Proposal #3537: Add missing NFData instances to parallel

[Roel van Dijk]

> This might be "dirty" but what about this instead?
>
>    instance (NFData a) => NFData (Data.IORef.IORef a) where
>        rnf r = unsafePerformIO $ modifyIORef r (`using` rnf)

Hmm, I don't know. The question is whether you want to evaluate the
value stored inside of the IORef. I think that any function that
evaluates a value inside of an IORef should be in the IO monad. I
wouldn't expect a function with the type "a -> ()" to be able to "look
into" an IORef.

The reason I made an NFData instance for IORef is for a situation like this:

test :: IO ()
test = do x <- newIORef ()
          y <- newIORef ()
          let z | hugeConditionalExpression = x
                | otherwise = y
          rnf z `seq` doSomething

You might want to force z to head normal form to force the evaluation
of the hugeConditionalExpression.