Test if a value has been evaluated to weak head normal form

Question

In Haskell, is it possible to test if a value has been evaluated to weak head normal form? If a function already exists, I would expect it to have a signature like

Answer 1

I'm not sure that there's anything pre-packaged for this. However, one can code it up:

import Data.IORef
import System.IO.Unsafe

track :: a -> IO (a, IO Bool)
track val = do
    ref <- newIORef False
    return
        ( unsafePerformIO (writeIORef ref True) `seq` val
        , readIORef ref
        )

Here's an example usage in ghci:

*NFTrack> (value, isEvaluated) <- track (undefined:undefined)
*NFTrack> isEvaluated
False
*NFTrack> case value of _:_ -> "neat!"
"neat!"
*NFTrack> isEvaluated
True

Of course, this will be tracking whether the wrapped write-and-then-return-the-original-value thunk is evaluated to WHNF, not whether the thing passed to track is evaluated to WHNF, so you'll want to put this as close to the thunk you're interested in as possible -- e.g. it will not be able to tell you whether a thunk made by somebody else has already been evaluated by somebody else before the tracking started. And of course consider using MVar instead of IORef if you need thread-safety.