.exe < nums
real    1m10.607s
user    0m0.031s
sys     0m0.015s

ghc -XBangPatterns -O2 --make dons_bstr.hs
time dons_bstr.exe < nums
real    0m1.500s
user    0m0.015s
sys     0m0.031s

so althought the -fstrictness still seems to have no effect in this case, using -O2 the dons version is ~2 times faster then the bytestring/readInt version -O2

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Gmane