I had a program I was working on lately (darcs) crash with a segmentation fault after I made a seemingly harmless refactoring. The original code was {-# INLINE linesPS #-} linesPS :: B.ByteString -> [B.ByteString] linesPS ps | B.null ps = [B.empty] | otherwise = BC.split '\n' ps which I wanted to optimize, pasting code from the definition of Data.ByteString.lines, to linesPS ps = case search ps of Nothing -> [ps] Just n -> B.take n ps : linesPS (B.drop (n + 1) ps) where search = BC.elemIndex '\n' So I looked at the bytestring library to see if there was something that could explain the crash. I found that it uses accursedUnutterablePerformIO all over the place. The dire warnings accompanying this "function" (including the citation of a number of problem reports against commonly used libraries) made me think that it may be worthwhile to offer an opt-out for users of libraries like bytestring or text. (Note that I am not claiming my particular crash is due to a bug in bytestring, I merely want to exclude the possibility.) For the libraries in question it would be simple to do this: just add a cabal flag to optionally disable use of accursedUnutterablePerformIO. That still leaves the question of how users can make their project depend on a bytestring that has been built with this flag. I know this can be done with manual installation of a new version of the library, but I would rather use cabal new-build (as I am used to) and let it figure out itself that it has to attach a new hash to the variant with the flag. Cheers Ben