opt out of accursedUnutterablePerformIO

[Ben Franksen]

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