An example of how Foreign.Marshal.ContT can make FFI code nicer

cont-regex.hs

compile :: ByteString -> [PCREOption] -> Either String Regex
compile str flags = unsafePerformIO $
  useAsCString str $ \pattern -> do
    alloca $ \errptr       -> do
    alloca $ \erroffset    -> do
        pcre_ptr <- c_pcre_compile pattern (combineOptions flags) errptr erroffset nullPtr
        if pcre_ptr == nullPtr
            then do
                err <- peekCString =<< peek errptr
                return (Left err)
            else do
                reg <- newForeignPtr finalizerFree pcre_ptr -- release with free()
                return (Right (Regex reg str))
                
-- becomes

compile :: ByteString -> [PCREOption] -> Either String Regex
compile str flags = unsafePerformIO $ lowerContT $ do
  pattern <- useAsCString str
  errptr <- alloca
  erroffset <- alloca
  pcre_ptr <- c_pcre_compile pattern (combineOptions flags) errptr erroffset nullPtr
  if pcre_ptr == nullPtr
    then do
        err <- peekCString =<< peek errptr
        return (Left err)
    else do
        reg <- newForeignPtr finalizerFree pcre_ptr -- release with free()
        return (Right (Regex reg str))