[Haskell-cafe] is value evaluated?

Hi, I am curious to know if there is a function in Haskell to find if a certain value has already been evaluated. The function I need would have the type:

(?!) :: a -> Bool

And I expect it to be such that the following terminates after printing the first 101 fibonacci numbers.

fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

main = do print $ fibs !! 100 print $ takeWhile (?!) fibs

Although I guess I can imagine the following not terminating:

print $ filter (?!) fibs

I would find such a function immensely useful in "printing out" my infinite lists. Thanks! nikhil