The appended snippet might help.. --sigbjorn -- whnf.hs import Foreign.StablePtr import System.IO.Unsafe isWHNF :: a -> Bool isWHNF a = unsafePerformIO $ do stl <- newStablePtr a rc <- isWhnf stl freeStablePtr stl return (rc /= 0) foreign import ccall safe "isWhnf" isWhnf :: StablePtr a -> IO Int /* whnf.c */ #include "Rts.h" int isWhnf(StgStablePtr st) { StgClosure* c = (StgClosure*)(stable_ptr_table[(StgWord)st].addr); return !(closure_THUNK(c)); } ----- Original Message ----- From: "Jan-Willem Maessen" <jmaessen@alum.mit.edu> To: "glasgow-haskell-users" <glasgow-haskell-users@haskell.org> Sent: Wednesday, November 23, 2005 08:10 Subject: 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

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