[Haskell-cafe] How to write such a code elegantly ?

[Ertugrul Soeylemez]

z_axis <z_axis at 163.com> wrote:

> betterStdGen :: IO StdGen
> betterStdGen = alloca $ \p -> do
>    h <- openBinaryFile "/dev/urandom" ReadMode
>    hGetBuf h p $ sizeOf (undefined :: Int)
>    hClose h
>    mkStdGen <$> peek p
>
> picoSec :: IO Integer
> picoSec = do
>     t <- ctPicosec `liftM` (getClockTime >>= toCalendarTime)
>     return t
>
> The pseudo-code is :
>
> if doesFileExist "/dev/urandom"
>     then myGen = betterStdGen
>     else myGen = (mkStdGen . fromTnteger) <$> picoSec
>
> How to write these pseudo-code elegantly ?

I would do this:

  {-# LANGUAGE ScopedTypeVariables #-}

  readFrom :: forall a. Storable a => Handle -> IO a
  readFrom h =
    alloca $ \ptr ->
      hGetBuf h ptr (sizeOf (undefined :: a)) >>
      peek ptr

  newStdGen' :: IO StdGen
  newStdGen' = do
    mh <- try $ openBinaryFile "/dev/urandom" ReadMode
    case mh of
      Left err -> ctPicosec <$> (getClockTime >>= toCalendarTime)
      Right h  -> mkStdGen <$> readFrom h `finally` hClose h

Warning: Untested code, but it should work and have a safer file
handling.  Also note that the current implementation (base >= 4) does
this already.

You should probably try one of the more sophisticated PRNG libraries out
there.  Check out mersenne-random and mwc-random.  If you want a pure
generator, there is also mersenne-random-pure64 and some other
libraries.


Greets,
Ertugrul


-- 
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/