Checking for WHNF (a horrible naughty thing)

[Jan-Willem Maessen]

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