This has nothing to do with <insane>PerformIO.
import Data.Char ( ord ) import qualified Data.ByteString as B import qualified Data.ByteString.Char8 as BC import qualified Data.ByteString.Internal as BI import Test.QuickCheck
-- | betweenLinesPS returns the B.ByteString between the two lines given, -- or Nothing if they do not appear. betweenLinesPS :: B.ByteString -> B.ByteString -> B.ByteString -> Maybe B.ByteString betweenLinesPS start end ps = case break (start ==) (linesPS1 ps) of -- replace this call here ^^^^^ with linesPS2 -- and it crashes (_, _:rest@(bs1:_)) -> case BI.toForeignPtr bs1 of (ps1, s1, _) -> case break (end ==) rest of (_, bs2:_) -> case BI.toForeignPtr bs2 of (_, s2, _) -> Just $ BI.fromForeignPtr ps1 s1 (s2 - s1)
Ouch. What if the elements returned by linesPS1 are not based off the same memory area? And indeed that happens. If add a bit of debug output, (ps2, s2, _) -> traceShow ("oops", s1, s2, ps1, ps2) $ Just $ BI.fromForeignPtr ps1 s1 (s2 - s1) then we get ("oops",2,4,0x0000004200107060,0x0000004200107060) with linesPS1 but ("oops",0,4,0x0000000000000000,0x0000004200107060) with linesPS2. The reason for the 0 pointer is that 'Data.ByteString.take' has a special case when the empty string is produced: take :: Int -> ByteString -> ByteString take n ps@(PS x s l) | n <= 0 = empty | n >= l = ps | otherwise = PS x s n {-# INLINE take #-} where empty = PS nullForeignPtr 0 0 You are kind of lucky that linesPS1 works. Cheers, Bertram