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" <[EMAIL PROTECTED]>
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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