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