Checking for WHNF (a horrible naughty thing)

I would like to do a horrible naughty thing (which I promise never to expose to the world). I would like to tell whether a term is in WHNF, without forcing evaluation of that term. Something like: isWHNF :: a -> Bool Is there a way of doing this? I can fake it with an IORef and much unsafeness, but I'm wondering if there's a safe-but-ugly way of doing the test in GHC. If you're curious, I'm trying to compact exactly the evaluated spine of a list without changing the list's laziness in any way. It remains to be seen whether this is even vaguely a good idea. :-) -Jan-Willem Maessen